HEAT STRESS AND STRAIN

HEAT STRESS AND STRAIN

HEAT STRESS AND STRAIN 

TLV®

Warning: While the TLV is based on the ability of most healthy, acclimatized workers to sustain a heat stress exposure, cases of heatstroke and other exertional heat illnesses may occur below the TLV. A program of heat stress management should include acclimatization, early recognition of symptoms with appropriate first aid, and recognition of personal risk factors. Further, there is evidence of a carry-over effect from a previous day’s exposure.

Personal risk factors include, among others, prior heatstroke, repeated heat exhaustion, cardiac or kidney disease, pregnancy, obesity, older age and certain medications. It is recommended that workers with personal risk factors consult a health care provider prior to working in a hot environment.

This TLV has a small margin of safety. Therefore, those working near the TLV should be warned to drink water regularly and be alert for dizziness, lightheadedness, nausea, and headache.

Goal: The goal of this TLV is to maintain body core temperature within + 1°C of normal (37°C) for the average person. For most individuals, body core temperature will be below 38.3°C. Body core temperature can exceed 38.3°C under certain circumstances with selected populations, environmental and physiologic monitoring, and other controls.

More than any other physical agent, the potential health hazards from work in hot environments depend strongly on physiological factors that lead to a range of susceptibilities depending on the level of acclimatization. Therefore, professional judgment is of particular importance in assessing the level of heat stress and physiological heat strain to adequately provide guidance for protecting nearly all healthy workers with due consideration of individual factors and the type of work. Assessment of both heat stress and heat strain can be used for evaluating the risk to worker safety and health. A decision-making process is suggested in Figure 1. The exposure guidance provided in Figures 1 and 2 and in the associated Documentation of the TLV represents conditions under which it is believed that nearly all heat acclimatized, adequately hydrated, unmedicated, healthy workers may be repeatedly exposed without adverse health effects. The Action Limit (AL) is similarly protective of unacclimatized workers and represents conditions for which a heat stress management program should be considered. While not part of the TLV, elements of a heat stress management program are offered. The exposure guidance is not a fine line between safe and dangerous levels.

FIGURE 1. Evaluating heat stress and strain. 

Heat Stress is the net heat load to which a worker may be exposed from the combined contributions of metabolic heat, environmental factors (i.e., air temperature, humidity, air movement, and radiant heat), and clothing requirements. A mild or moderate heat stress may cause discomfort and may adversely affect performance and safety, but it is not harmful to health. As heat stress approaches human tolerance limits, the risk of heat-related disorders increases.

Heat Strain is the overall physiological response resulting from heat stress. The physiological responses are dedicated to dissipating excess heat from the body.

Acclimatization is a gradual physiological adaptation that improves an individual’s ability to tolerate heat stress. Acclimatization requires physical activity under heat-stress conditions similar to those anticipated for the work. With a recent history of heat-stress exposures of at least two continuous hours (e.g., 5 of the last 7 days to 10 of 14 days), a worker can be considered acclimatized for the purposes of the TLV. Its loss begins when the activity under those heat stress conditions is discontinued, and a noticeable loss occurs after four days and maybe completely lost in three to four weeks. Because acclimatization is to the level of the heat stress exposure, a person will not be fully acclimatized to a sudden higher level; such as during a heatwave.

FIGURE 2. TLV® (solid line) and Action Limit (broken line) for Heat Stress. WBGTeff is the measured WBGT plus the clothing adjustment value.

 

The decision process illustrated in Figure 1 should be started if (1) a qualitative exposure assessment indicates the possibility of heat stress, (2) there are reports of discomfort due to heat stress,or (3) professional judgment indicates heat stress conditions.

Section 1: Clothing. Ideally, free movement of cool, dry air over the skin’s surface maximizes heat removal by both evaporation and convection. Evaporation of sweat from the skin is the predominant heat removal mechanism. Water-vapor-impermeable, air-impermeable, and thermally insulating clothing, as well as encapsulating suits and multiple layers of clothing, severely restrict heat removal. With heat removal hampered by clothing, metabolic heat may produce excessive heat strain even when ambient conditions are considered cool.

Figure 1 requires a decision about clothing and how it might affect heat loss. The WBGT-based heat exposure assessment was developed for a traditional work uniform of a long-sleeve shirt and pants. If the required clothing is adequately described by one of the ensembles in Table 1 or by other available data, then the “YES” branch is selected.

If workers are required to wear clothing not represented by an ensemble in Table 1, then the “NO” branch should be taken. This decision is especially applicable for clothing ensembles that are 1) totally encapsulating suits or 2) multiple layers where no data are available for adjustments. For these kinds of ensembles, Table 2 is not a useful screening method to determine a threshold for heat-stress management actions and some risk must be assumed. Unless a detailed analysis method appropriate to the clothing requirements is available, physiological and signs/symptoms monitoring described in Section 4 and Table 4 should be followed to assess the exposure.

Section 2: Screening Threshold Based on Wet-Bulb Globe Temperature (WBGT). The WBGT offers a useful first order index of the environmental contribution to heat stress. It is influenced by air temperature, radiant heat, air movement, and humidity. As an approximation, it does not fully account for all the interactions between a person and the environment and cannot account for special conditions such as heating from a radiofrequency/microwave source.

WBGT values are calculated using one of the following equations:

With direct exposure to sunlight:
WBGTout = 0.7 Tnwb + 0.2 Tg + 0.1 Tdb

Without direct exposure to the sun:
WBGTin = 0.7 Tnwb + 0.3 Tg

where:

Tnwb = natural wet-bulb temperature (sometimes called NWB)

Tg = globe temperature (sometimes called GT)

Tdb = dry-bulb (air) temperature (sometimes called DB)

 

Because WBGT is only an index of the environment, the screening criteria are adjusted for the contributions of work demands and clothing. Table 2 provides WBGT criteria suitable for screening purposes. For clothing ensembles listed in Table 1, Table 2 can be used when the clothing adjustment values are added to the environmental WBGT.

 

TABLE 1. Clothing Adjustment Values for Some Clothing Ensembles* 
Clothing Type  Addition to WBGT [°C] 
Work Clothes (long sleeve shirt and pants)  0 
 
Cloth (woven material) 
Coveralls 
0 
 
Double Layer Woven Clothing  3 
 
SMS Polypropylene Coveralls  0.5 
 
Polyolefin Coveralls  1 
Limited-Use Vapor-Barrier Coveralls  11 
 
* These values must not be used for completely encapsulating suits, often called Level A. Clothing Adjustment Valuescannot be added for multiple layers. The coveralls assume that only modesty clothing is worn underneath, not a second layer of clothing.

 

TABLE 2. Screening Criteria for TLV® and Action Limit for Heat Stress Exposure 
  TLV® (WBGT values in °C)  Action Limit (WBGT values in °C) 
Allocation of Work in a Cycle of Work and Recovery  Light  Moderate  Heavy  Very Heavy  Light  Moderate  Heavy  Very 

Heavy 

75 to 100%  31.0  28.0      28.0  25.0     
50 to 75%  31.0  29.0  27.5    28.5  26.0  24.0   
25 to 50%  32.0  30.0  29.0  28.0  29.5  27.0  25.5  24.5 
0 to 25%  32.5  31.5  30.5  30.0  30.0  29.0  28.0  27.0 
Notes:  

  • See Table 3 and the Documentation for work demand categories.  
  • WBGT values are expressed to the nearest 0.5°C.  
  • The thresholds are computed as a TWA–Metabolic Rate where the metabolic rate for rest is taken as 115 W and work is the representative (mid-range) value of Table 3. The time base is taken as the proportion of work at the upper limit of the percent work range (e.g., 50% for the range of 25 to 50%). 
  • If work and rest environments are different, hourly time-weighted averages (TWA), WBGT should be calculated and used. TWAs for work rates should also be used when the work demands vary within the hour, but note that the metabolic rate for rest is already factored into the screening limit.  
  • Values in the table are applied by reference to the “Work-Rest Regimen” section of the Documentation and assume 8-hour workdays in a 5-day workweek with conventional breaks as discussed in the Documentation. When workdays are extended, consult the “Application of the TLV®” section of the Documentation. 
  • Because of the physiological strain associated with Heavy and Very Heavy work among less fit workers regardless of WBGT, criteria values are not provided for continuous work and for up to 25% rest in an hour for Very Heavy work. The screening criteria are not recommended, and a detailed analysis and/or physiological monitoring should be used.  
  • Table 2 is intended as an initial screening tool to evaluate whether a heat stress situation may exist (according to Figure 1) and thus, the table is more protective than the TLV® or Action Limit (Figure 2). Because the values are more protective, they are not intended to prescribe work and recovery periods.

 

To determine the degree of heat stress exposure, the work pattern and demands must be considered. If the work (and rest) is distributed over more than one location, then a time-weighted average WBGT should be used for comparison to Table 2 limits.  

As metabolic rate increases (i.e., work demands increase), the criteria values in the table decrease to ensure that most workers will not have a core body temperature above 38°C. Correct assessment of work rate is of equal importance to environmental assessment in evaluating heat stress. Table 3 provides broad guidance for selecting the work rate category to be used in Table 2. Often there are natural or prescribed rest breaks within an hour of work, and Table 2 provides the screening criteria for three allocations of work and rest.  

Based on metabolic rate category for the work and the approximate proportion of work within an hour, a WBGT criterion can be found in Table 2 for the TLV and for the Action Limit. If the measured time-weighted average WBGT adjusted for clothing is less than the table value for the Action Limit, the “NO” branch in Figure 1 is taken, and there is little risk of excessive exposures to heat stress. If the conditions are above the Action Limit, but below the TLV, then consider general controls described in Table 5. If there are reports of the symptoms of heat-related disorders such as fatigue, nausea, dizziness, and lightheadedness, then the analysis should be reconsidered. 

If the work conditions are above the TLV screening criteria in Table 2, then further analysis is required following the “YES” branch. 

Section 3: Detailed Analysis. Table 2 is intended to be used as a screening step. It is possible that a condition may be above the TLV or Action Limit criteria provided in Table 2 and still not represent an exposure above the TLV or the Action Limit. To make this determination, a detailed analysis is required. Methods are fully described in the Documentation, in industrial hygiene and safety books, and in other sources.  

Provided that there is adequate information on the heat stress effects of the required clothing, the first level of detailed analysis is a task analysis that includes a time-weighted average of the Effective WBGT (environmental WBGT plus clothing adjustment value) and the metabolic rate. Some clothing adjustment values have been suggested in Table 1. Values for other clothing ensembles appearing in the literature can be used in similar fashion following good professional judgment. The TLV and Action Limit are shown in Figure 2.

 

TABLE 3. Metabolic Rate Categories and the Representative Metabolic Rate with Example Activities 
Category  Metabolic Rate [W]*  Examples 
Rest  115  Sitting 
Light  180  Sitting with light manual work with hands or hands and arms, and driving. Standing with some light arm work and occasional walking. 
Moderate  300  Sustained moderate hand and arm work, moderate arm and leg work, moderate arm and trunk work, or light pushing and pulling.  Normal walking. 
Heavy  415  Intense arm and trunk work, carrying, shoveling, manual sawing; pushing and pulling heavy loads; and walking at a fast pace. 
Very Heavy  520  Very intense activity at fast to maximum pace. 
*The effect of body weight on the estimated metabolic rate can be accounted for by multiplying the estimated rate by the ratioactual body weight divided by 70 kg (154 lb).

 

The second level of detailed analysis would follow a rational model of heat stress, such as the International Standards Organization (ISO) Predicted Heat Strain (ISO 7933, 2004; Malchaire et al., 2001). While a rational method (versus the empirically derived WBGT thresholds) is computationally more difficult, it permits a better understanding of the sources of the heat stress and is a means to appreciate the benefits of proposed modifications in the exposure. Guidance to the ISO method and other rational methods is described in the literature.

The screening criteria require the minimal set of data to make a determination. Detailed analyses require more data about the exposures. Following Figure 1, the next question asks about the availability of data for a detailed analysis. If these data are not available, the “NO” branch takes the evaluation to physiological monitoring to assess the degree of heat strain. 

If the data for a detailed analysis are available, the next step in Figure 1 is the detailed analysis. If the exposure does not exceed the criteria for the Action Limit (or unacclimatized workers) for the appropriate detailed analysis (e.g., WBGT analysis, another empirical method, or a rational method), then the “NO” branch can be taken. If the Action Limit criteria are exceeded but the criteria for the TLV (or other limit for acclimatized workers) in the detailed analysis are not exceeded, then consider general controls and continue to monitor the conditions. General controls include training for workers and supervisors, heat stress hygiene practices, and medical surveillance (Table 5). If the exposure exceeds the limits for acclimatized workers in the detailed analysis, the “YES” branch leads to physiological monitoring as the only alternative to demonstrate that adequate protection is provided.

 

TABLE 4. Guidelines for Limiting Heat Strain 
Monitoring heat strain and signs and symptoms of heat-related disorders is sound industrial hygiene practice, especially when clothing may significantly reduce heat loss. For surveillance purposes, a pattern of workers exceeding the heat strain limits is indicative of a need to control the exposures. On an individual basis, the limits represent a time to cease exposure and allow for recovery. 

One or more of the following measures may mark excessive heat strain, and an individual’s exposure to heat stress should be discontinued when any of the following occur: 

  • Sustained (several minutes) heart rate is in excess of 180 bpm (beats per minute) minus the individual’s age in years (180 age) for individuals with assessed normal cardiac performance; or 
  • Body core temperature is greater than 38.5°C (101.3°F) for medically selected and acclimatized personnel; or greater than 38°C (100.4°F) in unselected, unacclimatized workers; or 
  • Recovery heart rate at one minute after a peak work effort is greater than 120 bpm; or 
  • There are symptoms of sudden and severe fatigue, nausea, dizziness, or lightheadedness. 

An individual may be at greater risk of heat-related disorders if: 

  • Profuse sweating is sustained over hours; or 
  • Weight loss over a shift is greater than 1.5% of body weight; or 
  • 24-hour urinary sodium excretion is less than 50 mmoles. 

EMERGENCY RESPONSE:  If a worker appears to be disoriented or confused, suffers inexplicable irritability, malaise, or chills, the worker should be removed for rest in a cool location with rapidly circulating air and kept under skilled observation. Absent medical advice to the contrary, treat this as an emergency with immediate transport to a hospital. An emergency response plan is necessary.  

— NEVER ignore anyone’s signs or symptoms of heat-related disorders — 

 

TABLE 5. Elements to Consider in Establishing a Heat Stress Management Program 
Monitor heat stress (e.g., WBGT Screening Criteria in Table 2) and heat strain (Table 4) to confirm adequate control  

General Controls 

  • Provide accurate verbal and written instructions, annual training programs, and other information about heat stress and strain 
  • Encourage drinking small volumes (approximately 1 cup) of cool, palatable water (or other acceptable fluid replacement drink) about every 20 minutes  
  • Encourage employees to report symptoms of heat-related disorders to a supervisor 
  • Encourage self-limitation of exposures when a supervisor is not present  
  • Encourage co-worker observation to detect signs and symptoms of heat strain in others 
  • Counsel and monitor those who take medications that may compromise normal cardiovascular, blood pressure, body temperature regulation, renal, or sweat gland functions; and those who abuse or are recovering from the abuse of alcohol or other intoxicants 
  • Encourage healthy lifestyles, ideal body weight, and electrolyte balance 
  • Adjust expectations of those returning to work after an absence from hot exposure situations and encourage consumption of salty foods (with approval of physician if on a salt-restricted diet)  
  • Consider pre-placement medical screening to identify those susceptible to systemic heat injury  
  • Monitor the heat stress conditions and reports of heat-related disorders 

Job-Specific Controls 

  • Consider engineering controls that reduce the metabolic rate, provide general air movement, reduce process heat and water vapor release, and shield radiant heat sources, among others  
  • Consider administrative controls that set acceptable exposure times, allow sufficient recovery, and limit physiological strain  
  • Consider personal protection that is demonstrated effective for the specific work practices and conditions at the location  

— NEVER ignore anyone’s signs or symptoms of heat-related disorders — 

 

Section 4: Heat Strain. The risk and severity of excessive heat strain will vary widely among people, even under identical heat stress conditions. The normal physiological responses to heat stress provide an opportunity to monitor heat strain among workers and to use this information to assess the level of heat strain present in the workforce, to control exposures, and to assess the effectiveness of implemented controls. Table 4 provides guidance for acceptable limits of heat strain. 

