OccupationalExposure Limits

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For more than half a century, the toxicology and

industrial hygiene community and others have

used occupational exposure limits (OELs) to help to pre-vent work-related illnesses. The first attempt to formalizeexposure controls occurred in 1946, when the AmericanConference of Governmental Industrial Hygienists(ACGIH) adopted 148 exposure limits. These exposurelimits were initially referred to as maximum allowableconcentrations. In 1956 the terminology used today, thresh-old limit values (TLVs), was introduced.4

Today, the practicing occupational health professionalhas a variety of exposure limits to use in assessing variousexposures to chemical and physical agents. Although theobjective of each limit may vary (for example, from pro-tecting “nearly all workers”3 to ensuring that no “employeewill suffer diminished health”10), the overall goal is to pre-vent occupational exposures that may result in adversehealth effects.

This chapter focuses on OELs developed by organiza-tions in the United States. Other countries have also devel-oped limits. For example, Germany’s Commission for theInvestigation of Health Hazards of Chemical Compoundsin the Work Area has developed OELs known as MAKs

for 600 chemical compounds.6 Great Britain also has sev-eral hundred OELs. Some of these exposure limits havebeen approved by the United Kingdom’s Health and SafetyCommission, giving them the force of law. Others serve asrecommendations made by Britain’s Health and SafetyExecutive.8

Most of the OELs developed in the United States havebeen essentially voluntary guidelines or recommendations.These include the following exposure limits:

� OSHA’s permissible exposure limits (PELs) � The ACGIH’s TLVs � The AIHA’s Workplace Environmental Exposure

Level (WEELs) � NIOSH’s Recommended Exposure Limits (RELs) � The EPA’s New Chemical Exposure Limits (NCELs) � Internal company standards developed by chemical

manufacturers and users

These guidelines are based on industrial hygiene judgmentusing available toxicologic and epidemiologic research anddata resulting from field activities and occupational healthprograms. They are generally set at levels where adverse

40 GAYLA J. MCCLUSKEY

�

Warning sign. (Courtesy of Lydia

Forte Roberts Photography, with

permission.)

health effects are not expected to occur. The ACGIH isexplicit in stating that the TLV it recommends are not finelines between safe and dangerous concentrations or relativeindices of toxicity, and that any user should be trained in theprinciples of industrial hygiene.3 The ACGIH also indicatesthat the limits should not be used as legal standards, althoughthe current OSHA standards were largely derived from TLVs.OSHA’s compliance procedures indicate that overexposure toa TLV could be considered a violation.12 In addition, severalother countries have adopted the TLVs into law.

The Occupational Safety and Health Act of 1970 estab-lished the National Institute of Occupational Safety andHealth (NIOSH) to recommend standards and OSHA topromulgate and enforce standards.10 OSHA permissibleexposure limits (PELs) are currently the only standards inthe United States backed by the force of law. After passageof the OSHA act, the Secretary of Labor was given a 2-year window in which to adopt existing national consen-sus standards and any established federal standard. SinceTLVs were included as part of the Walsh Healy PublicContracts Act, OSHA for the most part adopted the 1968TLVs, which resulted in nearly 500 limits for approxi-mately 300 chemical substances.13 These were establishedin 1971 and are based on data that were obtained primarilyin the 1950s and early 1960s.

Since its inception, OSHA has promulgated new PELsfor only about two dozen chemicals over the last 30 yearsand has not updated the initial PEL list. In 1989, OSHApublished a final rule revising 212 existing exposure limitsand establishing 164 new OELs. Industry and labor groupschallenged the proposed rule, and in 1992 the 11th CircuitCourt of Appeals ruled that the agency did not sufficientlydemonstrate that the proposed changes were necessary orthat they were economically or technologically feasible.1

This decision to vacate both the new limits and the revised,more protective limits forced the agency to return to the useof the original 1971 limits, which are still in effect today.

