TLV Chemical Substances Introduction

  1. Introduction to the Chemical Substances TLVs®

    General Information

    The TLVs® are guidelines to be used by professional industrial hygienists. The values presented in this book are intended for use only as guidelines or recommendations to assist in the evaluation and control of potential workplace health hazards and for no other use (e.g., neither for evaluating or controlling community air pollution; nor for estimating the toxic potential of continuous, uninterrupted exposures or other extended work periods; nor for proving or disproving an existing disease or physical condition in an individual). Further, these values are not fine lines between safe and dangerous conditions and should not be used by anyone who is not trained in the discipline of industrial hygiene. TLVs® are not regulatory or consensus standards.

    Editor’s note: The approximate year that the current Documentation was last substantially reviewed and, where necessary, updated may be found following the CAS number for each of the adopted entries in the alphabetical listing, e.g., Chromium [7440-47-3] and inorganic compounds (2017). The reader is advised to refer to the “TLV® Chronology” section in each Documentation for a brief history of the TLV® recommendations and notations.

    Definition of the TLVs®

    Threshold Limit Values (TLVs®) refer to airborne concentrations of chemical substances and represent conditions under which it is believed that nearly all workers may be repeatedly exposed, day after day, over a working lifetime, without adverse health effects.

    Those who use the TLVs® MUST consult the latest Documentation to ensure that they understand the basis for the TLV® and the information used in its development. The amount and quality of the information that is available for each chemical substance varies over time.

    Chemical substances with equivalent TLVs® (i.e., same numerical values) cannot be assumed to have similar toxicologic effects or similar biologic potency. In this book, there are columns listing the TLVs® for each chemical substance (that is, airborne concentrations in parts per million [ppm] or milligrams per cubic meter [mg/m3]) and critical effects produced by the chemical substance. These critical effects form the basis of the TLV®.

    ACGIH® recognizes that there will be considerable variation in the level of biological response to a particular chemical substance, regardless of the airborne concentration. Indeed, TLVs® do not represent a fine line between a healthy versus an unhealthy work environment or the point at which material impairment of health will occur. TLVs® will not adequately protect all workers. Some individuals may experience discomfort or even more serious adverse health effects when exposed to a chemical substance at the TLV® or even at concentrations below the TLV®. There are numerous possible reasons for increased susceptibility to a chemical substance, including age, gender, genetic factors (predisposition), lifestyle choices (e.g., diet, smoking, abuse of alcohol and other drugs), medications, and pre-existing medical conditions (e.g., aggravation of asthma or cardiovascular disease). Some individuals may become more responsive to one or more chemical substances following previous exposures (e.g., sensitized workers). Susceptibility to the effects of chemical substances may be altered during different periods of fetal development and throughout an individual’s reproductive lifetime. Some changes in susceptibility may also occur at different work levels (e.g., light versus heavy work) or at exercise — situations in which there is increased cardiopulmonary demand. Additionally, variations in temperature (e.g., extreme heat or cold) and relative humidity may alter an individual’s response to a toxicant. The Documentation for any given TLV® must be reviewed, keeping in mind that other factors may modify biological responses.

    Although TLVs® refer to airborne levels of chemical exposure, dermal exposures may possibly occur in the workplace (see “Skin” on page 73 of the Definitions and Notations section).

    Four categories of TLVs® are specified: time-weighted average (TWA); short-term exposure limit (STEL); surface limit (SL); and ceiling (C). For most substances, a TWA alone or with a STEL is relevant. For some substances (e.g., irritant gases), only the TLV–STEL or TLV–C is applicable. If any of these TLV® types are exceeded, a potential hazard from that substance is presumed to exist.

    Threshold Limit Value–Time-Weighted Average (TLV–TWA)

    The TWA concentration for a conventional 8-hour workday and a 40-hour workweek, to which it is believed that nearly all workers may be repeatedly exposed, day after day, for a working lifetime without adverse effect. Although calculating the average concentration for a workweek, rather than a workday, may be appropriate in some instances, ACGIH® does not offer guidance regarding such exposures.

