Threshold limit values and their applicability in the realms of chemical exposure control

ARTICLE

Year : 2011  |  Volume : 34  |  Issue : 1  |  Page : 77-80

Threshold limit values and their applicability in the realms of chemical exposure control

S Soundararajan
Industrial Hygiene and Safety Section, Radiation Safety Systems Division, Health, Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai, India

Date of Web Publication

17-Mar-2012

Correspondence Address:
S Soundararajan
Industrial Hygiene and Safety Section, Radiation Safety Systems Division, Health, Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai
India

Abstract

How to cite this article: Soundararajan S. Threshold limit values and their applicability in the realms of chemical exposure control. Radiat Prot Environ 2011;34:77-80

1. Introduction

Industrial hygiene is a multidisciplinary subject dealing with control of health hazards that arise in or out of a work premise. Anticipation and recognition, evaluation, and control of health hazards are the essential components of industrial hygiene. The environmental stresses that could be present in an occupational set-up are categorized as (i) stresses due to chemical agents, (ii) stresses due to physical agents, (iii) stresses due to biological agents, and (iv) stresses due to ergonomic factors. Methods are available to measure these stresses and occupational exposure limits have been prescribed to each of these stresses. Threshold Limit Value (TLV) is one such limit that is developed as a guideline to assist in the control of health hazards. TLVs are developed to protect workers who are normal, healthy adults. Similar to the Derived Air Concentration (DAC) used in the field of radiological safety, the TLV is used as a guideline to control health hazards caused by different agents. However, the analogy stops here as both are based on different approaches. TLVs are developed as guidelines by the American Conference of Governmental Industrial Hygienists (ACGIH), a not-for-profit scientific association, to assist in the control of health hazards. TLVs are based solely on health factors; there is no consideration given to economic or technical feasibility. They are not developed for use as legal standards. The user must recognize the constraints and limitations subject to their proper use and bear the responsibility for such use.

2. Chemical TLVs

When we discuss about chemical hazards, 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. TLVs are developed to protect workers who are normal, healthy adults. These values are not fine lines between safe and dangerous conditions. These are intended for use in the practice of industrial hygiene, to be interpreted and applied only by a person trained in this discipline. Three categories of TLVs are specified: Time Weighted Average (TWA), Short-Term Exposure Limit (STEL), and a Ceiling (C). For most substances, a TWA alone (e.g. ethanol - TWA: 1000 ppm) or with a STEL (e.g. ammonia - TWA: 25 ppm and STEL: 35 ppm) is relevant. For some substances (e.g. irritant gases), only the TLV-Ceiling (e.g. formaldehyde - TLV-C: 0.3 ppm) is applicable. If any of these TLV types are exceeded, a potential hazard from that substance is presumed to exist. Where there is a hazard of absorption of a chemical through skin, the fact is indicated in the TLV document by the word "skin" (e.g. formamide - TWA: 10 ppm, Skin).

2.1 TLV-TWA

TLV-TWA is the limit on time-weighted average air concentration value for an 8-hour workday and a 40-hour workweek. This is 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. The TWA limit concept accommodates excursion of the concentrations above the limit, provided they are compensated by excursions below the limit in the same workday so that the average does not exceed the limit. This is done to accommodate some flexibility in operations where it can be done so.

2.2 TLV-STEL

The TLV-STEL indicates the concentration to which the workers can be exposed for short time without suffering from irritation, chronic or irreversible tissue damage, or narcosis that can cause accidents or reduce work efficiency. The STEL presupposes that the TLV-TWA is not exceeded during the workday though concentrations exceeding the limit are permitted for short duration. The duration of exposure at STEL level should not exceed 15 minutes. STELs are recommended only in the cases of chemicals that have toxic effects on exposure to high concentrations.

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 should be less than 15 minutes, should not occur 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.

2.3 TLV-C

In some cases, it has been found that the nature of effects is such that the exposure is not advisable to be exceeded a certain concentration even for short durations in a shift. These cases are given a "C" value TLV with the understanding that the concentrations for these chemicals are always kept to within the TLV-C (TLV-Ceiling) limit. This type of Ceiling limits is given to the chemicals which are fast acting. This is the limiting concentration on such a chemical 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.

3. Limitations

Chemical substances with equivalent TLVs (i.e. same numerical values) cannot be assumed to have similar toxicological effects or similar biologic potency. In the TLV Book, there are columns listing the TLVs for each chemical substance [i.e. airborne concentrations in parts per million (ppm) or milligrams per cubic meter (mg/m ³ )] and critical effects produced by the chemical substance. These critical effects form the basis of the TLV.

TLVs will not adequately protect all workers. Some individuals may be more susceptible to a certain chemical substance and experience discomfort or even more serious adverse health effects when exposed to that 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, ethnicity, 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). Some changes in susceptibility may also occur at different work levels (e.g. light vs. 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. Individuals who are hyper susceptible may not be adequately protected from adverse effects of certain chemicals at concentrations at or below the threshold limits. An occupational physician should evaluate the extent to which such workers require additional protection.

