University of North Texas 
OSHA currently has Permissible Exposure Limits (PELs) for nearly 500 hazardous chemicals as specified in 29 CFR Part 1910, subpart Z. The PEL is the eight-hour time weighted average concentration of contaminant in air to which a healthy person can be repeatedly exposed without reasonable expectation of adverse health effects.
Another measure of exposure limits is Threshold Limit Values (TLV), which is a reserved term of the American Conference of Governmental Industrial Hygienists (ACGIH). Similar to PELs, TLVs are the average concentration of a chemical that a worker can be exposed to over an 8-hour workday, 5 days per week, over a lifetime without observing ill effects. The difference between PELs and TLVs is that TLVs are advisory guidelines only and are not legally enforceable. Both PELs and TLVs can be found in SDSs.