Following good industrial hygiene sampling practice, which considers likely extremes and the less tolerant workers, the absence of any of these limiting observations indicates acceptable management of the heat stress exposures. With acceptable levels of heat strain, the “NO” branch in Figure 1 is taken. Nevertheless, if the heat strain among workers is considered acceptable at the time, consideration of the general controls is recommended. In addition, periodic physiological monitoring should be continued to ensure acceptable levels of heat strain. 

If limiting heat strain is found during the physiological assessments, then the “YES” branch is taken. This means that suitable job-specific controls should be implemented to a sufficient extent to control heat strain. The job-specific controls include engineering controls, administrative controls, and personal protection.  

After implementation of the job-specific controls, it is necessary to assess their effectiveness and to adjust them as needed.  

Section 5: Heat Stress Management and Controls. The elements of a heat stress management program including general and job-specific controls should be considered in light of local conditions and the judgment of the industrial hygienist. The recommendation to initiate a heat stress management program is marked by 1) heat stress levels that exceed the Action Limit or 2) work in clothing ensembles that limit heat loss. In either case, general controls should be considered (Table 5).  

Heat stress hygiene practices are particularly important because they reduce the risk that an individual may suffer a heat-related disorder. The key elements are fluid replacement, self-determination of exposures, health status monitoring, maintenance of a healthy lifestyle, and adjustment of expectations based on the acclimatization state. The hygiene practices require the full cooperation of supervision and workers. 

In addition to general controls, appropriate job-specific controls are often required to provide adequate protection. During the consideration of job-specific controls, Table 2 and Figure 2, along with Tables 1 and 3, provide a framework to appreciate the interactions among acclimatization state, metabolic rate, work-rest cycles, and clothing. Among administrative controls, Table 4 provides acceptable physiological and signs/symptoms limits. The mix of job-specific controls can be selected and implemented only after a review of the demands and constraints of any particular situation. Once implemented, their effectiveness must be confirmed and the controls maintained. 

The prime objective of heat stress management is the prevention of heatstroke, which is life-threatening and the most serious of heat-related disorders. The heatstroke victim is often manic, disoriented, confused, delirious, or unconscious. The victim’s body core temperature is greater than 40°C (104°F). If signs of heatstroke appear, aggressive cooling should be started immediately, and emergency care and hospitalization are essential. The prompt treatment of other heat-related disorders generally results in full recovery, but medical advice should be sought for treatment and return-to-work protocols. It is worth noting that the possibility of accidents and injury increases with the level of heat stress. 

Prolonged increases in deep body temperatures and chronic exposures to high levels of heat stress are associated with other disorders such as temporary infertility (male and female), elevated heart rate, sleep disturbance, fatigue, and irritability. During the first trimester of pregnancy, a sustained core temperature greater than 39°C may endanger the fetus. 

DOCUMENTATION

Rationale for the TLV® 

The TLV for heat stress and strain is a departure from past recommendations for the control of heat stress in the workplace. The expert panel on heat stress of the World Health Organization (WHO) recommended limiting body core temperature to 38°C for extended periods of time (WHO, 1969). This recommendation became the benchmark. Because no simple technology existed to assess core temperature in the workplace and because industrial hygiene methods for risk assessment concentrated on exposure evaluations, the natural course of action was to develop an exposure assessment metric for heat stress and to select a threshold to limit body core temperature. 

Since that time, exposure assessment methods have improved substantially. First, rational models for heat stress have evolved to consider not only the risk of excessive body temperature but also of excessive dehydration. Second, more experience with, and a greater need for, measures of heat strain have emerged. The TLV for heat stress and strain incorporates these established ideas. 

Exposure Limits to Occupational Heat Stress

In the 1960s, there were two widely held recommendations for the protection of workers during heat stress exposures. In 1963, Lind (1963a) recommended an upper limit of the prescriptive zone as an occupational exposure limit. In 1969, WHO recommended a limitation on the deep-body temperature of 38°C (100.4°F) for sustained exposures to heat stress. The upper limit of the prescriptive zone and the core temperature limit still guide the consideration of exposure limits.

Upper Limit of the Prescriptive Zone

Nielsen (1938) and Nielsen and Nielsen (1962) demonstrated that body core (rectal) temperature remains constant over a wide range of environments, which includes cool, comfortable, and warm conditions. There is a critical environment above which the steady-state core temperature increases with temperatures greater than the critical environment. This trend is illustrated in Figure 3, based on Lind’s classic study reported in 1963 (1963a). In this series of experiments, 3 mine rescue personnel completed about 45 trials each at a metabolic rate of 350 W, and 2 of these completed another 35 trials at 210 and 490 W. The participants were not acclima­tized and wore shorts and shoes only. At each metabolic rate, Lind looked for the critical environment that marked the middle of the transition from a straight flat line to a straight-sloped line for the data. Below the critical environment is the work-driven zone, and above the critical environment is the environmentally driven zone. The work-driven zone is a zone of compensable heat stress and the core temperature depends only on the relative demands of the work on the individual (Pandolf et al., 1986). The environmentally driven zone represents the early stages of uncompensable heat stress and a condition where small changes in the environment can lead to large changes in core temperature. Lind described this transition point as the Upper Limit of the Prescriptive Zone (ULPZ). When exposures are below the ULPZ, the body core temperature is comfortably below 38°C except for heavy work near 500 W. This physiologically based approach became the heart of the recommendations by the U.S. National Institute for Occupational Safety and Health (NIOSH) for occupational exposures to hot environments (U.S. NIOSH, 1997). While Lind (1963a, b) and others reported the environmental conditions as Effective Temperature, the values were converted to an equivalent WBGT for reporting here.

FIGURE 3. Illustration of trends among core temperature [°C], work demands [W] and environment [ET °C], adapted from Lind (1963a).

 

WBGT-Based Thresholds

The early use of the wet-bulb globe temperature (WBGT) was to limit the level of heat stress that produced unacceptable numbers of heat casualties among U.S. Marine recruits during vigorous outdoor activity in summer. To establish a threshold, the assumption was that one type of clothing was worn. When a criterion based on measurements of WBGT was set, a particular level of training exercise either could proceed or be interrupted. Use of this approach by Yaglou and Minard (1957), with criterion figures related to these specific activity levels, was immediately effective.

With the above principles in mind, the search began for a WBGT threshold to be used in industry to protect workers. Because the original work of Yaglou and Minard (1957) was based on military recruits, Lind’s proposal for the upper limit of the prescriptive zone in 1963 (1963a) was the starting point. These data were from three unacclimatized semi-nude mine rescue men while they walked on a treadmill for an hour to establish thermal equilibrium. Table 6 reports these initial values of ULPZ at the three metabolic rates.

The upper limit of the prescriptive zone was verified for a moderate rate of work (350 W) for 25 unacclimatized semi-nude men (soldiers), 24 of whom were able to complete a 3-hour exposure with a demonstrated thermal balance (Lind, 1970). In this study, 45 others were exposed to higher WBGTs and many were not able to establish thermal equilibrium, while all 25 participants at a lower WBGT completed the 3-hour exposure. Subsequent experiments using 2 unacclimatized men in similar physical condition indicated that there were no detrimental physiological effects at 29.1°C-WBGT for 8 hours of continuous and intermittent work (same total energy expenditure) (Lind, 1963b). These experiments by Lind collectively demon­strated the validity of the ULPZ and the utility for work/rest cycles.

TABLE 6. Values in °C-WBGT for the Upper Limit of the Prescriptive Zone (ULPZ) and Comparison to the NIOSH REL and RAL at Three Metabolic Rates
Metabolic Rate
[W]
ULPZ (Lind, 1963a) ULPZ
(5th %ile) (Lind, 1963b)
ULPZ (Bernard, 1995) NIOSH REL/ RAL
210 32.2 30.0 / 27.2
350 28.4 27.4 29.2 27.4 / 24.0
490 27.8 25.8 / 22.0

 

There were two factors, however, that must be addressed to be applicable to occupational exposure limits. One was the use of unacclimatized participants when many workers would be acclimatized. The other was the semi-nude dress, just shorts, and shoes. Lind et al. (1970) reported on trials with 12 acclimatized semi-nude men (6 older and 6 younger miners). They found no differences between the age groups and that the highest of 5 WBGTs at which all could complete the exposure was 29.2°C-WBGT at 350 W. This suggested that acclimatization could raise the upper limit by 2°C-WBGT. Belding and Kamon (1973) reported that the difference between semi-nude and work clothes was about 2°C-WBGT, which means that the effects of acclimatization and clothing appeared to be counter-balancing.

These efforts set the stage for ACGIH® to propose heat exposure guidelines in 1971 and to adopt them in 1974 and for NIOSH to present criteria for occupational exposure to heat stress in 1986 (U.S. NIOSH, 1997). As reported in the 1986 NIOSH criteria document, the continuous exposure thresholds were the Recommended Exposure Limit (REL) for acclimatized individuals and the Recommended Alert Limit (RAL) for unacclimatized workers (U.S. NIOSH, 1997). Equations that represent the REL and RAL were fit to the curves presented in the 1986 criteria document with metabolic rate (M) in Watts (range, 100–600 W) (U.S. NIOSH, 1985; Bernard, 1995).

TLV [°C-WBGT] = REL [°C-WBGT]            (1)
     = 56.7 – 11.5 log10 M [W]

AL [°C-WBGT] = RAL [°C-WBGT]              (2)
= 59.9 – 14.1 log10 M [W]

Using these equations, the REL and RAL values at the three metabolic rates of Lind (1963a, b) are provided in Table 6. The values selected for the REL were generally lower than the original values for the ULPZ and lower than the adjustment of 1°C-WBGT for the 5th percentile at the lower and higher metabolic rates.

The REL and RAL became the TLVs for acclimatized and unacclimatized workers in past TLVs, respectively. The recommended TLV remains at the NIOSH REL, and the recommended Action Limit (former TLV for unacclimatized) is the NIOSH RAL. The TLV and Action Limit are shown in Figure 2 in the TLV section. These thresholds represent the trade-off between environmental conditions as reflected in the WBGT and the heat generation due to work, all assuming lightweight work clothes. Specifically, the thresholds are set so that most workers can easily maintain thermal equilibrium at or below the threshold. Therefore, the threshold maintains an adequate level of protection.

COMPARING THE TLV® TO CRITICAL WBGTS

Because the TLV is unchanged from that established in the 1970s, which was based on the best available data, comparing the TLV to data not considered at the time is a worthwhile endeavor. Table 7 summarizes the data on the ULPZ for acclimatized individuals wearing work clothes, as reported in seven different studies. Where only the average values for the climates were available, the mean WBGT is reported in the table. If the standard deviation was provided, then the difference to the lower limit was computed as 1.65 times the standard deviation to represent 95% of the data in a one-tailed normal distribution. The lower limit was then computed as the mean minus the difference. The mean values, with a whisker if the lower limit is known, are presented in Figure 4, with the TLV line for reference.

The experimental protocol of Kuhlemeier et al. (1977) followed the method employed by Lind (1963a, b) to determine the ULPZ at three levels of metabolic rate. The data reported in the table and figure are for workers employed in hot industries during the summer so that they could be considered acclimatized. The one point below the line is from this study at the lowest metabolic rate. This point was lower than the next higher metabolic rate and well off the other data. While the value is based on an adequate number of data to suggest that it is real, it lacks biological plausibility. The two data points nearest the line are from the same study, suggesting that there was a systematic bias to lower values than the other studies.

 

TABLE 7. ULPZ for Acclimatized Individuals Wearing Work Clothes 
Reference Metabolic Rate Mean
WBGT
Difference to Lower Limit Lower Limit
for 95%
Kuhlemeier et al., 1977 270 27.1
Kuhlemeier et al., 1977 345 28.8
Kuhlemeier et al., 1977 490 26.6
Belding and Kamon, 1973 220 32.8
Belding and Kamon, 1973 280 32.6
Belding and Kamon, 1973 360 31.7
Kamon et al., 1978 305 33.4 1.1 32.3
Kenney, 1987 (reported) 300 31.8
Kenney et al., 1993 400 29.2 0.3 28.9
O’Connor and Bernard, 1999 290 34.2 0.9 33.3
Bernard et al., 2005 306 34.5 2.6 31.9

 

The other studies reported in the table and figure used a different protocol to identify the ULPZ, and this protocol is called the critical WBGT in this discussion. While the exact protocol varies among the studies, the underlying principle is a progressive increase in heat stress during one trial until there is a clear loss of thermal equilibrium. The point at which thermal balance is no longer maintained is the critical WBGT.  Belding and Kamon (1973) sought a critical vapor pressure at an air temperature of 36.5°C at three levels of metabolic rate and four levels of airspeed. The WBGT values were estimated from the dry-bulb temperature, mean vapor pressure, and airspeed and then averaged over the air speeds because there was little difference due to air movement. Kamon et al. (1978) reported the mean and standard deviation of the critical vapor pressures for acclimatized young men and women over seven air temperatures from 36° to 52°C.

These data allowed for the estimation of WBGT at the mean values and at 1.65 standard deviations below the mean to estimate a 5th percentile value (called lower limit in the table). Kenney (1987) reported a mean WBGT for work clothes in his laboratory. In another study, Kenney et al. (1993) looked at army fatigues (summer battle dress uniform) for which a mean and a lower limit WBGT were estimated from the data. O’Connor and Bernard (1999) reported mean WBGT values and standard deviations for work clothes in three different environments. In another study, Bernard et al. (2005) reported a mean and standard deviation of WBGT at a moderate rate of work for work clothes. As a note, no one using the progressive protocol for critical WBGT has demonstrated that the heat stress level can be maintained for a prolonged period of time. In fact, Lind argued that the critical WBGT may be greater than the ULPZ, but this was speculative on his part as well.

FIGURE 4. Reported WBGT limits for compensable heat stress as a mean (and when available, as a 5th %ile whisker) with the current TLVs as reference.

 

Some studies in India, Australia, and Saudi Arabia suggest that the TLV criteria may be unnec­essarily restrictive (Boyle, 1995; Khogali, 1983; Parikh et al., 1976; Rastogi et al., 1992; Tranter, 1998). For example, the WBGT criteria suggested for India appear to be higher than those recommended in the TLV (NIOH, 1996b). Other studies suggest that, at levels appearing unacceptable by the TLV criteria, the individual behavior reactions of those exposed can sufficiently modify physiological responses to avoid ill effect (Budd et al., 1991; Gunn and Budd, 1995). Overall, the TLV is protective. It appears to have a margin of protection of about 3°C-WBGT, but this margin of protection has not been sufficiently demonstrated to merit change at this time.

Notes on the Use of WBGT-Based Thresholds

Metabolic heat loads of many work activities make major contributions to heat stress and, therefore, to heat strain (Belding and Hatch, 1955; ISO 7933, 2004). At the same time, very wide variations were found in the levels of heat loads among workers carrying out a common task (Malchaire et al., 1984). This means that climatic chamber experiments that tightly control metabolic rate are less likely to support the epidemiological experience with WBGT (Ferres et al., 1954; Ramsey and Chai, 1954). Although there is always consid­erable uncertainty concerning metabolic heat load of the task for any individual, the validity of the work–rest regimens is supported (Malchaire, 1979).

