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Permissible Exposure Limit (PEL)|
Time-Weighted Average (TWA)
TWA levels are usually lower than ceiling values. Thus, a worker may be exposed to a level higher than the TWA for part of the day (but still lower than the ceiling value) as long as he is exposed to levels below the TWA for the rest of the day. See 1910.1000 for the formulas used in the calculations.
In 1989, OSHA decided that it was impractical to set PEL's for tens of thousands individual chemicals and enacted a generic rulemaking scheme that broke chemicals up into various categories. This methodology was challenged in court and in AFL-CIO v. OSHA, 965 F.2d 962 the 11th U.S. District Court of Appeals vacated (struck down) the 1989 PEL revisions stating "(1) OSHA failed to establish that existing exposure limits in the workplace presented significant risk of material health impairment or that new standards eliminated or substantially lessened the risk; (2) OSHA did not meet its burden of establishing that its 428 new permissible exposure limits (PELs) were either economically or technologically feasible."
Toxicological professionals generally disagree with both of those points. With respect to the first, there was substantial documentation to support OSHA's position, and with the respect to the second, performing an economic analysis for every chemical (there are well over 100,000 commodity chemicals in use in the US) for every industry is completely infeasible. In 58:35338-35351 OSHA noted that "...OSHA continues to believe that many of the old limits which it will now be enforcing are out of date (they predate 1968) and not sufficiently protective of employee health based on current scientific information and expert recommendations. In addition, many of the substances for which OSHA has no PELs present serious health hazards to employees."
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In theory, OSHA is continuing to perform risk assessment and feasibility analysis on air contaminants in light of this court ruling, using a risk-based prioritization to identify those substances which have the most significant impact on worker safety. But in the years since this ruling, OSHA has enacted or updated PEL's for just a handful of substances.
Just to confuse matters, this decision did not directly apply to the states and territories which administer their own OSHA-approved plans. Some of these states may retain the 1989 limits. Therefore, to maximize safety and minimize potential legal problems, it is best to compare the 1989 PELs for these substances with the ACGIH limits (TLV's), and comply with which ever is most protective.
Therefore, vacated PEL values may still appear on MSDS's because some states may have enacted statutes based on their values and because some employers or employees might prefer their larger margin of safety (or error, depending on your perspective) when setting their workplace exposure limits. Whatever values you find, always err on the side of safety and choose the lower one. Remember we always want to keep chemical exposures as low as reasonably achievable (ALARA).
In general, PEL's refer to substances that may be inhaled, although some can be absorbed through the skin or eyes. When working with materials that have an established PEL or TWA use administrative and engineering controls to minimize the generation of a vapor or dust in the first place. If those controls are not sufficient, use appropriate personal protective equipment (PPE) such as gloves, dust masks, and respirators to limit your exposure to the chemicals.
Remember, exposure limits are not some magic threshold that define the border between safe and dangerous. A PEL that was acceptable in 1950 may be recognized as dangerously high today. Therefore, always do everything reasonable to limit the airborne release of chemicals or dusts in the first place.
See also: action level, engineering controls, inhalation, Threshold Limit Values (TLV).
Additional definitions from Google and OneLook.
Entry last updated: Wednesday, August 10, 2016. This page is copyright 2000-2016 by ILPI. Unauthorized duplication or posting on other web sites is expressly prohibited. Send suggestions, comments, and new entry desires (include the URL if applicable) to us by email.
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