    Threshold Limit Value–Short-Term Exposure Limit (TLV–STEL)

    A 15-minute TWA exposure that should not be exceeded at any time during a workday, even if the 8-hour TWA is within the TLV–TWA. The TLV–STEL is the concentration to which it is believed that nearly all workers can be exposed continuously for a short period of time without suffering from 1) irritation, 2) chronic or irreversible tissue damage, 3) dose-rate-dependent toxic effects, or 4) narcosis of sufficient degree to increase the likelihood of accidental injury, impaired self-rescue, or materially reduced work efficiency. The TLV–STEL will not necessarily protect against these effects if the daily TLV–TWA is exceeded. The TLV–STEL usually supplements the TLV–TWA where there are recognized acute effects from a substance whose toxic effects are primarily of a chronic nature; however, the TLV–STEL may be a separate, independent exposure guideline. Exposures above the TLV–TWA up to the TLV–STEL (15-min TWA) should be less than 15 minutes, should occur no more than four times per day, and there should be at least 60 minutes between successive exposures in this range. An averaging period other than 15 minutes may be recommended when this is warranted by observed biological effects.

    *Threshold Limit Value–Surface Limit (TLV–SL)

    The concentration on workplace equipment and facility surfaces that is not likely to result in adverse effects following direct or indirect contact. The TLV–SL is intended to supplement airborne TLVs®, especially those with Skin, DSEN and RSEN notations, to provide quantitative criteria for establishing acceptable surface concentrations expressed as mg/100 cm2. For systemic effects, consistent with the use of the Skin notation, the TLV–SL will often correspond to the dose permitted by the TLV–TWA over an 8-hour period, unless chemical-specific data are available linking adverse effects with surface sample results. For certain dermal sensitizers, the surface limit may be established using potency estimates from animal studies, such as the effective concentration causing a 3-fold increase in lymphocyte proliferation (EC3) and applying an appropriate adjustment factor (Naumann and Arnold, 2019). For other sensitizers, including some respiratory sensitizers that cause induction of sensitization via dermal exposure, professional judgment may be required to supplement available surface and airborne monitoring results.

    Threshold Limit Value–Ceiling (TLV–C)

    The concentration that should not be exceeded during any part of the working exposure. If instantaneous measurements are not available, sampling should be conducted for the minimum period of time sufficient to detect exposures at or above the ceiling value.

    ACGIH® believes that TLVs® based on physical irritation should be considered no less binding than those based on physical impairment. There is increasing evidence that physical irritation may initiate, promote, or accelerate adverse health effects through interaction with other chemical or biologic agents or through other mechanisms.

    Peak Exposures

    The TLV® Committee recommends consideration of a TLV–STEL if there are supporting data. For many substances with a TLV–TWA, there is no TLV–STEL. Nevertheless, short-term peak exposures above the TLV–TWA should be controlled, even where the 8-hour TLV–TWA is within recommended limits. Limiting short-term high exposures is intended to prevent rapidly occurring acute adverse health effects resulting from transient peak exposures during a workshift. Since these adverse effects may occur at some multiple of the 8-hour TWA, even if they have not yet been documented, it is prudent to limit peak exposures. Therefore, the following default short-term exposure limits apply to those TLV–TWAs that do not have a TLV–STEL:

    Transient increases in workers’ exposure levels may exceed 3 times the value of the TLV–TWA level for no more than 15 minutes at a time, on no more than 4 occasions spaced 1 hour apart during a workday, and under no circumstances should they exceed 5 times the value of the TLV–TWA level when measured as a 15-min TWA. In addition, the 8-hour TWA is not to be exceeded for an 8-hour work period.