Clothing

There are two considerations in whatever decisions have to be made about clothing in the workplace. It is important to wear clothing that protects the person from the environment, and it is also important to wear clothing that does not impede the loss of heat. In most cases, the clothing is chosen to protect the individual from chemical, physical, or biological agents that might be in the work environment, and then the question of how the selection affects heat stress is asked. This is an opportunity to ask whether the clothing is over-protective and if it can be changed to reduce the level of heat stress.

The factors in clothing that most influence the degree of heat stress are the evaporative resistance and the insulation, which are largely properties of the fabrics and the construction of the clothing. Evaporative resistance (the inverse of water vapor permeability) is a factor that influences the maximum rate of evaporative cooling that can be established. Vapor-barrier clothing significantly reduces evaporative cooling and greatly increases the level of heat stress. Clothing insulation is the resistance to dry heat flow due to convection and radiation. A traditional expression of clothing insulation is in units of clo (Gagge et al., 1941). In hot environments, clothing insulation reduces heat gain by convection from hot air and by radiation from hot surfaces. Highly insulating ensembles, however, contribute significantly to the evaporative resistance, and the marginal reduction in heat gain may be over-whelmed by the associated increase in evaporative resistance that is associated with insulation. Both thermal insulation and evaporative resistance of individual garments and clothing assemblies are considered in an International Organization for Standardization (ISO) standard (ISO 9920, 1995). This information is especially useful for rational methods for heat stress assessment (ISO 7933, 2004).

Evaporative resistance and insulation are reduced by circulation of air around and through the clothing. This convective permeability of the clothing depends on the fabrics, body movements, and clothing construction. Loosely woven (or perforated) fabrics with a construction that is loosely fitted with generous openings allow more evaporative cooling when body motion forces air under the clothing. This effectively reduces the level of heat stress. Con­versely, clothing fabrics and construction that are designed to minimize convective air movement through and around clothing (e.g., encapsulating clothing and turnout gear) greatly increase the level of heat stress. Because WBGT is commonly used to set exposure limits and because these limits are based on a typical cotton work uniform, some investigators have explored WBGT adjustments to account for other clothing ensembles. While the reporting of how the adjustments should be used (either as a reduction in the exposure limit or as an effective increase in the ambient WBGT), the purpose is essentially the same. Ramsey (1978), as well as Bernard et al. (1996), used professional judgment to recommend clothing adjustment values for a few clothing ensembles. Paull and Rosenthal (1987) and Kenney (1987) provided the first empirically-based recommendations for clothing adjustments, followed by a number of other researchers. A summary of those values for clothing adjustments that have been reported, referenced as additions to the ambient WBGT to represent an effective WBGT, are provided in Table 8.

This table, however, does not include all garments and materials that are commercially available.

 

TABLE 8. Reported Clothing Adjustments*
Clothing as Reported by Investigators Adjustment [°C-WBGT]
Kenney (1987)
        Cotton anti-contamination coveralls with hood and gloves 3.6
        Double cotton anti-contamination coveralls with hood and gloves 6.2
        Vapor-barrier over anti-contamination coveralls with hood and gloves 10.6
Paull and Rosenthal (1987)
        Vapor-barrier 11
Reneau and Bishop (1996)
        Vapor-barrier with hood and gloves 10
O’Connor and Bernard (1999)
        Tyvek® 1422A coveralls w/hood #1 1.5
        Tyvek® 1422A coveralls w/hood #2 2.0
        SMS polypropylene coveralls w/hood #1 -1.3
        SMS polypropylene coveralls w/hood #2 -0.9
        Coveralls w/hood of microporous film laminate 2.6
        ProTech 2000 coveralls w/hood and scrub suit 3.0
        Zirpo® wool shirt and FR8 denim pant 0.5
        FR9 cotton shirt and pants 0.8
        FR9 cotton shirt and pants w/undershirt 1.3
        FR9 S&P w/single-layer jacket–closed 2.1
        FR9 S&P w/double-layer jacket–open 2.6
        FR9 S&P w/double-layer jacket–closed 3.2
Bernard et al. (2005)
        Cotton coveralls 0
        Tyvek® 1424 coveralls 1
        NexGen® coveralls 2
        Tychem QC® coveralls 10
*The names of clothing are provided as reported. There are other kinds of clothing for which values are not reported in the scientific literature.

 

Readers should be aware of a number of caveats associated with the studies referenced in Table 8 and the use of clothing-adjustment values in the derivation of the effective WBGT. There is significant variability among subjects in the response to heat stress, so those studies with larger sample sizes and those studies with participants acting as their own control provide a stronger statistical statement. The external environment can influence the clothing adjustment values. Since the studies referenced in Table 8 were not all conducted under similar conditions, direct comparisons of adjustment values for various products across independent studies are tenuous. Similarly, the individual’s metabolic heat load during a particular task will influence the clothing protection value; the studies in Table 8 were not conducted under similar conditions. Metabolic heat load is particularly important with encapsulating suits or microporous films when the water vapor transmitting characteristics are overwhelmed. The adjustment values for layered clothing are not additive, and the inclusion of hoods and gloves to the ensemble may significantly increase the adjustment values. Lastly, manufacturers are continually modifying their products, so it is not uncommon to have a range of values reported for a common material.

Examination of the data in Table 8 allows some recommendations for clothing adjustments to be made.

  1. Cotton Coveralls. Kenney’s data (1987) sug­gested an adjustment of 3.6°C-WBGT for coveralls with a hood and sealed cuffs. The comparison group was from a different study, which means different participants and less control over the metabolic rate. The Bernard et al. (2005) study used more participants than other studies and the participants acted as their own controls, both features provide a stronger statistical statement. They recommended treating the coveralls without a hood the same as work clothes; that is, a zero adjustment. The adjustment of 0°C-WBGT for coveralls was selected for the TLV.
  2. Polyolefin (polyethylene). O’Connor and Bernard (1999) reported an adjustment of 1.8°C-WBGT for Tyvek 1422A. Subsequently, Bernard et al. (2005) reported a value of 1.0°C-WBGT for Tyvek 1424. The recommended adjustment value for Tyvek (polyolefin) coveralls is 1.0°C-WBGT.
  3. For SMS (spunbond-meltblown-spunbond) polypropylene coveralls, the adjustment from two separate studies reported by O’Connor and Bernard (1999) had very similar values at -1.0°C-WBGT, although the negative value might be an artifact of the experimental conditions and the need to cross over two studies. Bishop et al. (2003) have reported less heat strain experienced by participants wearing an SMS coverall than Tyvek 1422A, which is consistent with the lower adjustment. ACGIH® recommends a 1.5°C-WBGT adjustment for SMS coveralls.
  4. Double-Layer Cotton. Kenney (1987) reported a difference between single- and double-layer coveralls of 2.6°C-WBGT, based on the same participants and work demands. O’Connor and Bernard (1999) reported adjustments of 2° to 3°C-WBGT for multiple layers of clothing (jacket) compared to 0.8°C-WBGT for the single layer based on the same subjects. A full second layer would likely contribute to a greater difference. These results suggest, however, that 2.5°C-WBGT for a second full layer of woven clothing is about right. Accepting that the first layer is equivalent to work clothes with a 0°C-WBGT, 3°C-WBGT for a double-layer of woven clothing is recommended for the TLV.
  5. Vapor Barrier Clothing. Reneau and Bishop (1996) considered vapor barrier clothing in a Level B configuration in environments in which there was compensable and uncompensable heat stress and concluded that the transition point was about 10°C-WBGT lower than for ordinary clothing. Paull and Rosenthall (1987) and Kenney (1987) reported comparable values. Bernard et al. (2005) found some independence of WBGT with vapor-barrier coveralls and, thus, recommended a higher adjustment value in the observed range. The recommended clothing adjustment value for vapor-barrier clothing is 11°C-WBGT.
  6. Microporous Films. There is no clear consensus for water vapor transmitting films with liquid water holdout characteristics, typically microporous films. Values range from 2° to 6°C-WBGT and appear to depend on the actual film and the test conditions. For this reason, no specific guidance is recommended for the TLV.
  7. Firefighters’ Turnout Gear. Traditional bunker gear is a tri-layered garment, similar to the U.S. military chemical, biological, and nuclear protective clothing. Based on the Committee’s judgment, a 10°C-WBGT adjustment could be applied, but this value is not included in Table 1.

Please note that the recommended adjustment values are based on the worker wearing a single-layer coverall over modesty clothing (e.g., shorts and tee-shirt). There are no sufficient data available to recommend if or how combinations of adjustment values might be combined for combined clothing ensembles. For instance, multiple layers cannot be added. For this reason, the absence of an adjustment value for a specific combination of clothing means that the WBGT analysis described in the TLV cannot be used.

The recommended clothing adjustment values are provided in Table 1.

Screening Criteria

There was an early recognition that typical work is not continuous and that a rapid method of assessing the level of heat stress was welcomed. For these reasons, screening criteria were developed for the TLV (see Table 2). The first consideration was a representative metabolic rate for four categories of work, and these were taken as midpoints in the range, as described in Table 9. Four allocations of work in a period (25%, 50%, 75%, and 100%) were then used. For computational purposes, the upper limit of the work allocation was taken as representative of the work/rest cycle. That is, 100% work was assigned for the 75% to 100% category. This provides some compensation to balance the fact that a midpoint metabolic rate was used instead of the upper limit for the metabolic demands category. Assuming a resting metabolic rate of 115 W, the TWA metabolic rate was determined for the four work allotments. Rest was assumed to occur in the same or similar environment as the work; that is, the WBGT is the same. Based on the average metabolic rate, the threshold WBGT was computed. If there is a difference between the work and rest WBGTs, a TWA of the WBGT should be used.

The WBGT values in Table 2 are rounded to the nearest 0.5°C-WBGT to recognize the most realistic precision. Because similar tables are often converted to °F, and sometimes back to °C, there will be errors.  For this reason, Table 10 is provided here that has the WBGT values to the nearest 0.1° in both °C and °F. These are for background and not considered the TLV and Action Limit values.

 

TABLE 9. Categories of Metabolic Rate in Watts as Suggested by ISO (1989)
Category Metabolic Rate
[W]*
Range
of Metabolic Rates [W]
Examples
Rest 115 Sitting
Light 180 115–235 Sitting with light manual work with hands or hands and arms, and driving. Standing with some light arm work and occasional walking.
Moderate 300 235–360 Sustained moderate hand and arm work, moderate arm and leg work, moderate arm and trunk work, or light pushing and pulling.  Normal walking.
Heavy 415 360–470 Intense arm and trunk work, carrying, shoveling, manual sawing; pushing and pulling heavy loads; and walking at a fast pace.
*The effect of body weight on the estimated metabolic rate can be accounted for by multiplying the estimated rate by the ratio of actual body weight divided by 70 kg (154 lb).

 

TABLE 10. Screening Criteria in °C- and °F-WBGT from Which the Values in Table 2 Were Derived
TLV °C-WBGT
%Work L M H VH
100 30.8 28.2 26.6 25.5
75 31.2 29.0 27.6 26.5
50 31.8 30.1 28.8 27.9
25 32.3 31.3 30.5 29.8
°F-WBGT
%Work L M H VH
100 87.4 82.8 79.9 77.8
75 88.2 84.3 81.7 79.8
50 89.2 86.1 83.9 82.3
25 90.2 88.4 86.9 85.7
ACTION LIMIT °C-WBGT
%Work L M H VH
100 28.1 25.0 23.0 21.6
75 28.7 26.0 24.2 22.9
50 29.3 27.2 25.7 24.6
25 30.0 28.8 27.8 27.0
°F-WBGT
%Work L M H VH
100 82.6 77.0 73.4 70.9
75 83.6 78.8 75.6 73.3
50 84.8 81.0 78.3 76.3
25 86.1 83.8 82.0 80.6

 

Heat Stress and Strain

It is useful to distinguish between the concepts of heat stress and heat strain. Heat stress is the composite of external demands placed on a person.

For example, air temperature, relative humidity, air velocity, the intensity of infrared thermal radiation, the type of clothing worn, and physical activity are all factors defining potential heat stress, which are accessed with varying degrees of precision and accuracy. Such measures provide valuable and useful information about the thermal load to which humans must adjust. Typically, the more factors evaluated, the more reliable the information.

Measuring just air temperature, for instance, seldom provides much insight. Additional data about ambient humidity, air velocity, infrared radiant intensities, and emissivities of clothing and nearby objects provide a much more detailed picture of the level of heat stress. Measurements of thermal stress, no matter how accurately they are assessed, only quantify the thermal demands that challenge thermoregulation. On the other hand, heat stress measures are relatively accessible for hazard evaluation.

Heat strain reflects the extent to which the individual has to marshal defenses to maintain deep-body temperature in an acceptable range. Heat strain is the cost of adjusting to heat stress. It is not a measure of how successfully the adjustment is made. Gross measures of heat strain include body core temperature, heart rate, and sweat loss. Other important responses are distribution of the fluid volumes in the body, electrolyte concentrations in the intra- and extra-cellular spaces, levels of hormones, and blood pressure. For a given heat stress exposure, heat strain is a characteristic that is unique to each person and will change for the same person from time to time (Havenith et al., 1995; Havenith and Van Middendorp, 1990).

Heat stress assessment is the traditional approach to evaluation of the hazard of working in hot environments, but there is a role for heat strain assessment as an alternative. The assessment of heat strain is especially important when there is a wide variability in the heat stress conditions and the work demands. In fact, the variability in work demands may be the greater contributor to heat stress than the environmental conditions.

Personal Factors

Heat strain is not reliably predicted from heat stress. The predictive gap is largely explained by personal factors. These are each person’s unique strengths and weaknesses for distributing heat in the body and for dissipating it to the surrounding environment. The likelihood and severity of heat strain experienced by an individual for a given level of heat stress depends on the physiological capacity of that individual to respond to the heat stress. Personal risk factors are those elements that may reduce an individual’s tolerance for heat stress. A critical personal factor that will determine tolerance for heat stress is a previous episode of heatstroke or repeated episodes of heat exhaustion. Other personal factors include age, obesity, state of hydration, use of medications and drugs, gender, and acclimatization state. Some of the personal risk factors can be addressed through interventions (e.g., hydration state and acclimation), while others represent the usual variability among individuals.

AGE

Like many factors associated with evaluating human vulnerability, those presumably related to age and susceptibility to heat strain must be interpreted carefully. Age itself is not the most important criterion. Physical condition and debilitations commonly associated with age are more significant. The distinction is that not everyone in an older age group will exhibit these conditions, while some of younger age might. The consequence is that some people, even of advanced age, defend themselves well in a heat stress situation.

Individuals of any age who have suffered peripheral nerve injuries may also have reduced sweating ability and reduced vasomotor control (Drinkwater and Horvath, 1979; Ferrer et al., 1995; Miescher and Fortney, 1989). Some physical disabilities associated with aging seriously weaken responses to heat stress. Limited control of the peripheral circulation reduces the ability to distribute heat in the body and bring it to the skin surface, as do compromised abilities to maintain full hydration (Evans et al., 1993; Kenney et al., 1997; Richardson and Shepherd, 1991). Similarly, chronic illnesses that weaken cardiac output or reduce circulating blood volume reduce ability to cope with heat stress.

