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Controlled exposure to hazardous substances as established by health and safety authorities. In California, PELs are set by the Occupational Safety and Health Standards Board, and enforced by the Division of Occupational Safety and Health (DOSH). There are three types of Cal/OSHA PELs:
1. The 8-Hour Time Weighted Average (TWA) is the average employee exposure over an 8-hour period, based on industrial hygiene monitoring. The measured level may sometimes go above the TWA value, as long as the 8-hour average stays below. All chemicals with PELs have a TWA value. Only a few chemicals have Ceiling and Excursion Limits.
2. The Ceiling Limit is the maximum allowable level. It must never be exceeded, even for an instant.
3. The Excursion Limit is a value that can be exceeded only for a specified short period of time (between 5-15 minutes), which is called the Excursion Duration. When there is an Excursion Limit for a substance, exposure must never exceed the Ceiling Limit, and the 8-hour average must remain at or below the TWA.
The National Institute for Occupational Safety and Health (NIOSH) has the statutory responsibility for recommending exposure levels that are protective to workers. NIOSH has identified Recommended Exposure Limits (RELs) for 667 hazardous substances (NIOSH Pocket Guide to Chemical Hazards. DHHS Publication No. 94-116). The American Conference of Governmental Industrial Hygienists (ACGIH) has developed hundreds of exposure limits that are more protective than OSHA’s (Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices – 1994-1995). Many U.S. companies use the current ACGIH levels or other internal and more protective limits.
For Permissible Exposure Limits see CFR 29 - 1910.1000, Subpart Z (Code of Federal Regulations 29 – Occupational Safety and Health Standards – Toxic and Hazardous Substances)
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