    This guidance on limiting peak exposures above the value of the TLV–TWA is analogous to that for the TLV–STEL, and both represent 15-minute exposure limits. The consistency in approach is intended to encourage minimizing process variability and ensuring worker protection. Good design and industrial hygiene practice ensures that processes are controlled within acceptable ranges. Historically, guidance on peak exposures (formerly excursion limits) has been based purely on statistical considerations: if log-normally distributed, short-term exposure values for a well-controlled process have a geometric standard deviation of 2.0, then 5% of all values will exceed 3.13 times the geometric mean. Processes that display greater variability are not under good control, and efforts should be made to restore control. Higher exposure levels also increase the possibility that acute health effects may occur, which were probably not factored into the TLV–TWA if it was based on prevention of chronic effects. The maximum peak exposure factor of 5 also reflects this concern about undesirable health effects. Limiting peak exposures reduces the probability of exceeding the TLV–TWA. When initial samples indicate peak exposures beyond these recommendations, more careful assessment is needed, especially when dealing with unusual work schedules.

    The so-called “3 by 5 Rule”, as described above, should be considered a rule of thumb, and a pragmatic precautionary approach. It is recognized that the geometric standard deviations of some common workplace exposures may exceed 2.0. If such distributions are known, and it can be shown that workers are not at increased risk of adverse health effects, recommended peak exposure guidelines may be modified based on workplace-specific and compound-specific health effects data. For example, consideration should be given to dose-rate effects and elimination half-times for the particular substance and for similar compounds. Special consideration should also be given to unusual work schedules and whether the peak exposure factors should be applied to the TLV–TWA (e.g., if concerns for acute health effects predominate) or the adjusted TWA (e.g., if the concern is with exceeding the adjusted TWA). The practicing hygienist must use judgment in applying this guidance on peak exposures. When a TLV–STEL or a TLV–C is available, this value takes precedence over the above guidance for peak exposures.

    TWA and STEL versus Ceiling (C)

    A substance may have certain toxicological properties that require the use of a TLV–C rather than a TLV–STEL or peak exposure guidance above a TLV–TWA. The amount by which the TLVs® may be exceeded for short periods without injury to health depends upon a number of factors such as the nature of the contaminant, whether very high concentrations — even for short periods — produce acute poisoning, whether the effects are cumulative, the frequency with which high concentrations occur, and the duration of such periods. All factors must be taken into consideration in arriving at a decision as to whether a hazardous condition exists.

    Although the TWA concentration provides the most satisfactory, practical way of monitoring airborne agents for compliance with the TLVs®, there are certain substances for which it is inappropriate. In the latter group are substances that are predominantly fast-acting and whose TLV® is more appropriately based on the concentration associated with this particular response. Substances with this type of response are best controlled by a TLV–C that should not be exceeded. It is implicit in these definitions that the manner of sampling to determine noncompliance with the TLVs® for each group must differ. Consequently, a single, brief sample that is applicable to a TLV–C is not appropriate to the TLV–TWA; here, a sufficient number of samples are needed to permit determination that the TLV-C is not exceeded at any time during a complete cycle of operation or throughout the workshift.

    Whereas the TLV–C places a definite boundary that exposure concentrations should not be permitted to exceed, the TLV–TWA requires an explicit limit to the number and duration of peak exposures which are acceptable above the recommended TLV–TWAs.


    Special consideration should also be given to the application of the TLVs® in assessing the health hazards that may be associated with exposure to a mixture of two or more substances. A brief discussion of basic considerations involved in developing TLVs® for mixtures and methods for their development, amplified by specific examples, is given in Appendix E.

    Deviations in Work Conditions and Work Schedules

    Application of TLVs® to Unusual Ambient Conditions

    When workers are exposed to air contaminants at temperatures and pressures substantially different than those at 25°C and 760 torr, care should be taken in comparing sampling results to the applicable TLVs®. For aerosols, the TWA exposure concentration (calculated using sample volumes not adjusted to conditions at 25°C and 760 torr) should be compared directly to the applicable TLVs® published in the TLVs® and BEIs® book. For gases and vapors, there are a number of options for comparing air-sampling results to the TLV®, and these are discussed in detail by Stephenson and Lillquist (2001). One method that is simple in its conceptual approach is 1) to determine the exposure concentration, expressed in terms of mass per volume, at the sampling site using the sample volume not adjusted to conditions at 25°C and 760 torr, 2) if required, to convert the TLV® to mg/m3 (or other mass per volume measure) using a molar volume of 24.4 L/mole, and 3) to compare the exposure concentration to the TLV®, both in units of mass per volume.