More important for older workers facing heat stress are general good health and level of physical fitness (Drinkwater et al., 1982; Kenney et al., 1990; Pandolf, 1994; Yousef et al., 1986). Older workers who have intact cardiovascular, respiratory, and sweating reflexes and are in general good health, fully hydrated, and retain thirst sensitivity, have balanced those other factors that may make them more vulnerable to heat stress (Andersen et al., 1976; Buono and Sjoholm, 1988; Inoue and Shibasaki, 1996; Phillips et al., 1991; Wenger, 1988).

GENDER

Gender differences in response to heat stress exposures are difficult to demonstrate between men and women in the adult years when physical fitness and levels of heat acclimatization are considered (Nunneley, 1978; Panolone et al., 1978; Frye and Kamon, 1981; Frye et al., 1982; Drinkwater et al., 1976). Common to both is the important ability to maintain a high cardiac output and peripheral blood flow when someone is physically fit. Differences in heat tolerance for women at different times in the menstrual cycle is similarly difficult to demonstrate even though there is an extravascular movement of body fluid that reduces plasma volume during the luteal phase and a shift in sweating threshold (Frye and Kamon, 1981; Carpenter and Nunneley, 1988; Kenney, 1985; Stephenson and Kolka, 1988; Kolka and Stephenson, 1989).

Although there are clear cardiovascular adjustments made during pregnancy, short-term heat stress has been demonstrated to be safe for women with normal pregnancies but may not be for those with high-risk pregnancies (Vaha-Eskeli et al., 1991; Pirhonen et al., 1994). Also, for women with normal pregnancies, moderate heat stress does not affect uterine contractility and does not appear to endanger the fetus (Vaha-Eskeli and Erkkola, 1991).

Temporary infertility in both females and males has been observed when core temperatures were above 38°C (100.4°F). During the first trimester of pregnancy, there is an increased risk of malformation of the unborn fetus if the mother’s core temperature exceeds 39°C (102.2°F) for extended periods (American Medical Association, 1984; Edwards et al., 1995; Milunsky et al., 1992).

OBESITY

Body fat is an impediment during heat stress (Chung and Pin, 1996; Gardner et al., 1996). Heavier people require a greater expenditure of energy to move around. Those people who are obese are commonly less fit and show comparatively higher heart rates during exercise and physical exertion (Epstein, 1990). Even so, many people who are chronically overweight perform as well in the heat as those with a leaner body mass (Gunn and Budd, 1995). The major reasons are related to experience on the job and to improvements in physical fitness that occurs with work.

HYDRATION STATE

The important job of distributing heat in the body falls to the cardiovascular system. Virtually all heat is transferred within the body and to the skin by forced convection mediated by blood flow. Keeping a large circulating blood volume is essential for safe heat exposures. Water accounts for much of the blood volume, and most water loss during heat stress is from sweating. Despite the heavy dependence on heat loss by the evaporation of water in sweat, remaining adequately hydrated requires conscious attention (Engall et al., 1987). Dehydration via heat exposure is a serious threat to thermoregulation by reducing blood volume and increasing hematocrit, which increases blood viscosity. Moderate dehydration reduces plasma volume but not blood osmolarity. More profound dehydration increases blood osmolarity, without further reductions in plasma volume (Sawka et al., 1985). These effects increase with age (Kenney et al., 1990). For all, dehydration during heat stress is associated with increased body heat storage and the greater incidence of heat strain for the same deep-body temperature (Sawka et al., 1982, 1984).

MEDICATIONS AND RECREATIONAL DRUGS

Many prescription drugs and some over-the-counter medications require caution when used during heat stress. Some anticholinergic medications, for example, inhibit sweating, especially for the elderly (Hassanein et al., 1992; Adubofour et al., 1996). In addition, some sedatives affect thirst thresholds, some behavioral-modifying drugs increase body temperature and some appetite suppressants increase metabolic heat production and reduce heat distribution by seriously affecting the peripheral circulation (Vassallo and Delaney, 1989; Shimizu et al., 1997; Kew et al., 1982). Selective serotonin reuptake inhibitors and lithium-based antidepressants reduce the ability to thermoregulate (Epstein et al., 1997).

Adverse drug effects involve thermoregulatory status, even in the absence of heat stress or exercise. Amphetamine-like drugs, for example, characteristically increase body heat loading and produce hyperthermia (Watson et al., 1993). They also induce intravascular coagulation, sometimes leading to renal damage. Anyone exposed to heat stress must pay attention not only to possible adverse effects of a single drug but also to the interactions among multiple medications. The prescribing physician should be advised about heat stress exposures.

Alcohol in high doses induces many physiological and perceptual disorders. Not only is it a central nervous system depressant, but alcohol also produces peripheral vasodilation and diuresis. Aside from affecting perception and judgment, demonstrating that low doses of alcohol seriously reduce thermoregulatory responses, including vasomotor and sweating reflexes, or that it may serve as a diuretic, is difficult (Allison and Reger, 1992; Desruelle et al., 1996; Shirreffs and Maughan, 1997). Alcohol use will adversely affect cardiovascular regulation of blood pressure, leaving some people more susceptible to a fall in arterial pressure during heat stress (Ylikahri et al., 1988).

HEAT ACCLIMATIZATION

Heat acclimatization allows the worker to withstand heat stress with a reduction in heat strain. Acclimatization to heat involves a series of compensations that occur in an individual that favor survival in a changed environment (NIOH, 1996c; Prosser and Brown, 1961; Wyndham et al., 1954). These include more finely tuned sweating reflexes with increased sweat production rate and lower electrolyte concentrations, lower rectal and skin temperatures, more stable and better regulated blood pressure with lower pulse rates, and  improved productivity and safety (ISO 7933, 2004; Houdas and Ring, 1982; Schmidt-Nielsen, 1983; Havenith and Van Middendorp, 1990).

Working at a moderate rate under heat stress conditions brings about physiological changes that substantially improve comfort and safety for those who are in general good health (Aoyagi et al., 1997; Houmard et al., 1990; Regan et al., 1996; Nielsen et al., 1997). The heat stress does not have to be severe. Even wearing thermally insulating clothing while working in cold weather provides some degree of heat acclimatization (Dawson et al., 1989). The effects of acclimatization are often dramatic and begin with as little as 30 minutes of physical activity each day for as short a time as one week (Hyland, 1987). Mere exposure to heat does not confer acclimatization; an elevated metabolic rate for about two hours per day is required (Bass, 1963). Acclimatization is specific to the level of heat stress. Acclimatization to one heat-stress level does not confer full acclimatization to a higher level of heat stress (Laddell, 1964). Physical training before starting work with heat exposures increases the rate of heat acclimatization in the initial phase.

Many of the physiological changes accompany­ing heat acclimatization are associated with cardiovascular and peripheral vascular phenomena (Aoyagi et al., 1997; Regan et al., 1996; Nielsen et al., 1997). There is an increase in plasma volume, which expands circulating blood volume and improves the convective distribution of heat in the body (Nielsen et al., 1997). There is also a greater cardiovascular reserve, which means that the heat acclimatized person is working further below the upper limit during heat stress (Aoyagi et al., 1997). Both the improved cardiovascular potential and the increased circulating blood volume greatly reduce the likelihood of decreases in systemic pressure and the incidence of heat exhaustion and heat syncope. These effects are perhaps the most readily noticed by the heat-acclimatized persons themselves. Because blood pressure is more readily maintained, brain blood flow is preserved; therefore, the person feels more comfortable, is less stressed, and is less likely to feel dizzy, nauseated, or faint while working in a heat stress condition. As a bonus, the heat-acclimatized person also develops a greater ability to shunt blood from the viscera to the general circulation during heat stress, which keeps arterial blood pressure elevated (Rhoades and Tanner, 1995).

Body water and electrolyte maintenance is also improved with heat acclimatization. Individuals who are heat acclimatized increase their abilities to sweat greater volumes, but the electrolyte concentration in their sweat decreases (Precht et al., 1973; Nielsen et al., 1997). Moreover, they improve the precision by which sweat rates match the body’s need to evaporate water and lose heat (Houdas and Ring, 1982). Acquisition of heat acclimatization is a continuum. Not all functional body changes occur at the same rate within the continuum, and no given physiological parameter dominates throughout the continuum (Mitchell et al., 1976; Senay et al., 1976; Wyndham et al., 1976). Internal body temperature, skin temperature, heart rate and blood pressure, sweat rate, shifts in internal body fluid, and renal conservation of fluid each progress at different rates.

Rapid changes in the first few days of acclimatization instill most of the benefits of lower core and skin temperatures and lower heart rate associated with a faster onset of sweating and higher rate of sweat production. These benefits are seen in week one, with further improvements in water and electrolyte management in weeks two and three (Wenger, 1988).

Although heat acclimatization for most individuals begins early in a period of working in the heat, it is also lost quickly if this regimen of exposure is discontinued (Pandolf et al., 1986). This loss is transitory and is quickly made up so that a two-day loss is made up in the first day back at work. If there is a week or two of no exposure, re-acclimatization requires four to seven days (Bass, 1963).

The net effects of heat acclimatization are to improve the effectiveness and efficiency of physiological heat distribution and loss systems, improve comfort in the heat exposure, and delay the development of heat strain. Acquiring heat acclimation is an effective way of improving safety and comfort during a heat stress exposure.

Heat-Related Disorders

Heat-related disorders are manifestations of untoward outcomes of heat stress exposures. They vary in degree of severity, although all but heatstroke resolve readily and have no lasting effects. The TLVs for heat stress and strain attempt to provide a framework to control heat-related disorders.

Risk of accidents and injuries increases with increasing levels of heat stress, but occupational exposure limits are not directed toward controlling these effects (Ramsey et al., 1983).

HEAT SYNCOPE

Heat syncope is a problem with the cardiovascular regulation of arterial pressure, especially systolic pressure. It is preceded by a fall in blood pressure that is caused by marked peripheral vasodilation and/or a rapid change in posture that causes blood to pool in the lower extremities. Diminished levels of consciousness, even fainting, are early warning signs of excessive heat strain. Heat syncope occurs more often in unacclimatized individuals, which is the reason that many workers experience it when hot weather first appears. Heat syncope is not always linked with hyperthermia or changes in tissue osmolarity.

DEHYDRATION AND LOSS OF ELECTROLYTES

Heavy and prolonged sweating brings large volumes of body water and electrolytes, principally sodium, to the skin surface. Dehydration reduces blood volume, impairs the normal regulation of blood pressure, and sets the stage for heat syncope. Because dehydration diminishes the effectiveness of blood circulation for distributing heat in the body, even mild dehydration (about 1% of total body water) is correlated with the development of hyperthermia and as much as 5% dehydration impairs workability (Terrados and Maughan, 1995). Adequately replenishing water lost in sweat during heat stress is difficult. Thirst is not usually sensed until dehydration is already well established (about 2% of body weight), and even people well experienced with heat strain often fail to drink enough water to regain full hydration. Most drink just enough to satisfy thirst.

Many organs depend on maintaining precise ionic gradients across cell membranes to support their electrical activity. Depletion of electrolytes by sweating threatens the ability of the myocardium to retain its contractile rhythm, as it does for muscles of the gastrointestinal tract to keep normal levels of motility and for skeletal muscle to maintain tone. Normal blood and tissue osmolarity are no less important for the coordinated contractions of skeletal muscle and the normal functioning of nerve cells, all of which depend on controlled electrical activity at the cellular level. Failure to re-establish normal, whole-body hydration and electrolyte levels is often seen as gastrointestinal disturbances and muscle cramps.

HEAT EXHAUSTION

Heat exhaustion is a heat disorder caused by cardiovascular insufficiency. Mild dehydration (water loss in excess of 1% of body weight) reduces blood volume and the ability of the cardiovascular system to respond fully to the demands of work plus thermo-regulation, which may explain the hyperthermia associated with mild dehydration.

Heat exhaustion is more likely to occur with greater levels of dehydration. As the dehydration becomes greater, the impairment of the cardiovascular system has more immediate effects. The symptoms include a loss of work capacity, diminished psychomotor skills, lightheadedness, nausea, and fatigue.

Fundamentally, heat exhaustion is manifested through the cardiovascular system. Rehydration with rest usually produces immediate recovery in the less severe cases. In the more severe cases, intravenous (IV) fluid replacement is required along with overnight bed rest.

EFFECTS OF CHRONIC HEAT EXHAUSTION

While the acute effects of high levels of heat stress are well documented, less data are available on chronic, long-term effects and appear generally less conclusive. Psychological effects in subjects from temperate climates, following long-term exposure to tropical conditions have been reported (Leithead and Lind, 1964). Following years of daily work exposures at high levels of heat stress, chronic lowering of full shift urinary volumes appears to result in a higher incidence of kidney stones despite greatly increased work shift fluid intake (Borghi et al., 1993). There appears to be an increased risk for serious gastrointestinal disease, while chronic heat stress may be protective for cardiovascular disease (Redmond et al., 1977).

HEAT STROKE

Heatstroke is a failure of the thermoregulatory system at the central nervous system and results in multi-organ damage and failure due to elevated temperatures. Classical heat stroke is a leading contributor to death for thousands who have impaired cardiovascular and cerebrovascular abilities or who are diabetic (Halle and Repasy, 1987; Khogali and Hales, 1983). Occupationally, exertional heatstroke is more likely, and it is marked by an inability to remove enough metabolic heat. Heatstroke is characterized by elevated body temperature, often greater than 40.5°C (105°F). The person is delirious, semi-conscious, or unconscious, while others will be hyperactive and manic (Auerbach et al., 1993). Many individuals will have seizures or become comatose (Khogali and Hales, 1983). Most will be dizzy, confused, and disoriented at some stage. Rapid breathing, with a rapid, weak pulse is common. The signs and symptoms of exertional heat stroke often appear with others that are related to high levels of physical exertion. For example, rhabdomyolysis (the breakdown of skeletal muscle from overuse), intravascular coagulation, increased potassium ion concentrations in blood, hypoglycemia, increased catecholamines, and major organ damage are common (Knochel, 1989; Bouchama et al., 1996; Hart et al., 1982; Al-Hadramy and Ali, 1989; Lumlertgul et al., 1992).

Without exception, the appearance of heatstroke signs and symptoms signal an emergency. Developing an emergency response plan with the consultation of a healthcare provider is an important feature of a heat stress management program. Removing the person from the heat stress, loosening clothing, flooding the body surface with tepid water, and fanning vigorously will start the important initial phase of cooling. Some investigators recommend the more drastic measure of partial immersion in cool or cold water as an effective means of reducing body core temperature (Khogali and Hales, 1983; Armstrong et al., 1996; Hadad et al., 2004; Khogali and Weiner, 1980; Pandolf and Burr, 2001). The deleterious effects of heatstroke depend on how high and how long the time course of body core temperature proceeds. The greater the number of degree-minutes above 40°C, the more severe the heatstroke, which is the reason fast and effective cooling is important. The important feature of the first aid response is to do something to cool the person while waiting for emergency transport. Emergency medical care and hospitalization are essential. Even if body temperature is reduced and the person regains consciousness, dangers of organ damage and even death will remain for several days. The high internal body temperature and loss of thermoregulation, which characterize heat stroke, are not only dangerous in themselves, but they also precipitate life-threatening failure in many organs and tissues. The damaging and lethal consequences may appear hours to days after body temperature has returned to a normal range. In heatstroke, for example, reflex-induced sustained, increased cardiac output often leads to multiple electrocardiographic abnormalities and abnormal heart rhythms, sometimes with heart damage (Akhtar et al., 1993; Garcia-Rubira et al., 1995). Some victims die later of heart failure and some have pulmonary edema (Anderson et al., 1983; Zahger et al., 1990). Renal failure is common for about 30% of those individuals recovering from exertional heat stroke because damage to skeletal muscles with overuse releases myoglobin into the circulation and increases its uric acid concentration (Anderson et al., 1983; Zahger et al., 1990; Sharma et al., 1983). The kidney itself may also be damaged directly by hyperthermia and suffer from a reduced blood supply when large volumes of blood are directed to the skin in an attempt to dissipate heat. Clots may also form spontaneously in blood vessels because of direct thermal damage to their endothelial walls (Bowie and Owen, 1983). No body organ is immune from damage during heatstroke. Because organ damage and failure may occur after the heatstroke episode is over, hospitalization and continued medical supervision are essential to recovery. Long-term damage to the liver is common (Hassanein et al., 1992; Lumlertgul et al., 1992; Sort et al., 1996). The danger depends not only on how high body temperature climbed but also on how long it remained elevated. Damage directly to formed elements in the blood, for example, maybe avoided for short-term exposures but not for longer periods; even if the temperature level is lower (Utoh and Harasaki, 1992; Costrini et al., 1979). Damage to the brain, spinal cord, and peripheral nerves is also common in heat stroke (Delgado et al., 1985; Upadhayay and Venkatraman, 1990). Damage to the cerebellum leaves permanent problems with maintaining body posture, walking, and other coordinated maneuvers. Some individuals will have personality changes, leading even to dementia, while others will have diminished abilities to defend themselves against subsequent heat stress (Hales and Richards, 1987; Royburt et al., 1993). Even for the short-term, individuals who have suffered heat stroke should not be allowed to re-enter a heat stress environment until there is medically verified full recovery.

