Exposure limits: PELs and TLVs
What are the differences between a permissible exposure limit and a threshold limit value?
Responding is Gregg Bako, product manager, Industrial Scientific Corp., Oakdale, PA.
Although both terms have been around for quite a while, they sound remarkably similar at first blush. So what is the difference? Let’s start off with the generally accepted definitions of permissible exposure limits and threshold limit values.
Permissible exposure limits are set by OSHA to protect workers against the adverse effects of exposure to chemical substances. In the world of gas detection, PELs limit the amount or concentration of a substance in the air and generally are based on an 8-hour time-weighted average exposure.
A threshold limit value, set by the American Conference of Governmental Industrial Hygienists, is the limit of exposure to a chemical substance that a worker can be exposed to, day after day, without adverse health effects. TLVs are estimates based on the known toxicity of a chemical substance in humans or animals given the currently available analytical and technological resources. To determine TLVs, ACGIH uses committees to review various published literature in disciplines such as industrial hygiene, toxicology and occupational medicine. TLVs are then developed as recommendations or guidelines and are intended to be interpreted and used by a person trained in the discipline of industrial hygiene.
Given the above definitions, the terms still sound remarkably similar. However, their differences are beginning to emerge. PELs are legal limits, meaning OSHA can enforce their use and any non-compliance in the United States. In contrast, TLVs are recommendations. Although ACGIH is a well-known and respected scientific organization that contributes to the PELs set by OSHA, its TLVs are not legal limits.
Although industrial hygienists must ensure they meet the PELs set by OSHA, they and their organization may choose to adopt TLVs for certain chemical substances – provided they are appropriate for the application – because TLVs generally are more stringent than PELs. One must keep in mind, though, that TLVs are based solely on health factors, not economic or technical feasibility. In my world of gas detection, I have heard of TLVs for certain gases where the required level of detection to accurately measure against the TLV is difficult, if not impossible, to achieve given the current gas detection technology available.
While OSHA and ACGIH are both doing commendable work in setting their respective limits for the greater good of protecting the workforce, differences in opinion regarding which limit one should adopt are likely to continue. I cannot definitively say which limit is best; again, both limits are set by their respective organization with the most noble of intentions. However, I can say that the PELs set by OSHA are legal limits, where the TLVs set by the ACGIH are not legal limits. As far as adopting the generally more stringent requirements of TLV, an industrial hygienist will know his or her application best and is the person most suited to determine the limits for the organization.
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