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| Module 1: Occupational Hygiene - Section 3: Evaluation of Airborne Contaminants |
| OH3.5: Occupational Exposure Limits |
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| Module 1: Occupational Hygiene - Section 3: Evaluation of Airborne Contaminants |
| OH3.5: Occupational Exposure Limits |
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Air sampling data is usually compared to recommended or mandated occupational exposure limits.
Threshold Limit Values:
Threshold Limit Values, TLVs, refer to airborne concentrations of substances and represent conditions under which it is believed that nearly all workers may be repeatedly exposed, day after day, without adverse effect. 4 The data used to establish TLVs comes from animal studies, human studies, and industrial experience. The limit may be selected based on the fact that the substance is:
TLVs are not:
Occupational Exposure Limits:
The Occupational Exposure Limits,OELs, are published by the Department of Labour. They were first published in 1995 and are contained in Tables 1 and 2 of the Regulations for HCS. OELs are enforceable by law.
The regulations set two types of Occupational Exposure Limits for workers who are exposed hazardous chemical substances, namely the OEL-RL and OEL-CL.
Occupational Exposure Limit - Recommended Limit (OEL-RL) is an exposure limit that is based on health considerations only and it 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 would have negligible risk.
Occupational Exposure Limit - Control Limit (OEL-CL) is an exposure limit for which health based standards have been compromised by economic considerations such as the cost to industry of implementing control measures. Workers who are exposed at concentrations below the OEL-CL may still be at significant risk of developing adverse health effects. Therefore, it is the responsibility of employer to bring down concentration to the lowest level possible.
Time-weighted Average (TWA)
Concentration of airborne contaminants may vary considerably in the course of one 8-hour working period. By calculating the time weighted average concentration, the exposure level of a worker can be determined more readily as a summary measure over that 8-hour shift. The TWA then means the average airborne concentration of a particular substance when calculated over a normal 8-hour working day.
The time-weighted average is calculated as follows:
Where Ci is the air concentration during period i, and Ti is the duration of exposure at the concentration Ci.
Short Term Exposure Limit (STEL):
For many hazardous substances, peak levels that go momentarily above the TWA are permissible provided that high exposure is indeed only for short periods of time. These concentration levels are known as short term exposure levels.
The Short term exposure limit is the maximum concentration of a hazardous substance to which a worker may be exposed for 15 minutes without suffering irritation, chronic or irreversible damage to tissue. No more than 4 excursions above STEL are permitted per day with at least 60 minutes between exposure periods.