Evaluation Methods

Not only is it important to distinguish between the phenomena of heat stress and heat strain but also necessary to choose carefully between how each is to be measured and have reasonable expectations for the kind of information each provides.

Evaluation of Heat Stress

By asking what is the level of heat stress exposure, the occupational safety and health professional is asking how might the combination of environmental conditions, work demands, and clothing requirements affect the ability to maintain thermal balance. A discussion of heat stress often begins with the concepts of heat generation inside the body from metabolic work and heat exchange between the body and the environment. The generalized description of this exchange is shown in Equation 3 (Belding and Hatch, 1955; ISO 7933, 2004).

S = M + R + C – E                                                 (3)

A positive value for S, R, and C signifies that the body gained heat, and a negative value indicates that heat is lost from the body. Each term represents a rate of energy transfer. Sometimes the rates are reported as values normalized to body surface area.

S is Heat Storage Rate. If the value for S is zero, the body is in thermal equilibrium, and heat gain is balanced by loss from the body. If S is positive, the body is gaining heat at the rate indicated by the value of S.

M is Metabolic Rate. The rate of metabolism depends directly on the rate and type of external work demanded by the job.

R is Radiant Heat Exchange Rate (Radiation). The rate of heat transfer by radiation depends on the average temperature of the surrounding solid surfaces, skin temperature, and clothing. Radiant heat exchange depends on the equivalent black body temperatures of the person and the overall surroundings.

C is Convective Heat Exchange Rate (Convection). The exchange of heat between the skin and the surrounding air is referred to as convection. The direction of heat flow depends on the temperature difference between the skin and air. If air temperature is greater than skin, C is positive and heat flows from the air to the skin. The rate of convective heat exchange depends on the magnitude of the temperature difference, the amount of air motion, and clothing.

E is Rate of Evaporative Heat Loss. Sweat on the skin surface will absorb heat from the skin when evaporating into the air. The process of evaporation cools the skin and, in turn, the body. The rate of evaporative heat loss depends on the amount of sweating, air movement, ambient humidity, and clothing.

Under ideal conditions, the gain rate, S, is zero because evaporative cooling is sufficient to remove the heat generated by metabolism plus any heat gained from (or lost to) the environment through R + C. The required evaporative cooling is denoted as Ereq. Then Equation 3 becomes

Ereq = M + R + C                                                    (4)

The maximum rate of evaporative cooling is called Emax. It depends on the water vapor pressure in the air (humidity), the amount of air movement, and the clothing involved. Heat storage with the concomitant increase in body core temperature is the imbalance when Emax is less than Ereq, such that

S = Ereq – Emax                                                                    (5)

The assessment of heat stress in the framework of heat exchange then needs to consider the environment, work demands and clothing requirements.

QUALITATIVE EXPOSURE ASSESSMENT

Qualitative exposure assessment is designed to identify potential exposures to heat stress with little instrumentation or expertise. The traditionally accepted job risk factors of hot environments, physically demanding work, and protective clothing represent an intuitive basis for qualitative judgment about the presence of heat stress. Malchaire et al. (1999) have proposed a more structured qualitative exposure assessment tool that can guide the decision.

EVALUATING ENVIRONMENTAL FACTORS

Merely measuring air temperature has been recognized since 1775 as grossly inadequate to quantify heat stress (Blagden, 1775). In 1905, Haldane (1905) recognized the relationship between humidity and heat stress. In fact, the vapor pressure of water in the air (absolute humidity) is what influences the maximum rate of evaporative cooling that might be achieved (Emax). Emax is also influenced by the speed at which air moves over the body, causing airspeed to be an important parameter.

Air temperature and speed provide insight into the convective exchange of heat. In a similar fashion, the black body temperatures of the surroundings and the person influence the radiant heat exchange via infrared radiation.

As a rule, the more characteristics of the environment that can be accurately assessed, the more useful is the information. Practicality moves toward fewer measurements and the trade-off is simplicity versus information. Various indices of the thermal environment implicitly address these tradeoffs. Several indices of the environment are discussed in the following sections.

Heat Index

Calculations of the Heat Index take an important step forward by including two environmental measures for defining heat stress: ambient temperature and relative humidity. The Heat Index is widely promoted by the U.S. National Weather Service as a means to combine the effects of air temperature and humidity on “how hot it feels.” Relative humidity plays an important role in thermal perceptions, and its use in the Heat Index has intuitive appeal (Holmes and Adams, 1975). As expected, the Heat Index increases with air temperature and relative humidity.

The Heat Index is not influenced by air velocity, which is a major factor in removing heat and water vapor by convection from skin and clothing surfaces, nor is an evaluation of infrared radiant heat exchange included. The guidance offered by the U.S. National Weather Service in the interpretation of the levels is not appropriate to occupational exposures. Heat Index is predictive of heat stress exposure in circumstances in which the relationship between the Heat Index and a conventional method of exposure assessment has been established for a particular environment (Logan and Bernard, 1999).

Effective Temperature and Corrected Effective Temperature

The Effective Temperature (ET) index of Houghton and Yaglou (1923) and Yaglou (1927) was devised as a measure of indoor comfort for sedentary activities using the psychrometric wet-bulb and dry-bulb temperatures and the rate of air movement. A nomograph was constructed to relate the three factors into an equivalent thermal sensation referenced to 100% relative humidity. It was subsequently used as an index of the environment for the assessment of heat stress.

Bedford (1946) modified the ET nomograph for use in naval vessels as an extension of the index for use in heat stress exposure assessment. He substituted globe temperature for the dry-bulb temperature to better reflect radiant heat effects. This became the Corrected Effective Temperature (CET). In environments where the temperatures of the solid surroundings are approximately the same as air temperature, the values for ET and CET are the same.

The dry-bulb temperature of ET and the globe temperature of CET combined with airspeed reflect the potential for heat gain or loss by convection and radiation. The wet-bulb temperature and airspeed combination is an index of the potential for evaporative cooling.

Wet-Bulb Globe Temperature

The wet-bulb globe temperature (WBGT) index was developed as an approximation of effective temperature. Globe thermometer was proposed to incorporate the effects of solar radiant heat and the convective heat exchange due to air temperature and movement. Rather than applying the modified ET nomogram proposed by Bedford (1946) for naval vessels, globe temperature and psychrometric wet-bulb temperature were combined by a simple arithmetic calculation into an index (Yaglou and Minard, 1957).

Although the Bedford index appeared to give a slightly better correlation with sweating than WBGT, any advantage was considered to be outweighed by the simplicity of WBGT versus air motion measurement and the use of a nomograph (Minard, 1961). Several combinations of similar instrumental data, using either natural (both shaded and sun-exposed) or psychrometric wet-bulb temperature and shaded dry-bulb temperature with the globe thermometer temperature, were subsequently considered (Minard, 1964; Minard et al., 1957). The index adopted for management of U.S. Marine training combines the globe thermometer temperature, natural (exposed) wet-bulb temperature, and shaded dry-bulb temperature. As finally configured, the WBGT considers the effect of ambient humidity and air motion by including the temperature of a thermal sensor from which water is freely evaporating (natural wet-bulb) and the important factors in dry heat exchange (i.e., convection and infrared radiant heat) by measuring the globe temperature.

WBGT is calculated for an outdoors environment in direct sunlight as Equation 6:

WBGTout = 0.7 Tnwb + 0.2 Tg + 0.1 Tdb               (6)

where:

  • WBGT = wet-bulb globe temperature
  • Tnwb = natural wet-bulb temperature
  • Tg = globe temperature
  • Tdb = dry-bulb temperature

 

WBGT is calculated for an indoor, shaded or cloudy environment as Equation 7:

WBGTin = 0.7 Tnwb + 0.3 Tg                                                  (7)

WBGT integrates four important environmental factors: air temperature, humidity, air motion, and radiant heat. The two conditions (i.e., with and without solar radiation) are distinguished because the black-globe thermometer accepts all radiant energy in the visible and infrared spectrum. Skin and clothing of any color are essentially “black bodies” to the longer-wavelength infrared radiation; but at the shorter infrared wavelengths of solar radiation, there are differences in absorption due to fabric and skin color (Yaglou and Minard, 1957; Kerslake, 1972). To represent most outdoor work situations, factoring in shielded air temperature has reduced the over-estimation of radiant heat from the globe temperature at the shorter wavelengths.

Although WBGT is not a complete calculation for the many environmental and physical factors influenc­ing heat strain, it provides useful information. Since WBGT calculations are in wide use and have proven utility, others continue to investigate its value as an index and are looking for alternatives (Pandolf et al., 1986; Bricknell, 1997; McCann and Adams, 1997; Nag et al., 1997; Reardon et al., 1997).

Even though WBGT is an often-quoted index and is commonly referenced in evaluations of work settings, it is still a simple approximation of the environmental factors. When used to set limits on heat stress, it may understate the level at both high and low humidities.

Environmental Data for Rational Indices

Rational indices of heat stress attempt to evalu­ate heat stress via biophysical models of heat exchange between a person and the environment (see following). Common to the rational models are the environmental data required to perform an analysis. The list includes air (dry-bulb) temperature, ambient water vapor pressure (humidity), air speed, and equivalent black-body temperature of the surround­ings inferred from globe temperature and airspeed.

METABOLIC HEAT

Skeletal muscles are relatively inefficient, and 75% to 90% of the total energy released is heat. The role that metabolic heat plays can be readily seen in Equation 4, where it is a direct driver of the required evaporative cooling. Often M is 90% of Ereq, which means that convection and radiation then play a relatively small role in heat stress.

Methods of metabolic heat assessment include direct measurement, prediction from task analysis, table look-up, prediction from other physiological variables, and perceived exertion. Because predic­tion from physiological variables and perceived exertion are confounded by heat stress, these will not be discussed further.

Direct Method

Direct methods to assess metabolic rate are those methods that measure the amount of oxygen consumed by an individual, which is then converted to energy units by a simple scaling factor (Consolazio et al., 1963; ISO 8996, 2004; Rodgers, 1978). Each liter of oxygen consumed is equivalent to 5 kcal of energy (Rodgers, 1978). Therefore, if a job requires 1.0 liter of oxygen per minute, the metabolic rate is 5 kcal/min, 300 kcal/h or 350 W. (The accuracy can be improved by also measuring carbon dioxide produced and delivered to the expired air.) The improvement in accuracy is most appropriate for laboratory measurements where other work conditions are more tightly controlled than in field measurements (Consolazio et al., 1963). Portable equipment for field measurements is available and has repeatable and valid results. Special attention needs to be paid to the direct measurement of metabolic rate because of the transient dynamics of oxygen uptake in response to changing work demands. An ISO standard for measuring oxygen consumption describes techniques for assessing the oxygen demand of a task (ISO 8996, 2004). In the simplest case of constant work demands under very repeatable conditions, a three-minute sample may be enough. In most work situations, one or more cycles of work plus a recovery period must be measured to obtain a reasonable value for the work cycle. But variations in task speed, weight handled, or other task factors can have a significant effect on metabolic rate (Malchaire et al., 1984). In addition, there is wide variability in the energy expenditure among different people carrying out the same task (Malchaire et al., 1984). Multiple measurements representative of a range of task variables are needed to see the whole picture on metabolic rate for task variations. Someone familiar with direct measurements of changing work demands should supervise field measures of metabolic rate to obtain reliable results.

In summary, direct measurement of metabolic rate in the field is accurate but requires special expertise and equipment. It can also interfere with the workers. In addition, direct measurement requires performance on an actual task, real or simulated, by a number of workers to obtain information on group metabolic demands.

Predictive Methods

Predictive methods rely on a task analysis of a real or hypothetical job. The task analysis is done in sufficient detail that every component contributing to the metabolic demand is reasonably well defined. The structure of the task analysis depends on the prediction method that is used. Predictive methods have intuitive appeal and demonstrated value. It is a logical extension of the direct measurement of oxygen consumption for a short-term task, where the entire task and recovery oxygen consumption is measured and the oxygen consumption is assigned to the task demonstrated that it is applicable to a broad range of tasks, and Garg et al. (1978) widened the application to industrial tasks (ISO 8996, 2004; Kamon et al., 1975). The predictive methods can be further divided into two categories. The first category is largely quantitative, while the second category is a mix of qualitative and quantitative information.

In the quantitative methods, the oxygen demand is estimated from previously developed relationships between task activities and metabolic rate. For instance, the metabolic demand of lifting a box of known weight from the floor to a table of known height can be estimated (Garg et al., 1978). The person performing the task analysis must have some expertise and be familiar with the kinds of information that the predictive methods require. When there is a task for which a predictor does not exist, one must be developed or an educated estimate made.

The mixed qualitative and quantitative methods allow the use of some judgment in the assessment of the demands. As an example, the Systematic Workload Estimation (SWE) assigns a code that indicates the class of activity (i.e., stationary, walking, extra exertion) and then a subclass code that depends on the amount of body involvement and degree of effort (Tayyari et al., 1989). The class and subclass codes are cross-referenced to a table that indicates the metabolic rate. While it was designed to record activities on a periodic basis for work sampling, the SWE is applicable to an analysis of individual job activities. A variation of the activity analysis is a checklist of job components with a score assigned to each component (Eastman Kodak Company, 2004). The estimated metabolic rate is determined from the total score.

The ISO published a method that provides good structure to the estimation process with a fair blend of qualitative and quantitative data (ISO 8896, 2004). It is based on the summation of components that contribute to metabolic rate (M) in Equation 8. The method described in the criteria document published by NIOSH in 1986 was an earlier version of this ISO method. The components are the basal metabolic rate (B), posture (P), kind of activity (A) that are adjusted for the degree of involvement of the body and the level of effort, average rate of horizontal travel (H), and average rate of vertical travel (V). B, H, and V are quantitative data and P and A are qualitative. Table 11 describes the components, the factors that determine the metabolic rate of the component and value to assign the component based on the factors.

M = B + P + A + H + V                                         (8)

While the potential for assessment error is greater for the predictive methods than for the direct method, the principal value of predictive methods in a heat stress program is that they do not intrude upon the workers, require less expertise, and do not require expensive equipment as well as pointing to the largest contributors of the metabolic rate. Accuracy of a prediction depends on the refinement of the predictors in the method and the ability of the analyst. Another significant advantage of the predictor methods is their role as a resource that is available to evaluate jobs that are in the planning stage.

