Module 1: Occupational Hygiene - Section 3: OH Standards
OH2.3: Limit Values


Threshold Limit Values (TLVs) were first published by the ACGIH in 1950 3. They are the most widely used limits and continue to be used in countries where there are no compulsory standards.

TLVs refer to airborne concentration of materials and represent conditions under which it is believed that nearly all workers may be repeatedly exposed day-after-day without adverse effects. Because of wide variation in individual susceptibility, however, a small percentage of workers may experience discomfort from substances at concentrations at or below the threshold limit or may be affected more seriously and develop an occupational illness.

The earlier version of TLVs was based on irritation, sensitisation and acute toxicity. According to the ACGIH, TLVs are based on best available information from industrial experience including human epidemiological studies, from experimental animal studies, from chemical and physical analogy, and where possible, from a combination of the three. The basis on which the values are established may differ from substance to substance. Protection against impairment of health may be a guiding factor for some, whereas reasonable freedom from irritation, narcosis, nuisance or other forms of health effects may apply for others.

The assertion by the ACGIH that TLVs are purely health based has in the past been vigorously contested by others5, 6claiming that the TLV Committee was heavily influenced by industry which not only had their representatives sitting on the Committee, but also dictated the agenda and outcomes of the meetings. For an informative exchange between the ACGIH and their critics read Castelman and Cox:


The OELs are published by the Department of Labour in South Africa. They were first published in 1995 and are contained in Tables 1 and 2 of the Regulations for Hazardous Chemical Substances. Because the OELs are published by a government department, they can be enforced. There are two types of OELs and they are the OEL-CL and the OEL-RL.

The OEL-RL is an exposure limit that is based on health considerations only and applies to all substances listed in Table 1. This standard is considered to be more protective because workers who are exposed below the OEL-RL should have a negligible risk of developing adverse effects. According to the regulations, workers exposed at or below the OEL-RL should be monitored at least once every two years.

OEL-CL is an exposure limit for which health information has been compromised by socio-economic considerations. This includes the cost of compliance, severity of adverse health effects, availability of technology for exposure reduction, and number of workers potentially exposed. Workers who are exposed to concentrations at or below the OEL-CL may therefore still be at risk of developing adverse health effects. Therefore, it is prudent for employers to bring down concentrations to the lowest level possible. Exposures at or below the OEL-CL should be monitored at least once every 12 months.


For airborne exposures, there are three categories of exposure limits:

Time-weighted average (TWA): Concentration of airborne contaminants may vary considerably in the course of an 8-hour working period. By calculating the average time-weighted concentration (TWA) over an 8-hour shift of concentrations during shorter time periods within the 8-hour shift, a useful measure of worker exposure can be determined more readily. The TWA is ideal for use in protection against chronic health effects.

The TWA is calculated as follows:

Where: Ci is air concentration during period i, and Ti is the duration of exposure during period i.

Short Term Exposure Limits: For many hazardous substances peak levels that go momentarily above the TWA are permissible provided that exposure is indeed for a short period of time. These concentration levels are known as STELs.

STEL is therefore the maximum concentration of a hazardous substance to which a worker may be exposed for 10 or 15 minutes without suffering irritation, chronic or irreversible tissue damage. It is ideal for substances that have acute health effects. No more that 4 excursions above the STEL are permitted per day with at least 60 minute intervals between exposure periods.

Ceiling Limit: Ceiling limit is a concentration that should never be exceeded even instantaneously. It is ideal for fast acting hazardous substances. Ceiling values are best determined with direct reading instruments.

For some substances, for example irritant gases, only one category of exposure limit may be relevant. For other substances, either two or three categories may be relevant depending upon their physiological action.


5. Ziem GE, Castleman BI. Threshold limit values: historical perspectives and current practice. J Occup Med. 1989 Nov;31(11):910-8.

6. Castleman BI, Ziem GE. Corporate influence on threshold limit values. Am J Ind Med. 1988;13(5):531-59.

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General Introduction to Occupational Health: Occupational Hygiene, Epidemiology & Biostatistics by Prof Jonny Myers is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License