    A number of assumptions are made when comparing sampling results obtained under unusual atmospheric conditions to the TLVs®. One such assumption is that the volume of air inspired by the worker per workday is not appreciably different under moderate conditions of temperature and pressure as compared to those at 25°C and 760 torr (Stephenson and Lillquist, 2001). An additional assumption for gases and vapors is that absorbed dose is correlated to the partial pressure of the inhaled compound. Sampling results obtained under unusual conditions cannot easily be compared to the published TLVs®, and extreme care should be exercised if workers are exposed to very high or low ambient pressures.

    Unusual Work Schedules

    Application of TLVs® to work schedules markedly different from the conventional 8-hour day, 40-hour workweek requires particular judgment to provide protection for these workers equal to that provided to workers on conventional workshifts. Short workweeks can allow workers to have more than one job, perhaps with similar exposures, and may result in overexposure, even if neither job by itself entails overexposure. 

    Numerous mathematical models to adjust for unusual work schedules have been described. In terms of toxicologic principles, their general objective is to identify a dose that ensures that the daily peak body burden or weekly peak body burden does not exceed that which occurs during a normal 8-hour/day, 5-day/week shift. A comprehensive review of the approaches to adjusting occupational exposure limits for unusual work schedules is provided in Patty’s Industrial Hygiene (Paustenbach, 2000). Other selected readings on this topic include Lapare et al. (2003), Brodeur et al. (2001), Caldwell et al. (2001), Eide (2000), Verma (2000), Roach (1978), and Hickey and Reist (1977).

    Another model that addresses unusual work schedules is the Brief and Scala model (1986), which is explained in detail in Patty’s Industrial Hygiene (Paustenbach, 2000). This model reduces the TLV® proportionately for both increased exposure time and reduced recovery (i.e., non-exposure) time, and is generally intended to apply to work schedules longer than 8 hours/day or 40 hours/week. The model should not be used to justify very high exposures as “allowable” where the exposure periods are short (e.g., exposure to 8 times the TLV–TWA for 1 hour and zero exposure during the remainder of the shift). In this respect, the general limitations on peak exposures above the TLV–TWA and TLV–STELs should be applied to avoid inappropriate use of the model with very short exposure periods or shifts.

    The Brief and Scala model is easier to use than some of the more complex models based on pharmacokinetic actions. The application of such models usually requires knowledge of the biological half-life of each substance, and some models require additional data. Another model developed by the University of Montreal and the Institute de Recherche en Sante et en Securite du Travail (IRSST) uses the Haber method to calculate adjusted exposure limits (Brodeur et al., 2001).  This method generates values close to those obtained from physiologically based pharmacokinetic (PBPK) models. 

    Because adjusted TLVs® do not have the benefit of historical use and long-time observation, medical supervision during initial use of adjusted TLVs® is advised. Unnecessary exposure of workers should be avoided, even if a model shows such exposures to be “allowable.” Mathematical models should not be used to justify higher-than-necessary exposures.

    TLV® Units

    TLVs® are expressed in ppm, mg/m3 or mg/100 cm2. An inhaled chemical substance may exist as a gas, vapor, or aerosol. 

    • A gas is a chemical substance whose molecules are moving freely within a space in which they are confined (e.g., cylinder/tank) at 25°C and 760 torr. Gases assume no shape or volume.
    • A vapor is the gaseous phase of a chemical substance that exists as a liquid or a solid at 25°C and 760 torr. The amount of vapor given off by a chemical substance is expressed as the vapor pressure and is a function of temperature and pressure.
    • An aerosol is a suspension of solid particles or liquid droplets in a gaseous medium. Other terms used to describe an aerosol include dust, mist, fume, fog, fiber, smoke, and smog. Aerosols may be characterized by their aerodynamic behavior and the site(s) of deposition in the human respiratory tract.

