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Permissible Exposure Limit (PEL) Time-Weighted Average (TWA)

Definition

• A Permissible Exposure Limit (PEL) is the maximum amount or concentration of a chemical that a worker may be exposed to under OSHA regulations.
• A Time-Weighted Average (TWA) is explained below.
• A VPEL or Vacated PEL is an older PEL set by OSHA but later "vacated" (retracted) under court order. However, some states may continue to enforce these lower levels. See below.

Additional Info

PEL's can be defined in two different ways as discussed in the OSHA regulation on air contaminants, 1910.1000: Ceiling values - at no time should this exposure limit be exceeded. Sometimes denoted with the letter C. 8-hour Time Weighted Averages (TWA) - are an average value of exposure over the course of an 8 hour work shift. 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.

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Vacated PEL's
Analyzing the specific health risks and performing the cost/benefit analyses needed to set a PEL is a time-consuming and labor-intensive endeavor. As a result, only ~470 substances have OSHA PEL's.

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."

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OSHA was therefore forced to revert to the older limits, which, in general, afford lesser protection (or "protection" as some might argue). This case clearly established that to set new PEL's OSHA would need to collect specific evidence for each substance so the risk/benefit analysis could be reviewed. More information on this legal case can be found under Further Reading below. 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. The absence of a PEL does not necessarily mean that the substance is harmless, it merely means that OSHA has not yet had a chance to assess that chemical! The vacated PEL values may still appear on MSDS's because some states may have enacted statutes based on these older 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.

MSDS Relevance

PEL's are defined by OSHA in 3 Tables: Table Z-1 Limits for Air Contaminants. Table Z-2 TWA's and Ceiling Concentrations. Table Z-3 Mineral dusts. 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.

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Further Reading

• Chemical Sampling Information at OSHA lists the PEL's and/or TWA's for many substances, health effects, and equipment/manufacturers that can monitor concentration for PEL/TWA compliance.
• OSHA's Compliance and Enforcement Activities affected by the PELs Decision.
• AFL-CIO v. OSHA: Admitting Scientific Opinion as Scientific Evidence dicusses the vacated PEL case from a risk management perspective.
• Congressional testimony of Richard Schwartz, an attorney on industry's side in AFL-CIO v. OSHA.

See also: action level, engineering controls, inhalation, Threshold Limit Values (TLV).

Disclaimer: The information contained herein is believed to be true and accurate, however ILPI makes no guarantees concerning the veracity of any statement. Use of any information on this page is at the reader's own risk. ILPI strongly encourages the reader to consult the appropriate local, state and federal agencies concerning the matters discussed herein.