The various exposure limits other than the PELs are notsubject to technological or economic feasibility tests. Inaddition, some TLVs have been challenged in recent liti-gation. The amount and nature of the information used toestablish OELS vary from chemical to chemical, and someOELs are based on limited research data. And, becausethere are so few OELs, the vast majority of chemicals usedin commerce have no recommended limit. That said, thefew existing OELs are indispensable tools for the occupa-tional health professional. However, they must be used andapplied with the recognition that there are inherent limita-tions as well as benefits.

Organizations That SetExposure Limits

OCCUPATIONAL SAFETY AND HEALTHADMINISTRATION

OSHA’s track record in revising and setting new PELs hasnot been very effective. This is largely a consequence of

the burdens that were established by the OccupationalSafety and Health Act and by other requirements that havebeen added to the standards development process overtime. The legal criteria the agency must meet include prov-ing that the hazards pose a significant risk of materialimpairment, that the standards and limits are technologi-cally and economically feasible, and that the benefits arecost-effective.

NATIONAL INSTITUTE OF OCCUPATIONALSAFETY AND HEALTH

NIOSH was formed to conduct research on the healtheffects of exposure and to develop criteria for dealing withtoxic materials, including safe levels of exposure. In 1974,NIOSH joined OSHA in developing a series of occupa-tional health standards for substances with existing PELs,known as the Standards Completion Program. This effortresulted in the development of 380 substance-specific draftstandards with supporting documentation for use in prom-ulgating new health regulations. Because of the require-ments associated with standards setting, these documentsand the RELs remain as recommendations today. They areupdated periodically and are published annually in theNIOSH Pocket Guide.

NIOSH also developed levels for the 380 chemicals atwhich exposure is considered to be immediately dangerousto life and health (IDLH). These are included in the PocketGuide and are used for respirator selection.9

AMERICAN CONFERENCE OFGOVERNMENTAL INDUSTRIAL HYGIENISTS

The ACGIH is a membership organization that developsOELs. Three committees—the Chemical Substances TLVCommittee, the Biological Exposure Indices Committee,and the Physical Agents TLV Committee—create the TLVsand BEIs. Candidate chemicals and physical agents arelisted on a Notice of Intended Change for at least a yearbefore adoption. New and revised TLVs and BEIs areapproved by the ACGIH board of directors. The list isupdated annually and includes TLVs for about 700 chemi-cals and 40 BEIs.

AMERICAN INDUSTRIAL HYGIENEASSOCIATION

The AIHA, also a membership organization, began the useof a consensus process for the development of airbornechemical exposure limits (WEEL Guides) in 1980. TheAIHA WEEL Committee concentrates on chemicals forwhich there are no existing guidelines and has developedguides for about 80 chemicals. The WEEL Committeemakes every effort to cooperate with the ACGIH and sim-ilar organizations to avoid duplication.

In addition, the AIHA first set emergency exposure lim-its in 1964 and formed the current Emergency ResponsePlanning Guidelines (ERPG) Committee in 1987. Limitscurrently exist for more than 90 chemicals.5

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ENVIRONMENTAL PROTECTION AGENCY

The EPA recently began setting OELs under the authorityvested in that agency by the Toxic Substances Control Act (15USC s/s 2601). Anyone who plants to manufacture or importa new chemical substance must submit notice to the EPA. TheEPA conducts a risk assessment and, if the agency determinesthat the chemical may present an unreasonable risk of injuryvia inhalation exposure, it will set a new chemical exposurelimit (NCEL). These limits are for informational purposesonly and are not legally enforceable limits. However, chemi-cal manufacturers are bound by a TSCA Section 5(e) ConsentOrder to follow the agency’s recommendations. Currently,more than 20 limits are posted on the EPA web site.7

CHEMICAL MANUFACTURERS AND USERS

Chemical manufacturers often set OELs for the chemicalsthey manufacture or formulate. These internal limits areusually published on the material safety data sheet docu-ment (MSDS) provided to the customer.

Definitions

The terminology used to define various exposure limits isfairly consistent among the organizations that set OELs.Specific definitions are provided in the following discus-sion, although readers should refer to the referenced docu-mentation for the basis of each OEL.

