Hazardous Exposure Limits and Terms

TLV ACGIH Threshold limit value—The airborne concentration of a substance to which most workers can be exposed on a daily basis without adverse health effects. 

TWA ACGIH Time-weighted average—The average concentration of a substance for a normal 8 h workday and 40 h workweek. 

STEL ACGIH OSHA Short-term exposure limit—A 15 min TWA that should not be exceeded at any time during the workday. There should be 60 min between each 15 min exposure, up to four times a day. 

CEILING ACGIH Ceiling limit—The airborne concentration that is not to be exceeded at any time during 

PEL OSHA Permissible exposure limit—Limit based on 8 h TWAs. Exposures below the PEL do not require respiratory protection. 

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