Table Look-Up

Table look-up methods for assessing metabolic rate are a popular approach (U.S. NIOSH, 1997; Passmore and Durnin, 1955). There are three approaches to table look-up assessment of metabolic rate. For the direct look-up method, an analyst matches the job under consideration to a list of jobs or job descriptions that are common to the industry. The table value for the best match is taken for the job being assessed.

 

TABLE 11. Components of the ISO 8996 Method (2004) to Estimate Metabolic Rate, Factors that Determine the Estimated Value of Metabolic Rate*
Component Factor Metabolic Rate [W]*
Base (B) 80
Posture (P) Sit 20
Stand 45
Activity (A) Body Involvement See Activity Matrix Below
Effort
Average Horizontal Rate of Travel (H) Estimate H [m/min] 3.3 × H
Average Vertical Rate of Travel (V) Estimate V [m/min] 56 × V
Activity Matrix Effort
Body Involvement Light Moderate Heavy Very Heavy
None 0 0 0 0
Hand(s) Only 25 55 70 80
One Arm 65 100 135 170
Both Arms 115 155 190 230
Whole Body 225 340 505 700
Can be performed indefinitely
with ease
Can be performed indefinitely with some effort Can be performed for about 30 min before
a break
Can be performed for about 15 min before
a break
* The effect of body weight on the estimated metabolic rate can be accounted for by multiplying the estimated rate by the ratio of actual body weight divided by 70 kg (154 lb).

 

A second method uses subjective matching where the analyst seeks the best match between the job in question and standard activities for which the metabolic demand is known. The standard activities may be very similar to the job, but most often they are unrelated and a subjective judgment is used to make the match. The more the job can be sub-divided and analyzed, the more accurate the assessment maybe, but also the more time-consuming. For instance, a variation on subjective matching is analogous to predictive methods. In this case, the match is made to different components of the job, and then the component metabolic rates are summed together (ISO 8996, 2004). Another variation is to divide the job into tasks, to assign metabolic rate values to each task, and to do a time-weighted average of the tasks for overall metabolic demand.

A major problem with table look-up methods is the relevance and accuracy of the data. The first major attempt to collate metabolic rates associated with occupational demands was a review article by Passmore and Durnin in 1955. While it was exhaustive and their work is still cited, the relevance of many of the descriptions of work is suspect. Important factors to consider in using these kinds of data are the number of subjects, how well the subjects match to employee population, and the accuracy of the methods.

The remaining method is category assignment. The purpose of category assignment is to place the job in one of three to five categories of metabolic demand (e.g., light, moderate, heavy, or very heavy) (ISO 8996, 2004; Eastman Kodak Company, 2004). There are usually descriptors of specific work that may be included in the category or broad descriptors of activities (e.g., light handwork or heavy work with the whole body). Category assignment is the usual method for many heat stress management programs. The range of metabolic demands that represents a category is usually about 175 W, which is not precise. In fact, it has equivalent precision to 3°C-WBGT (Bernard and Joseph, 1994). Table 9 is an adaptation of the ISO classification of metabolic rate for use with WBGT-based heat stress exposure limits (ISO 7243, 1989). For comparison, the reference metabolic rates in the previous TLVs were 115, 230, 350, 465, and 580 W, which was generally higher by 50 W. Table 3 in the TLV follows from Table 9.

Under comfortable thermal conditions, most workers could be expected to work eight hours at the Light and Moderate metabolic rates with nominal breaks. At the Heavy metabolic rate, the least fit workers (maximum aerobic capacity of 660 W) would require a break at about 30 minutes; and at the Very Heavy metabolic rate, they would need a break at about 10 minutes (Eastman Kodak Company, 2004).

A common error in most subjective matching and category assignment methods is a tendency to overestimate the metabolic demands of the job. There are two likely reasons for the overestimation. One is that the peak demands of the job may dominate the judgment of the analyst so that the lower demands and recovery periods within a task are not fully accounted for. Another reason is the role of static work. Isometric muscle contractions cause a greater sense of physiological strain without contributing to the metabolic demand. In simplest form, table lookup methods are relatively easy, intuitive, and require little skill. They are, however, among the least accurate.

Detailed Analysis of Heat Stress

WBGT Thresholds

Table 2 in the TLV section provides criteria for screening thresholds. These were designed to take a coarse cut at the evaluation. The WBGT thresholds in Figure 2 are based on continuous relationships between WBGT and metabolic rate. Where the WBGTs of various work and recovery areas are different, a TWA of the WBGT for the hourly period should be used, as in Equation 9.

     (9)

where WBGT1, WBGT2, … and WBGTn are the values of WBGT for the various work and rest areas occupied during total time periods, t1, t2 … and tn are the elapsed times spent in the corresponding areas which are determined by a time study. The TWA for environmental conditions is calculated in hourly segments, i.e., where t1 + t2 + … + tn = 60 minutes for regular work patterns or continuous work, or up to two hours for irregular patterns.

The estimated TWA–M for the period of work within each hour, using either tables or otherwise determined (e.g., by oxygen consumption measurements) should be determined by Equation 10:

    (10)

where M1, M2, … and Mn are estimated or measured metabolic rates for the various activities of the worker during the total time periods t1, t2, … and tn, as determined by a time study.

The TLVs for continuous work are applicable where there is a work-rest regimen of a five-day workweek and an eight-hour workday with short morning and afternoon breaks (approximately 15 minutes each) and a longer lunch break (30 to 60 minutes). However, higher exposure limits can be permitted if personal monitoring is used and any necessary additional resting time is allowed. All breaks, including unscheduled pauses and administrative or operational waiting periods during work, may be counted as rest time when additional rest allowance must be given.

The thresholds in Figure 2 are quite conservative, provided that the estimated workload category has been correctly determined. Therefore, the TLVs apply to nearly all fully acclimatized workers wearing summer work clothes (i.e., lightweight pants and shirt) with adequate water and dietary electrolyte intake. The TLVs minimize the risk of a deep-body temperature greater than 38°C (100.4°F). When heat stress exceeds the TLV criteria levels recommended for continuous work, making provision for rest time based on the detailed WBGT analysis maintains the protective nature of the limits.

In general, when the work on a job is self-paced, the workers will spontaneously limit their hourly workload to 30% to 50% of their maximum physical performance (aerobic) capacity through the interspersing of unscheduled breaks or the setting of an appropriate work speed. One report indicates that by limiting continuous oxygen consumption demands to less than 50% of the predicted maximum for age, the resulting sustained heart rates will be less than those proposed in the TLV (NIOH Table 18, 1996). In this manner, the daily average of their metabolic rate will seldom exceed 380 W, but their hourly average metabolic rate might be higher. Under very hot conditions, any practice of allowing a compression of the work schedule should be discouraged, a suitable distribution of unscheduled breaks must be encouraged, and time-weighted average values used of both environmental heat (WBGT) and metabolic heat (M) over the 60-minute periods (Equations 9 and 10 above) should be below the TLVs. When exceeding those values, a rational analysis or physiological monitoring is the next step.

When workdays are longer than eight hours, greater emphasis on concerns about dehydration and fluid replacement may be necessary, even when the TLV and Action Limit criteria in Figure 2 are satisfied (Borghi et al., 1993).

WBGT environmental values should not be considered as a basis for extrapolating the guidance given by Figure 2 to situations where the values are above those for either continuous work or the specified work–rest regimens. An upper limit to their applicability is indicated in two ways: (1) when the WBGT exceeds 33°C and recovery is not possible, and (2) the same WBGT values can result in substantial differences in physiological heat strain (Malchaire, 1979; Ramanathan and Belding, 1973).

Rational Evaluation of Heat Stress

Rational evaluations of heat stress consider the biophysics involved in heat exchange. They are evaluation methods that are derived from first principles rather than empirical relationships like the WBGT-based limits. Equations 1 through 3 in the section on Factors in Heat Stress are the elemental description of rational models. For the evaluation of heat stress, the various models describe methods to compute values for M, R, C, and Emax, as well as criteria for interpretation.

The Heat Stress Index

The Heat Stress Index (HSI) was devised about 50 years ago (Belding and Hatch, 1955). The HSI is the percent ratio of Ereq divided by Emax. The dimensionless number from 0 to 100 (and higher) reflects interactions among ambient temperature, humidity, regional air velocity, infrared radiant temperature, work rate, and representative limits for sweat production. It also predicts a so-called safe exposure time to account for the rate of heat accumulation when HSI is greater than 100.

The HSI was an important step forward to include many more factors in the calculation of an exposure index than the WBGT and other empirical indices (e.g., effective temperature). By providing a heat balance framework, the HSI assesses the important effects of metabolic heat production and environmental contributions to heat stress. Examples of data required by and computed for the HSI analysis, along with some conclusions that might be reached, are readily available (Adams, 1994; MacPherson, 1960).

A major difficulty in using the HSI is the complex­ity of its calculations. Several equations must be solved for different sets of environmental factors as independent variables. The use of programmable calculators or computer programs facilitates the evaluation of heat stress with rational models (Adams, 1994; Adams and Potter, 1988). This allows quick and easy iterative solutions to deter­mine major operating factors in the defined environment (Adams, 1994). The tabletop evaluation of controls remains a value of the HSI.

However, there are other limitations of the HSI. Like other models of heat stress (as compared to heat strain), the HSI does not consider the important factors presented by unique differences among individuals in their abilities to cope with heat stress. The HSI assumes that the person for whom the calculations are made is healthy and dressed in a cotton work shirt and pants. The calculations of a safe exposure time are to be used with care. The HSI is a valuable way to learn about the interaction of environmental and personal factors in heat stress, but its application is necessarily restricted as an indicator of heat strain. The HSI does not replace the need for sound judgment by informed professionals.

Predicted Heat Strain

The ISO has recommended the Predicted Heat Strain (PHS) as a rational method for heat stress evaluation (ISO 7933, 2004; Malchaire et al., 2001). The approach is a developmental evolution from the HSI. This method is fundamentally a heat balance technique that estimates the contributions of metabolic rate, air temperature, humidity, air motion, and overall radiant conditions to the tendency to cause heat storage. PHS improves on the HSI in a number of ways: (1) it accounts for a loss of sweat evaporation efficiency at higher sweat rates, (2) it limits exposures based on the maximum rate of cooling that can be achieved after adjusting for sweating efficiency, (3) it limits exposures based on the risk for dehydration, and (4) it includes a broader range of clothing ensembles and effects. There are ongoing efforts to improve the method, in addition to consideration of the estimation of clothing effects and based on extensive studies (British Occupa­tional Hygiene Society, 1999).

Computationally, PHS is even more complex and demanding than the HSI. Again, the use of personal computers makes it practical for an off-line evalua­tion and trade-off analyses. The method appears to cross the line from heat stress analysis to heat strain prediction. It is, however, fundamentally an evaluation of heat stress. This method recognizes dehydra­tion as a cause of heat-related disorders and ele­vated core temperature and sets thresholds accord­ingly. Because this method examines only environ-mental conditions, work demands and clothing made from woven cloth, it sets protective limits to account for the individual variations seen in a healthy workforce.

Evaluation of Heat Strain

Human responses to heat stress vary greatly, not only from person to person in the same environment, but also for the same person in different exposures and from time to time. This makes it impossi­ble to predict with any reliability who is going to suffer a heat-related disorder in a specific setting. Despite its low predictability, excessive heat strain is easily avoided because there are usually early warning signs and symptoms. The appearance of a single early warning sign deserves attention. The appearance of more than one demands it. Knowing what signs there are and knowing what to do when they appear is a most important first line of defense.

There is a simple, yet valuable, two-step process for avoiding excessive heat strain. The first is to recognize personal characteristics that predispose its development and understand why they are important on a biological basis (Adams, 1988; Faunt et al., 1995). The second is to watch for the presentation of signs and symptoms of excessive heat strain, then do something to remediate them as soon as they appear. Ignoring the signs and symptoms, even in their early stages, guarantees their further development and progressively increased danger (Sharma et al., 1983; Bell, 1985; Enander, 1989; Keatinge et al., 1986; Wyon et al., 1966, 1979). Almost without exception, disabilities and deaths from excessive heat strain result from ignoring this two-step process. This process is just as effective for those who are highly vulnerable to heat stress, as for those who are well prepared for it.

Because someone has physical or physiological characteristics that predict a high risk for heat strain does not preclude their working in the heat. Human variability in response to heat stress is so great, it is reasonable and safe to expect that because of experience, training, and unrecognized compensa­tory mechanisms, someone with high risk factors may in fact do better than others in a heat stress (Gunn and Budd, 1995). Nevertheless, seeing high risk factors justifies paying particularly close atten­tion to the person under heat stress and making frequent checks for the early warning signs and symptoms of heat strain.

Early Warning Signs

Heat stress triggers an immediate sweating response in normal people. Early stages of reflex sweating often go unnoticed because the water in sweat readily evaporates. Sweat-gland secretion rates are tuned to the body’s need to lose heat by evaporation. Thermoregulatory responses that produce sweating continue to match sweat production rates as total body heat content increases. Sweating then becomes so active that water is brought to the skin surface faster than it can evaporate from it, and water begins to appear in a liquid phase, beading on the skin, dropping from it, and soaking into clothing. The presence of profuse sweating is an indicator of high heat stress and the likely escalation of heat strain.

Increasing cardiac output to circulate blood more vigorously through the body and to the skin for heat loss to the environment is a normal reflex response even to mild heat strain. Elevating both heart rate and stroke volume, as well as reducing peripheral vascular resistance, increases the volume flow of blood many fold, directed mostly to the skin (Khogali and Hales, 1983). Increasing heart rate is a response common to exercise, to work, and to heat stress. It is also common for it to diminish once the exercise or heat stress is reduced. Most individuals pay little attention to their increase in heart rate when working or facing heat stress. When heart rate is maintained in a heat stress to the point of sensing a pounding pulse or remaining elevated during rest periods, the heat stress is likely to be high.

Despite increases in cardiac output during heat stress, diastolic pressure commonly falls, especially in those people who are not heat acclimatized, are dehydrated, and have a low circulating blood volume (McConnell et al., 1924). The fall in peripheral vascular resistance and the increase in muscle blood flow, when the heat stress is accompanied by work, places demands on cardiovascular reflexes that challenge maintenance of blood pressure. A fall in blood pressure especially endangers the normal blood circulation of the brain. When brain blood flow falls, brain function declines also. Compromised brain blood flow is the reason why many people experience the early warning signs and symptoms of heat stress. These include dizziness, unexplained irritability, lightheadedness, nausea, blurred vision, loss of peripheral visual fields, postural instability, ringing in the ears, a “funny taste”, shaking of the hands, and the development of vague, flu-like symptoms, in addition to unexpected clumsiness on the job and misplacement of tools and equipment. These responses occur for many people early in a heat stress exposure, long before either total body heat or body temperature have increased.

Core Temperature

The majority of heat-related illnesses and disabilities do not involve much of an increase in deep-body temperature. Most heat strain problems stem from limitations in distributing and storing heat in the body but do not necessarily involve profound hyperthermia. Understanding how this happens requires insight into how body tissues serve as a thermal buffer and how they protect vital organs (e.g., brain, liver, and other abdominal viscera) from changes in deep-body temperature that would compromise their normal function.

