PELs Are Based Partly on Scientific Evidence and Partly on Politics
The OSHA permissible exposure limits (PELs) (29 CFR 1910.1000) are typically the least restrictive exposure values and serve as a minimum performance standard in the United States. It should be noted that when PELs are established, it is a political process mixed with scientific evidence. Economic factors related to compliance are presented by industry groups and influence the final selection of exposure limits as a compromise value. OSHA limits may also lag behind new scientific literature as the entire political rulemaking process must be followed to make changes to a PEL. PELs are typically time-weighted average concentrations that must not be exceeded during any 8-hour workshift. Short term exposure limits (STEL) have been established for some materials and are usually measured over a 15-minute period. OSHA ceiling concentrations must not be exceeded during any part of the workday.
The American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVs) are also expressed as time-weighted averages and short term limits. These are determined by ACGIH committees of experts in public health and related sciences through review of existing scientific literature. TLVs are based only on health factors and not subjected to a political process. The TLVs can more rapidly adapt to new scientific information than OSHA PELs. Most health and safety professionals we have known rely on the more conservative ACGIH TLVs as minimum protective standards for their clients.
From: Chemical Safety: Part 2 by The Safety Guys