    TLVs® for aerosols are usually established in terms of mass of the chemical substance in air by volume. These TLVs® are expressed in mg/m3.

    TLVs® for gases and vapors are established in terms of parts of vapor or gas per million parts of contaminated air by volume (ppm), but may also be expressed in mg/m3. For convenience to the user, these TLVs® also reference molecular weights. Where 24.45 = molar volume of air in liters at 25°C and 760 torr, the conversion equations for gases and vapors [ppm:mg/m3] are as follows:

    TLV in ppm = (TLV in mg/m3) (24.45)

    (gram molecular weight of substance)


    TLV in mg/m3 = (TLV in ppm) (gram molecular weight of substance)


    When converting values for volatile forms of inorganic compounds (e.g., as Fe, as Ni), the molecular weight of the element should be used, not that of the entire compound.

    In making conversions for substances with variable molecular weights, appropriate molecular weights should be estimated or assumed (see the TLV® Documentation).

    User Information

    Each TLV® is supported by a comprehensive Documentation. It is imperative to consult the latest Documentation when applying the TLV®.

    Additional copies of the TLVs® and BEIs® book and the multi-volume Documentation of the Threshold Limit Values and Biological Exposure Indices, upon which this book is based, are available from ACGIH®. Documentation of individual TLVs® is also available. Consult the ACGIH® website ( for additional information and availability concerning these publications.

    ACGIH® disclaims liability with respect to the use of TLVs®.

    References and Selected Readings

    1. Brief RS; Scala RA: Occupational health aspects of unusual work schedules: a review of Exxon’s experiences. Am Ind Hyg Assoc J 47(4):199-202 (1986).
    2. Brodeur J; Vyskocil A; Tardif R; et al.: Adjustment of permissible exposure values to unusual work schedules. Am Ind Hyg Assoc J 62:584-594 (2001).
    3. Buringh E; Lanting R: Exposure variability in the workplace: its implications for the assessment of compliance. Am Ind Hyg Assoc J 52:6-13 (1991).
    4. Caldwell DJ; Armstrong TW; Barone NJ; et al.: Lessons learned while compiling a quantitative exposure database from the published literature. Appl Occup Environ Hyg 16(2):174-177 (2001).
    5. Eide I: The application of 8-hour occupational exposure limits to non-standard work schedules offshore. Ann Occup Hyg 34(1):13-17 (1990).
    6. Hickey JL; Reist PC: Application of occupational exposure limits to unusual work schedules. Am Ind Hyg Assoc J 38(11):613-621 (1977).
    7. Lapare S; Brodeur J; Tardif R: Contribution of toxicokinetic modeling to the adjustment of exposure limits to unusual work schedules. Am Ind Hyg Assoc J 64(1):17-23 (2003).
    8. Leidel NA; Busch KA; Crouse WE: Exposure measurement action level and occupational environmental variability. DHEW (NIOSH) Pub. No. 76-131; NTIS Pub. No. PB- 267-509. U.S. National Technical Information Service, Springfield, VA (December 1975).
    9. Paustenbach DJ: Pharmacokinetics and Unusual Work Schedules. In: Patty’s Industrial Hygiene, 5th ed., Vol. 3, Part VI, Law, Regulation, and Management, Chap. 40, pp. 1787-1901. RL Harris, Ed. John Wiley & Sons, Inc., New York (2000).
    10. Roach SA: Threshold limit values for extraordinary work schedules. Am Ind Hyg Assoc J 39(4):345-348 (1978).
    11. Stephenson DJ; Lillquist DR: The effects of temperature and pressure on airborne exposure concentrations when performing compliance evaluations using ACGIH TLVs and OSHA PELs. Appl Occup Environ Hyg 16(4):482-486 (2001).
    12. Verma DK: Adjustment of occupational exposure limits for unusual work schedules. Am Ind Hyg Assoc J 61(3):367-374 (2000).