Deep and superficial tissues of the body play different roles in thermal protection. Knowing what these roles play is important when evaluating heat strain. Metabolic processes in the liver, kidney, brain, and other vital organs depend heavily on their temperature. The physiological functions of these organs are impaired when temperature increases; the functions of these organs cease, sometimes irreversibly, when their temperatures reach 41°C (106°F) or higher. Vital organs are located deep inside the body where they are protected more than are superficial tissues from changing environmental temperature. They are considered to reside in the thermal “core”. A thermal “shell” is envisioned to surround the thermal “core”, which consists largely of the skin and extremities. The thermal “core” and “shell” are functionally, not anatomically, defined. At times, muscles, fat, bone, and connective tissue of the extremities are included in the thermal “core”, along with deeper tissues and organs. When a person stores heat during a heat stress exposure, heat is stored in tissues of the thermal “shell”. The thermal “core” expands and the thermal “shell” decreases, so that total body heat content increases while deep-body temperature still remains unchanged. Adjustments to hot environments typically reduce the volume of tissue in the thermal “shell” to a small part of the total body mass. When the person returns to a thermoneutral environment, heat drains from the thermal “shell”, it expands; the thermal “core” once again shrinks, and total body heat content decreases. Deep-body temperature, however, is relatively stable throughout the process.

The thermal capacitance of the thermal “shell” plays an important role in buffering demands on the thermal “core”. The cardiovascular system and its distribution of blood flow play a pivotal role in controlling heat distribution in the body and its exchange with the environment. For this reason, decreases in circulating blood volume because of dehydration, for example, seriously impede convective heat transfer within the body and markedly add to thermal strain. The majority of heat strain is primarily associated with the attempts by the body to control total body heat content and only secondarily related to stabilizing deep-body temperature. When the limits of this ability are compromised and the thermal “core” has expanded as much as it can, any continued increase in heat gain increases the amount of heat stored in deep-body tissues. Deep-body temperature then rises to levels of hyperther­mia. Hyperthermia driven by heat stress is a result of the progressive inability of the body to maintain thermal balance.

An elevated core temperature greater than 39°C normally indicates a marginal ability to tolerate greater levels of heat stress or a loss of thermal regulatory control. For sustained exposures to heat stress, core temperature should be below 38°C (WHO, 1969). Peak, transient core temperatures of 39°C can be acceptable, but values below 38.5°C provide a margin for measurement error and time to reduce the exposure (WHO, 1969; Bernard and Kenney, 1994).

Heart Rate

The stronger and more capable the cardiovascu­lar system is for distributing heat in the body and delivering it to the skin, and the more effective is sweating for bringing water to the skin surface for heat loss by evaporation, the less the expected increase is in deep body temperature during heat stress.

For acute, high exposures to heat stress, the sustained peak heart rate is a useful metric of physiological strain. It is a leading indicator that thermal regulatory control may not be adequate and that increases in body core temperature have occurred, or will soon occur. The usual guidance is to avoid maximum heart rates. For the purposes of the TLV, a sustained peak heart rate is present when the heart rate spends several minutes at or above a value equal to 180 minus age (Brotherhood, 1998). For example, the sustained peak threshold for a 40-year-old person would be 140 beats per minute (bpm). These values represent an equivalent cardiovascular demand of working at about 75% of maximum aerobic capacity.

The longer-term demands placed on the cardiovascular system may be viewed in terms of the average heart rate over a day. While the sustained peak demand may not be seen, there may still be an excessive demand. Minard et al. (1971) reported drops in physical performance capacity at the end of an 8-hour shift when the daily average heart rate exceeded 115 bpm. This level is equivalent to working at roughly 35% of maximum aerobic capacity, a sustainable level for 8 hours.

Recovery heart rate is a simple technique to assess cardiovascular strain at a moment in time. This was first proposed more than 40 years ago and has evolved over time (Brouha, 1960). The concept behind the method is simple. If the cardiovascular system is not over-extended, the heart rate will quickly return to a low level after a work shift. Following the recommendation of the four-agency document for hazardous waste sites (U.S. NIOSH, 1985) and the laboratory experience of Bernard and Kenney (1994), a rate after one minute at rest of less than 110 bpm is indicative of acceptable strain and greater than 120 bpm usually meant that there was excessive strain or that it would occur soon.

Dehydration and Blood Pressure

Most weight change during the course of a day is due to water loss. As a general guideline, a loss of 1.5% of body weight from the beginning to the end of a shift is the upper limit of acceptability. Greater losses indicate dehydration.

Over shorter time intervals, weight change, corrected for food consumption and voiding, can be assessed. For example, over an hour or two, voids might be avoided; thus, the weight loss plus food consumed equals sweat loss. A loss of 1 kg per hour is the upper limit of acceptability, which is the same as a loss of 1 liter per hour and is a practical limit to fluid replacement.

While rare, significant losses of sodium can lead to salt depletion. A 24-hour urine collection can be used to monitor sodium excretion. If the amount is less than 50 mmoles, there is an insufficient amount of sodium in the body.

Maintaining sufficient blood pressure is neces­sary to support adequate blood flow. With losses of plasma volume or extensive vasodilation, blood pressure may fall precipitously. To assess ortho-static blood pressure, blood pressure can be taken lying down and then standing. A significant drop in blood pressure indicates that an exposure should be stopped until there is some recovery.

Application of the TLV® for Heat Stress and Strain

Assessing safety during a heat stress exposure requires more than just evaluating environmental conditions. There must be careful and insightful professional judgments made that include infor­mation about the level of heat stress and consideration of personal risk factors.

Clothing

Clothing clearly affects the level of heat stress. Where there are sufficient data to make a recommendation, the TLV provides clothing adjustment values to account for the added burden of the clothing on heat stress. If evaporative cooling is sufficiently restricted or there is a different fabric or clothing construction, the WBGT criteria may not be applicable. For instance, Pandolf and Goldman (1978) showed that the usual physiological responses in encapsulating clothing cannot be related to WBGT criteria as valid determinants of safety. Conditions became intolerable when deep body temperature and heart rate were still well below the levels at which subjects were normally able to continue activity, the determinant being the approaching convergence of skin and rectal temperature. A contribution to this by radiant heat, above that implied by the environmental WBGT, has been suggested by a climatic chamber study, and the importance of this in outdoor activities in sunlight and in cool weather has been indicated (Dessurealt et al., 1995; Coles, 1997).

When clothing adjustment values are not available from the TLV or the literature, then professional judgment must be used or physiological monitoring becomes necessary.

Wet-Bulb Globe Temperature Index

The fundamental instruments for the wet-bulb globe temperature index are dry-bulb, natural wet-bulb, and globe thermometers, and a means of suspension that does not impede the flow of air from the surrounding environment and is without heat transfer from or to adjacent equipment and/or structures. The measurement of the environmental factors should be performed as follows (Minard, 1961; Minard and O’Brien, 1964).

The range of the dry-bulb and the natural wet-bulb thermometers should be -5°C to 50°C (23° to 122°F) with an accuracy of ± 0.5°C. The importance of good measurement practices has been demonstrated by Malchaire (1976). The dry-bulb thermometer must be shielded from the sun and other radiant surfaces of the environment without restricting the airflow around the bulb. The wick of the natural wet-bulb thermometer should be wetted by direct application of water 0.5 hour before each reading, be kept wet with distilled water, and not depend on capillary action from a reservoir alone. The wick should extend over the bulb of the thermometer, covering the stem about one additional bulb length. The wick should always be clean, and new wicks should be washed and rinsed in distilled water before using. Note that use of the wet-bulb reading of a sling or aspirated psychrometer may be an alternative to natural wet-bulb when other factors are taken into account. In this case, the psychrometric wet-bulb temperature also provides data for the absolute water vapor content of the air. In addition, the integrated air movement at the worker position and globe temperature are needed (Kerslake, 1972; Ellis et al., 1972). Malchaire (1976) and Bernard and Pourmoghani (1999) provide equations to estimate natural wet-bulb temperature.

A globe thermometer, consisting of a 15 cm (6 in.) diameter, hollow copper sphere, painted on the outside with a matte black finish or equivalent, should be used. The bulb or sensor of a thermo-meter [range, 5°–100°C (23°–212°F) with an accuracy of ± 0.5°C (± 0.9°F)] must be fixed in the center of the sphere. The globe thermometer should be exposed at least 25 minutes before it is read. Smaller and faster responding spheres are commercially available, but there should be some assurance that such a substitute hollow copper device yields values equivalent to the standardized 15 cm (6 in.) copper sphere. The differences between the standard and smaller globes are small in indoor measurements related to thermal comfort rather than heat stress (Humphreys, 1977). The corrections that may be needed for smaller globe readings for use in the WBGT have been determined, and the relevance of black-body devices to the radiant heat exchanges between man and the environment were analyzed by Hatch (Hey, 1968; Hatch, 1973). In cases where HSI have been devised to use a standard globe thermometer as the measure of the mean radiant temperature of the surroundings (which can be calculated from there) and where globe temperature is used as input to an index calculation, the use of other devices may be inappropriate (Kerslake, 1972; Ellis et al., 1972). The differences between smaller and standard globes become considerable at high air velocities, and large differences between dry-bulb air and globe temperatures (e.g., outdoor work in the sun and in some metal industries) necessitate corrections being applied (Oleson, 1985).

A stand, or similar structure, should be used to suspend the three thermometers so as not to restrict free airflow around the bulbs and the wet-bulb and globe thermometer are not shaded.

Other types of temperature sensors are permitted if the reading is identical to that of a mercury thermometer under the same conditions. The thermometers must be placed so that the readings are representa­tive of the condition where the employees work or rest, respectively. Caution must be taken to prevent too close proximity of the thermometers to any nearby equipment or structures, yet the measure-ments must represent where or how personnel actually perform their work. While a single location of the sensors at thorax or abdomen level is commonly acceptable, in some circumstances (e.g., if the exposures vary appreciably at different levels), more than one set of instrumental readings may be required (e.g., at head, abdomen, and foot levels) and combined by weighting, thus (Oleson, 1985):

There are now many commercially available devices that provide direct readout of WBGT according to one or more of the equations that have been recommended for integration of the individual instrument outputs. In some cases, the individual readings can also be output, together with a measure of the local air movement. The majority employs small globe thermometers that provide more rapid equilibration times than the standard globe, but care must then be taken that valid natural wet-bulb and globe temperatures are also assessed (see previous discussion). The possibility of distor­tion of the radiant heat field, which would otherwise be assessed by the standard globe, should be considered and may require adequate separation of the sensors and integrator and their supports. Adequate calibration procedures are mandatory.

Heat Strain: Physiological Monitoring

Because heat strain can be easily measured, some argue this justifies personal monitoring of those working in the heat. Physiological monitoring described here is intended to be as noninvasive as possible.

Body Temperature

Core temperature is the basic measurement for protection against hyperthermia. While rectal temperature is the usual method of assessment, ingested transmitters, called radio pills, are available and are being used to monitor deep body tempera­ture in field measurements. If sufficient time is allowed (about two hours), the pill moves out of the stomach and far enough down the intestine that it is not influenced by ingestion of hot or cold drinks or food. The transmitted value for temperature based on the individual calibration can be taken as the true value.

Oral temperature is the most commonly meas­ured surrogate of core temperature. Electronic and disposable sensors are available. Care must be taken that the individual has not ingested any food or drink for 15 minutes prior to the reading and that the mouth was kept closed during the reading. Repeated measures are useful to determine the reliability of the method and instrumentation. Because the difference between core and oral temperatures is less than 0.5°C during work, adding 0.5°C should be a safe approximation. An oral temperature less than 38.0°C (less than 38.5°C core temperature) is acceptable in a closely monitored situation with experienced and acclimatized workers. Otherwise, 37.5°C oral temperature is a good threshold, based on the WHO objective of keeping chronic elevations of core temperature below 38°C (WHO, 1969).

Another surrogate measure is tympanic membrane temperature. Although limited in value for continuous measurements, infrared measurements at the base of the external auditory canal have been proposed (Beaird et al., 1996; Hansen et al., 1996). This method may be susceptible to external temperatures, but in a protective way.

Ear canal temperature can be effective if care is taken to insulate the ear canal from the outside environment and to “calibrate” against the oral result once environmental effects have equilibrated (Decker et al., 1992). In this way, an increase of 1.0° to 1.5°C may be considered acceptable for inexperienced and unacclimatized, and for acclimatized workers, respectively.

Bernard and Kenney (1994) have proposed a method of setting an alert based on a surface mounted device. While Reneau and Bishop (1996) questioned the approach as a stand-alone, it provides a protective limit when used with heart rate monitoring.

For encapsulated suit work, skin temperature greater than 36°C is a concern, and work should cease at 37°C, even if the core temperature rise has been assessed to exceed the appropriate criterion (WHO, 1969; Goldman, 1985).

Heart Rate

Heartrate monitoring provides an early indicator of excessive physiological strain. Assessing sus­tained peak heart rate, average heart rate, or recovery heart rate are all practical and useful approaches. A sustained peak heart rate for several minutes above a value equal to 180 minus age is one limit (Brotherhood, 1998). The daily average heart rate should not exceed 115 bpm. A recovery heart rate after 1 minute at rest of less than 110 bpm is acceptable and above 120 bpm indicates that excessive strain may be present or will be if the work continues. Heart rates above these values do not mean that collapse from heat stress is imminent, but that the cardiovascular system is being taxed and that the heat stress may be high.

The use of heart rate criteria must be applied to individuals with some care to their health status. The assumption is that they are healthy and that they are not on medications that may influence the heart rate.

As a simple practice, oral temperatures and recovery heart rates can be taken concurrently to provide more useful information (Logan and Bernard, 1999; Fuller and Smith, 1981).

Special Case Encapsulating Clothing

These are situations where no guidance from WBGT is applicable to the situation. One group in this category includes wearing encapsulated suits for which the external environmental WBGT data are irrelevant (Coles, 1997). Field studies among workers wearing encapsulated suits and self-contained breathing apparatus have confirmed that the sweat-drenched physical condition commonly observed among such outdoor workers following even short work periods suggest the probable, complete saturation of the internal atmosphere (Paull and Rosenthal, 1987). Skin-temperature monitoring was proposed because the physiological effects in exposed subjects indicated the early studies by Pandolf and Goldman (1978) were correct in suggesting that convergence of mean skin with core temperature was likely to have resulted in the other serious symptoms noted, notwithstanding modest heart rate increases and minimal rises in core temperature. Through subsequent follow-up to those studies, Goldman (1985) proposed that a single location (medial thigh) of a temperature sensor for the skin could provide an adequate indication of the mean value within such suits. This suggests that personal monitoring devices for heat strain could be readily calibrated to furnish the most suitable in suit warnings to users at either one of Goldman’s (1985) proposed values (36°C skin temperature for difficulty in maintenance of heat balance and 37°C as a stop-work value) or at the subject’s own selected, age-adjusted, moving time average, limiting heart rate (Budd et al., 1991). Alternatively, means are now available for such values to be telemetered to a control point from which a withdrawal signal could be radioed to the user.

Independent treadmill studies by NIOSH of humans wearing encapsulated suits showed that, even in milder indoor environments (21°C and 26.7°C) without solar radiant heat, most subjects in similar personal protection equipment (PPE) had to stop exercising in less than one hour (Bélard and Stonevich, 1995). It is clear, however, that the influence of any radiant heat is great, and when it is present, the ambient air temperature alone is an inadequate indication of strain in encapsulating PPE. This has been reported to be the case especially when work is outdoors with high solar radiant heat levels, again with mild dry-bulb temperatures (Coles, 1997). Dessurealt et al. (1995) used multisite skin-temperature sensors in climatic chamber experiments, including radiant heat sources, and sug­gested that Goldman’s (1985) proposal of a single-selected skin temperature site was likely to be adequate for monitoring purposes. In addition, Dessurealt and co-workers (1995) showed that conditions of globe temperature about 8°C above an external dry-bulb of 32.9°C resulted in the medial thigh skin temperature reaching Goldman’s (1985) suggested value for difficulty of working in a little over 20 minutes. (The WBGT calculated for the ambient conditions was 27.4°C and would have permitted continuous work for an acclimatized subject in a non-PPE situation at the 255-W metabolic rate.) In another subject from the same study, the actual mean of six skin temperatures reached 36°C in less than 15 minutes at a heart rate of only 120 at the dry-bulb of 32.5°C, wet-bulb of 22.4°C, and globe temperature of 39.5°C (i.e., WBGT of 26.8°C) when rectal temperature was 37°C. The study concluded that, for these reasons and because no equilibrium rectal temperature was reached when the exercise was continued, “the adaptation of empirical indices like WBGT is not viable.” Nevertheless, the use of skin temperature as a guide parameter does not seem to have been considered. These subjects were indeed young, fully acclimatized, and engaged in heavy physical training programs. The conclusion may be that the absence of all but one case of collapse in 6 subjects in 4 different work conditions during three exercise-rest cycles over 85 minutes cannot be regarded as a guide to the management of a less stringently selected workforce. In all cases, the percentage of maximum heart rate for age exceeded the value of 40% at which performance decrements can be expected and only 7 of the 18 analyses were below the value (> 60%) suggested at which tolerance time limits must be applied to the situations (Goldman, 1985).

