Generally nearly all substances have been assigned Threshold Limit Values (TLVs). The use of the term Threshold Limit Value refers to the maximum exposure to a toxic substance that is allowed by an appropriate regulatory body.
Three types of TLVs for chemical substances are defined:
Threshold Limit Value – Time Weighted Average (TLV-TWA) is the average exposure on the basis of a 8h/day, 40h/week work schedule.
Threshold Limit Value – Short-Term Exposure Limit (TLV-STEL) is the spot exposure for duration of 15 minutes that cannot be repeated more than 4 times per day.
Threshold Limit Value – Ceiling Limit (TLV-C) is the absolute exposure limit that should not be exceeded at any time.
Ref: Safeopedia website