4. Applicability

4.1 Excursion Limits

For many substances with a TLV-TWA, there is no TLV-STEL. Nevertheless, excursions above the TLV-TWA should be controlled, even where the 8-hour TLV-TWA is within recommended limits. The TWA which is a limit on 8-hour average concentration 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 TLV-TWA excursions should be applied to avoid inappropriate use of the model with very short exposure periods or shifts. Excursion limits apply to those TLV-TWAs that do not have TLV-STELs.

Excursions in worker exposure levels may exceed 3 times the TLV-TWA for no more than a total of 30 minutes during a workday, and under no circumstances should they exceed 5 times the TLV-TWA, provided that the TLV-TWA is not exceeded.

When the toxicological data for a specific substance are available to establish a TLV-STEL or a TLV-C, these values take precedence over the excursion limit.

4.2 TLVs for Mixtures

In addition to consideration of exposure to a single material, combined effects of different materials also should be considered. For example, a synergistic effect (enhancement of toxicity of a substance when it is taken with other substances) will be produced by exposure to a mixture of sulphur dioxide and sodium chloride crystals. Antagonistic effects are produced by inhaling fumes of iron and gaseous nitrogen oxides; the reduction effect in this case is explained on the basis of a firmly combined layer of nitrogen oxides on iron oxide particles. When purely local effects on different organs of the body are produced by various compounds of a mixture, it may be assumed that the effects would be independent. When two or more hazardous chemical substances have a similar health effect on the same target organ or system, their combined effect should be considered. Unless it is clearly known to be otherwise, it may be assumed that the effects of exposure to such chemical agents are additive. In such a case, if C _i is the measured atmospheric concentration and T _i is the corresponding threshold limit value, the sum of (C _i /T _i ) is determined. If the value of Σ(C _i /T _i ) exceeds unity, the threshold limit of the mixture is considered as being exceeded. Thus, for the combined threshold limit of the mixture to remain within the limiting value.



Example 1: Combination of chemical agents that have TLV-TWA alone

Consider a work environment of a solvent extraction process using tributylphosphate (TBP) as solvent and kerosene as diluent where airborne TBP and kerosene are present together. Both these substances are classified as upper respiratory tract (URT) irritants. Only TLV-TWA values have been prescribed as recommended occupational limit for these substances. The measured full-shift air-borne concentrations of these substances and their TWA values are given in [Table 1].

Table 1: Mixture of substances with only TLV-TWA values Click here to view

Applying the additive formula,



Thus, the threshold limit of the mixture is considered as being exceeded though the individual TLVs are not exceeded.

As we have seen earlier, some substances are prescribed only TWA values as occupational limit; some substances are prescribed both TWA and STEL values as recommendatory occupational limits; some fast-acting substances are prescribed only Ceiling value as occupational limit. Where a substance with a STEL or Ceiling limit is mixed with a substance with a TLV-TWA but no STEL, comparison of the short-term limit with the applicable excursion limit is considered. A value of five times the TLV-TWA is taken as the excursion limit. Thus, for such a case of exposure to chemical with TLV-TWA alone along with a chemical that has STEL value also, the modified additive formula is:



where T ₁ = the TLV-TWA of the chemical agent with no STEL and T _2STEL = the TLV-STEL of the chemical agent for which STEL is prescribed.

The example given below explains utility of this formula.

Example 2: Application of modified additive formula

Consider a work environment with acetic anhydride and acetic acid. Both are irritants and have similar toxicological effects. Hence, the additive rule can be applied to determine whether the concentration of the mixture exceeds the effective TLV. In addition to TWA, acetic acid has a STEL value also. The measured full-shift concentrations and the short-term concentrations of these chemical agents and their TLVs are given in [Table 2].

Table 2: Mixture of a substance with only TLV-TWA with that having TLV-TWA as well as TLV-STEL

Independently, the chemical agents are well below the respective TLVs. Full-shift analysis of this case of mixture of chemicals would use Equation (1), and accordingly we get:

2/5 + 5/10 = 0.9

This value is less than 1. Thus, the full-shift mixture limit is not exceeded.

For the short-term analysis, Equation (2) is applied. Accordingly,

[4/(5)(5)] + 14/15 = 0.16 + 0.933 = 1.093

This value is more than 1. Thus, the short-term mixture limit is exceeded.

5. Conclusion

In the field of industrial hygiene practice, the TLV prescribed by ACGIH can be used as recommendatory guideline values in the control of occupational exposures to environmental stresses. However, it requires a judicious approach. Additive rule can be useful in determining exposure to a mixture of chemicals. However, a thorough knowledge about the limitations of TLV concept and its application are essential.

6. Acknowledgments

Constant encouragement provided by Dr. A. K. Ghosh, Director, Health, Safety and Environment Group, and continuous support in writing this review paper extended by Dr. D. N. Sharma, Associate Director, Health, Safety and Environment Group, BARC, are gratefully acknowledged. The author expresses his sincere thanks to Dr. Pushparaja for his persistent stimulation in writing this paper.

7. Reference

1. American Conference of Governmental Industrial Hygienists (ACGIH): Threshold Limit Values for Chemical Substances and Physical Agents & Biological Exposure Indices. 2010.