For physiological monitoring beyond real-time skin temperature, oral temperature and recovery heart rate after doffing clothing and prior to donning the clothing are useful when trends are considered. That is, is there a steady increase in any of these measures from one entry to the next entry? If so, consideration should be given to reducing work times and/or increasing recovery times.

Heat Stress Management and Controls

A heat-stress management program should look to general and job-specific controls as necessary. The general controls are appropriate when the workplace conditions can be classified as above the Action Limit based on the screening criteria of Table 2 or the curve of Figure 2, or when the clothing dictates physiological monitoring. Job-specific controls are required when the heat stress conditions exceed the thresholds of a detailed analysis or when physiological monitoring dictates controls. Demonstration to the workforce of organizational commitment to the most appropriate program of heat-stress management is essential (British Occupational Hygiene Society, 1990; NIOH, 1996a).

General Controls

Training

Workers exposed to heat stress above the Action Limit or for whom physiological monitoring is the method of evaluation should have heat stress training. The training should include the concepts of heat stress and strain, signs and symptoms of heat-related disorders, heat stress hygiene practices, procedure for seeking medical help, and information on site-specific controls.

Heat-Stress Hygiene Practices

Heat-stress hygiene practices are those actions that an individual can take to lessen the risk of heat-related disorders. Self-determination and fluid replacement must be continuously exercised. In like manner, the individual must maintain a healthy lifestyle and monitor individual health status. Heat exposures during acute illness should be avoided because of the increased risk for heat stroke. Finally, individuals must adjust expectations during periods of acclimatization. All hygiene practices are an individual responsibility, but they require supervisory support.

The importance of a fully hydrated state cannot be overemphasized. This is very clear from studies of hydration state in workers in hot mines in tropical Australia and the effect of commencing work in a dehydrated state on the incidence of heat illness (Brake, 2001). Remaining adequately hydrated is difficult because thirst alone is a poor guide. Dehydration is already well established by the time most people experience thirst. Drinking small amounts of water frequently is an essential habit when being exposed to heat stress. Paying attention to the volume and color of voided urine and to the frequency of urination during the workday are useful guidelines (Armstrong et al., 1994). Not only is there concern for replenishing lost water but also for replacing body electrolytes, especially sodium and chloride, lost in sweat. Those on an unrestricted diet and who live in the United States are likely to ingest adequate amounts of sodium, potassium, chloride, and other electrolytes, unless they are regularly involved in activities associated with heavy sweating. In cases where the resulting rate of fluid loss is approaching the upper limit of gastric absorption of fluid, and notably in the case of inadequately acclimatized subjects, the electrolyte losses from body fluids and tissues may be more rapid than their replacement from the dietary intake, and electrolyte supplementation in the fluid intake may be necessary to reduce the incidence of gastrointestinal disturbances, muscle cramps, or salt depletion heat exhaustion. Where dietary salt restriction has been recommended to individuals, consultation with their physician is a necessary first step to decide how lost electrolytes are to be replaced.

Health status is important in the tolerance to heat stress. Most chronic illnesses or their treatment can have an effect on heat tolerance. Those receiving treatment for chronic disease should inform their physician of the occupational exposure to heat stress. Those suffering an acute illness, especially with a fever, nausea, vomiting, or diarrhea, should consider taking the time to recover, and if they report to work, supervision should be very cautious about assigning hot work because of the risk for heat stroke.

While ACGIH® does not recommend acclimatization as an administrative control, it is a hygiene practice over which management has some control. It requires at least five days and up to two weeks of heat exposure for at least two continuous hours per day to develop acclimatization. As a rule of thumb, one day is lost for every four days away from work. Workers and supervisors must adjust expectations during periods of acclimatization. For returning workers, starting with about 50% of a normal exposure and ramping up over the five days is acceptable. New workers should be introduced to hot working conditions from a lower starting point of about 20% of the normal working demands and ramped up over the five days. This allows them to adjust to the new job as well as the heat stress (NIOSH, 1997).

Medical Surveillance

Pre-placement and periodic medical examina­tions of employees routinely exposed to significant levels of heat stress are advisable. Consultation with a medical authority to establish inclusion criteria is appropriate (ISO 12894, 2001).

Sentinel health events for individuals would be a serious heat-related disorder or repeated complaints of heat disorders. Medical review of these cases is advisable. Sitewide monitoring of all heat-related disorders provides a means to assess the management of heat stress.

Job-Specific Controls

Engineering Controls

In considering engineering controls, it is worthwhile recalling the avenues of heat exchange reflected in Equations 2 and 3. From Equation 4, it is clear that anything that reduces Ereq lowers the level of heat stress. The greatest contributor, often by an order of magnitude, is the metabolic rate. Mechaniz­ing some of the work, sharing the work, or spacing out the most demanding aspects can reduce the metabolic rate. Mechanization is an engineering control, while sharing and spreading the work are administrative controls.

Convective heat gains can be reduced or convec­tive heat losses increased by decreasing the air temperature and increasing the air motion when air temperature is less than 35°C. Heat loss by forced convection from the skin surface increases in a nonlinear fashion as a function of air velocity, with the greatest changes occurring at the lowest increments of airflow. Even in presumable still air with no airflow, heat is exchanged at the body surface as long as skin and air temperatures are not equal to one another. So-called, passive (free or natural) convection carries heat upward from the body, just as warm air rises above a candle flame. When air next to the skin surface is warmed, it expands and rises because this air has a lower density than surrounding cooler air. Passive convection is a slow, but effective, avenue for heat transfer. Its importance is seen in the poor responses to heat stress for individuals who have barriers to airflow over the skin. For example, those wearing casts or bandages, those with heavy scarring from burns, and those with skin disorders like ectodermal dysplasia and scleroderma do not lose heat by convection to the surrounding air (Liebowitz et al., 1991). Increasing airflow over the body surface greater than that of passive convection enormously increases heat loss for the same temperature gradient. For example, increasing airflow from 5 to 6 mph produces an increase in heat loss of only 42.3% more than increasing it from 1 to 2 mph. Increasing airflow from 10 to 11 mph increases the loss only 8.3%. Changing airflow at its lower velocities is far more effective in increasing convective heat transfer than increasing it by the same amount at an initially high flow rate. This means that only a small fan that produces no more than about 5 to 6 mph in the immediate area where people are heat stressed is more effective than having them exposed to just still air.

Radiant heat gain and loss depends on the temperatures of the solid surroundings. Working indoors where there are high levels of infrared heat gain requires unique strategies to reduce heat stress. If an individual works in a hot environment (80°F, for example) near a vat of hot liquid, bright incandescent lights, or hot machinery, heat is lost by convection to the surrounding cooler air but is gained by infrared heat emitted from the surface of hot objects (150°F, for example). Blocking radiant heat gain in such a situation is comparatively easy. In fact, just a thin sheet of material between the worker and the hot objects changes the heat transfer situation considerably. The sheet of material soon comes to nearly air temperature. The shield need not be heavily insulated because its function is to block infrared radiation. If shielding of infrared heat sources in the hot work environment is impractical, then covering bare skin surfaces with long sleeves and full-length trousers is an alternative. A balance must be struck between establishing infrared heat barriers near the skin surface (e.g., using clothing) and impeding free air circulation over the skin.

Facing heat stress indoors at air temperatures less than about 35°C (95°F), the average tempera­ture of the skin surface requires clothing selection that optimizes heat loss, assuming work or exercise takes place where there are no sources of intense infrared heat (e.g., bright, incandescent lights; hot machinery; vats of hot liquids; ovens; or similar machinery). Wearing no more clothing than required for safety exposes large areas of bare skin to the surrounding air. In addition, increasing airflow across these surfaces with fans not only greatly increases convective heat loss, but it also greatly aids the removal of heat by the evaporation of the water in sweat. There is a different challenge for reducing heat strain working indoors when air temperature is greater than skin surface temperature. Then, thermal gradients for conductive, convective, and infrared radiative heat transfer are all in a direction for the body to gain heat. Skin surface evaporation of water is then the only avenue for heat loss. Reducing airflow across the skin to minimize convective heat gain cannot be allowed to impede the convective removal of water vapor. Shielding against infrared heat gain, however, is especially important because all objects in the environment are heat sources. Unless the infrared radiant heat sources are intense, the color of clothing is not an important factor. Its water permeability and ability to allow free circulation of airflow, however, are important. Although wearing as little clothing as possible is a practical strategy for some indoor conditions, it is not outdoors if activities are either in direct sunlight or around hot machinery. Clothing that shields radiative heat gain from direct sunlight will include a broad-brimmed hat, long sleeves, and long trousers. For exposures to direct sunlight, light-colored, loosely fitting, and water-permeable clothing are good choices.

In summary, for clothing, Equation 5 demon­strates the importance of Emax in comparison to Ereq. Where the preceding paragraphs emphasized the value of reducing Ereq, increasing Emax through changes in the environment and clothing is also important. Emax is increased when the humidity in the air is reduced. This can be accomplished by bringing in outside air to dilute water vapor that enters the air through the process. If there is no process water, then the water vapor must be removed through mechanical refrigeration or chilling mechanisms. Emax is also increased when the vapor resistance of the clothing ensemble is decreased (increased permeability). By choosing clothing with lower adjustment values, the evaporative resistance is lowered.

Consequently, when environmental data ade­quate for feasibility assessments and later evalua­tion have been obtained, the benefits of applying environmental controls should never be overlooked. Although the feasibility and cost of fully air condition­ing a workplace might appear unacceptable, product quality considerations in fixed work situations may, in fact, provide the justification, while small scale “spot” air-conditioning of individual workstations has been found to be an acceptable alternative in large volume, low occupancy situations, particularly when extreme weather conditions are periodic but infrequent short-term occurrences (Coles, 1998).

Administrative Controls

While work is occurring above an occupational threshold, administrative controls provide a means to limit the exposures and manage the risk. Administrative controls include work time limits, work–rest cycles, scheduling and sharing of work, and the buddy system.

Work time limits are often employed during heat stress conditions above the TLV. The most common is based on self-determination of the exposure time. Waiting for symptoms of heat-related disorders or exhaustion to drive the decision is likely to result in high levels of heat strain. While individual discretion is important, specified work times and/or physiologi­cal monitoring should also be used. Safe exposure times to obtain up to 99% probability of no occur­rences of heat collapse in some such cases have been developed (Bell et al., 1971). The U.S. Navy published physiological heat exposure limit (PHEL) curves to limit heat exposures aboard ship (U.S. Navy Bureau of Medicine and Surgery, 1988). The ISO PHS method prescribes a maximum exposure time (ISO 7933, 2004). The U.S. Navy recommends that the recovery time in a cool environment be as long as the exposure time. Physiological monitoring such as oral temperature, sustained heart rate and recovery heart rate can also be used to inform the decision for stopping an exposure.

Work–rest cycles are designed to bring an exposure to the TLV by prescribing what portion of the work cycle should be spent in recovery. A manual method to determine the amount of recovery time (tr) expressed as the fraction of a cycle time (tc) after a period of work (tw) follows. Its basis is via plotting the work conditions (WBGTw and Mw) and the recovery conditions (WBGTr and Mr) on the TLV graph. Cycle time is the period of time allowed for a TWA and generally falls into the range of 60 to 120 minutes. The WBGT values should reflect adjustments for clothing.

Given:

      (11)

Then:

           (12)

TLV is found by the value of WBGT at the intersection of the TLV curve and the line connecting the work and recovery locations. This is illustrated in Figure 5. The intersection is at 28°C-WBGT (inside the hexagon). Using the values in Figure 5,

At this proportion of rest (or fatigue allowance) in the overall cycle, the TWA metabolic rate is about 310 W, read from the graph as well. Using Equation 1 or similar curve fitting equation for the TLV, a computer-based implementation of this method can be developed.

FIGURE 5. Illustration of the method to find a TLV® for a combination of work and rest conditions.

 

Figure 6 is an example of a recovery curve following a similar thought process as above where moderate work is performed in clothing with an adjustment value of 2°C and the recovery occurs in a comfortable environment (WBGTr = 18°C) in work clothes.

Scheduling of work to cooler times can reduce the level of heat stress. This means to cooler times of the day as well as cooler seasons. Sharing the work can be accomplished by adding people to the crew, which is often seen when summer help is added. In other words, the work can be accomplished under temperate conditions with one crew size but another size is used to take up some of the burden. Anytime work is occurring above the TLV, workers should be working in teams of two or more and maintain contact with one another.

FIGURE 6. Example work–recovery curve for moderate work with recovery in a cool environment.

Personal Protection

For heat stress, personal protection is personal cooling. Personal cooling should be used as an interim measure while engineering or administrative controls are being explored or are impractical. The possibility remains of using personal cooling devices both by coolant delivered from auxiliary plant or by cooled air from an external supply (Quigley, 1987; Coles, 1984). When the restrictions imposed by external supply lines become unacceptable, commercially available cool vests with appropriate coolants remain a possible alternative, as do suit-incorporated cooling mechanisms when the additional workloads imposed by their weight are acceptable (Coleman, 1989). The evaporative cooling provided by wetted oversuits has also been investigated (Smith, 1980).

TABLE 12. TLV® Chronology: Heat Stress and Strain
Date Action Proposal
1971 proposed New physical agent addition with proposed TLVs®
1974 Adopted
1976 Revised Recommended TWA of both WBGT and metabolic load; Specified hourly TWAs for continuous exposures and 2-hour TWAs for intermittent exposures
1980 Editorial Expanded time-acclimatization and fitness paragraph
1982 Editorial Activity examples compiled into Table
1990 proposed Replaced Figure depicting TLVs® for acclimatized and unacclimatized workers; Added Table of WBGT correction factors for clothing and discussion of adverse health effects and reproductive risks
1991 Adopted Discussion of adverse health effects and reproductive risks
1991 proposed Further revisions to TLV® figure; revised Table and added text for clothing correction factors
1992 Adopted Clothing correction factors Table and text
1993 Adopted TLV® Figure
1997 proposed Name change to “Heat Stress and Strain,” with substantive revisions
1998 proposed Further substantive revisions
1999 proposed Further substantive revisions
2000 Adopted With revisions
2004 Errata Under Early Warning Signs, paragraph 3, line 2, changed systolic-arterial to diastolic-arterial
2006 proposed Clothing adjustment factors adjusted and expanded. Changed Unacclimatized to the concept of Action Limit; adjusted metabolic rate reference points; updated Documentation
2007 Adopted Clothing adjustment factors adjusted and expanded. Changed Unacclimatized to the concept of Action Limit; adjusted metabolic rate reference points; updated Documentation
2009 Editorial Documentation Update
2017 Editorial Documentation Update

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