TIME-WEIGHTED AVERAGE

Many exposure limits are based on time-weighted aver-age (TWA) calculations, which apportion the measuredexposure to the time period in which the exposureoccurred. Thus, an employee could be exposed to con-centrations above or below the exposure limit during theworkday.

This practice is not intended to subject an employee toan unreasonably high level of exposure. The ACGIH hasrecognized that excursions above the exposure level shouldbe held to reasonable limits. The ACGIH suggests thatexcursions during a workday may exceed three times theTLV-TWA for no more than a total of 30 minutes, and forno more than five times the TLV-TWA, provided that theTWA-TLV is not exceeded.3

In a typical workday, an employee is often exposed toseveral different short-term average concentrations as aresult of changes in job assignment, processes, and venti-lation conditions. The TWA method calculates the full-shiftaverage concentration by weighting different short-termaverage concentrations by exposure time. The TWA can becalculated using the following formula:

TWA = C1T1+C2T2+CnTn

8 hours

where:

TWA = time-weighted average concentration, usuallyin parts per million or milligrams per cubic meter.

C = concentration of contaminant during the incremen-tal exposure time.

T = time, T1, T2, T3....Tn are the incremental exposuretimes at average concentrations C1, C2 + Cn.

Eight hours is generally used as the denominator, asmost standards are based on an 8-hour workday.14

The application of OELs to workers on work sched-ules significantly different than the 8-hour day, 40-hourweek should be done only by industrial hygiene profes-sionals.

8-HOUR TWA LIMIT

Most exposure limits apply to the entire 8-hour workday.Using the TWA concept above, the resultant calculatedconcentration could then be compared with the variousoccupational exposure limits (OSHA’s PELs, NIOSH’sRELs, ACGIH’s TLVs, AIHA’s WEELs, EPA’s NCELs,and industry standards). The averaging time for all of theseis up to 480 minutes (8 hours) for a 40-hour work week,except for the RELs, which use up to 10 hours during a40-hour week.

SHORT-TERM EXPOSURE LIMITS

The ACGIH defines a short-term exposure limit (STEL) asa concentration that would not result in irritation, chronicor irreversible tissue damage, or narcosis that would impairthe worker or materially reduce work efficiency. Theselimits are set for chemicals that have recognized acuteeffects in addition to toxic effects that primarily are of achronic nature. STELS are considered to be a requirementin addition to the TWA limits.

A STEL is calculated as a TWA using an averaging timeof 15 minutes. It should not be exceeded at any time dur-ing a workday, even if the 8-hour TWA is not exceeded.Exposures should not be longer than 15 minutes andshould not occur more than four times per day, with 60minutes between successive exposures.2

CEILING

A ceiling limit (c) indicates a concentration that should notbe exceeded during any part of a workday. According toOSHA, if instantaneous monitoring is not feasible, then theceiling shall be assessed as a 15-minute time-weightedaverage exposure that shall not be exceeded at any timeduring the working day.12 The ACGIH indicates that thesampling time should not exceed 15 minutes, except forsubstances that may cause immediate irritation when expo-sures are short.2

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SKIN NOTATION

This notation appears with other exposure limits andindicates that a substance may be absorbed in toxicamounts through the skin or mucous membranes. OSHA,ACGIH, AIHA, and NIOSH exposure limits include thisdesignation.

SENSITIZER NOTATION

Both the ACGIH and the AIHA note chemicals that causesensitization. For ACGIH, the designation “SEN” is usedfor these agents based on human or animal data.3 TheAIHA uses two designations: “DSEN” indicates the poten-tial for dermal sensitization resulting from the absorbedagent and ultraviolet light, and “RSEN” indicates respira-tory sensitization.5

PHYSICAL AGENTS

OSHA and the ACGIH set OELs for physical agents of anacoustic, electromagnetic, ergonomic, mechanical, or ther-mal nature. These limits differ from inhalation TLVs inthat many are based on actual industrial experience andhuman experimental studies. The limits are not single num-bers and should only be applied by users who have train-ing and experience in the specific measurement andevaluation techniques.3

EMERGENCY RESPONSE LIMITS

The following guidelines are technically not considered tobe actual OELs but are important in emergency responseplanning.

