Permissible Exposure Limits

OSHA sets enforceable permissible exposure limits (PELs) to protect workers against the health effects of exposure to hazardous substances. PELs are regulatory limits on the amount or concentration of a substance in the air. They may also contain a skin designation. OSHA PELs are based on an 8-hour time weighted average (TWA) exposure. Permissible exposure limits (PELs) are addressed in specific standards for the general industry, shipyard employment, and the construction industry.PEL values are not as numerous and are not updated frequently. The lowest value on the response versus dose curve is called the threshold dose. Below this dose the body is able to detoxify and eliminate the agent without any detectable effects. In reality the response is only identically zero when the dose is zero, but for small doses the response is not detectable. The American Conference of Govt. Industrical Hygienists (ACGIH) has established threshold doses, called threshold limit values (TLVs), for a large number of chemical agents. The TLV refers to airborne concentrations that correspond to conditions under which no adverse effects are normally expected during a worker’s lifetime. The exposure occurs only during normal working hours, eight hours per day and five days per week. The TLV was formerly called the maximum allowable concentration (MAC). There are three different types of TLVs (TLV-TWA, TTLV-STEL and TLV-C). Reference: 1. D.Crowl and J. Louvar, Chemical Process Safety: Fundamentals with Applications, Prentice Hall (2002) Chapter 2; 2. MSDS sheet from online; 3.

Management/Process Safety Information and Knowledge/Hazards Management