Threshold Limit Value

The Threshold Limit Value (TLV) of a chemical substance defines the reasonable level to which a worker can be exposed without adverse health effects. Strictly speaking, TLV is a reserved term of the American Conference of Governmental Industrial Hygienists (ACGIH). It is however sometimes loosely used to refer to other similar concepts used in occupational health and toxicology. TLVs (along with biological exposure indices or BEIs) are published annually by the ACGIH.
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the maximum permissible concentration of a substance typically expressed in ppm that a worker may be exposed to for 8 hours (sometimes 40 hours). TLV is a legally mandated safety limit.

see Threshold Limit Value

The concentration of an airborne substance to which an average person can be repeatedly exposed without adverse effects. TLVs may be expressed in three ways: (1) TLV-TWA--Time weighted average, based on an allowable exposure averaged over a normal 8-hour workday or 40-hour work- week; (2) TLV-STEL--Short-term exposure limit or maximum concentration for a brief specified period of time, depending on a specific chemical (TWA must still be met); and (3) TLV-C--Ceiling Exposure Limit or maximum exposure concentration not to be exceeded under any circumstances. (TWA must still be met.)