NIOSH, as part of the Standards Completion Program,developed values for chemicals that represented exposurelevels that were immediately dangerous to life and health(IDLH). An IDLH levels is defined as a condition thatposes a threat of exposure to airborne contaminants whenthat exposure is likely to cause death or immediate ordelayed permanent adverse health effects, or to preventescape from such an environment. These limits are for a30-minute period.9

The AIHA issues ERPGs, which are community-basedexposure limits for use in planning for accidental chem-ical releases. They are intended to be used to assess theadequacy of preventative and response measures under-taken for chemical releases and transportation emergen-cies. The ERPGs include three levels for a 1-hour periodat which progressive levels of adverse health effects areexpected.5

BIOLOGICAL EXPOSURE INDICES

Technically not OELs, BEls are guidance values for assess-ing biological exposure results and are set for nearly 40chemicals and chemical classes by the ACGIH. These val-ues represent the levels of determinants that are most likelyto be observed in specimens collected from healthy work-ers who have been exposed to chemicals to the same extentas workers with inhalation exposure at TLV levels. Theexceptions are for chemicals with inhalation TLVs basedon protection against nonsystemic effects such as irritation.In this case the BEls are necessary, as these chemicals havethe potential for absorption via another route of entry (usu-ally the skin).3

Summary

OELs are based on the concept of a dose–response rela-tionship between the chemical or physical agent and result-ant health effects that results in the conclusion that there isa lower level of exposure where adverse health effects willnot occur. They do not afford a fine distinction betweensafe and dangerous conditions, owing to individual sus-ceptibility and other factors.

OELs should be considered guidelines for good practice.Their application requires knowledge of the basis of thelimit and an understanding of the methods and proceduresfor monitoring exposure.

REFERENCES

1. AFL-CIO v. OSHA, 965 F2d 962 (11th Cir 1992). 2. American Conference of Governmental Industrial Hygienists: 2000

Threshold Limit Values for Chemical Substances and Physical Agentsand Biological Exposure Indices. Cincinnati, Ohio, ACGIH, 2000.

3. American Conference of Governmental Industrial Hygienists: 2001Threshold Limit Values for Chemical Substances and PhysicalAgents and Biological Exposure Indices. Cincinnati, Ohio, ACGIH,2001.

4. American Conference of Governmental Industrial Hygienists on line:Available at http://www.acgih.org/About/History.htm/

5. American Industrial Hygiene Association: The AIHA 2001Emergency Response Planning Guidelines and Workplace Environ-mental Exposure Level Guides Handbook. Fairfax, Va, AIHA, 2001.

6. Deutsche Forschungsgemeinschaft: MAK and BAT Values 2001(Report 37). Weinheim, Germany, Commission for the Investigationof Health Hazards of Chemical Compounds in the Work Area, 2001.(Available in the United States from John Wiley, 605 Third Ave, NewYork, NY 10158-0012.)

7. Environmental Protection Agency on line: Available athttp://www.epa.gov/opptintr/newchems/ncelmain.htm/

8. Health and Safety Executive: Occupational Exposure Limits 2001.Sudbury, Suffolk, U.K., Health and Safety Executive Books, 2001.

9. National Institute for Occupational Safety and Health (NIOSH):NIOSH Respirator Decision Logic (DHHS [NIOSH] Publication No

422 SECTION I / Occupational Toxicology

87-108, NTIS Publication No PB-91-151183). Washington, DC,NTIS, 1987.

10. Occupational Safety and Health Act, 29 USC Chap 15. 11. Occupational Safety and Health Administration: 29 Code of Federal

Regulations, 1910. 1000. Washington, DC, OSHA, 1971. 12. Occupational Safety and Health Administration: CPL 2.445B—

Changes to the Field Operations Manual (FOM). Washington, DC,OSHA, 1993.

13. Olishifski JB: Fundamentals of Industrial Hygiene, 2nd ed. Chicago,National Safety Council, 1971.

14. Plog BA: Fundamentals of Industrial Hygiene, 4th ed. Itasca, Ill,National Safety Council, 1996.

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