Threshold Limit Value

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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.

The TLV is an estimate based on the known toxicity in humans or animals of a given chemical substance and the reliability & accuracy of the latest sampling and analytical methods. It is not a static definition since new research can often modify the risk assessment of substances and new laboratory or instrumental analysis methods can improve analytical detection limits. The TLV is a recommendation by ACGIH, with only a guideline status. As such, it should not be confused with exposure limits having a regulatory status, like those published and enforced by the Occupational Safety and Health Administration (OSHA). Many of the OSHA regulatory exposure limits (all PELs published in 29CFR 1910.1000) are based on the 1969 ACGIH TLVs when the OSH Act was adopted by Congress and so are dated/over 30 years old and are not considered by the industrial hygiene community as protective levels of exposure to chemicals. The National Institute of Occupational Safety and Health (NIOSH) publishes RELs (recommended exposure limits) which OSHA takes into consideration when promulgating new regulatory exposure limits.

[edit] Definitions

The TLV for chemical substances is defined as a concentration in air, typically for inhalation or skin exposure. Its units are in ppm for gases and in mg/m³ for particulates (aerosols such as dust, fumes and mist).

Three types of TLVs for chemical substances are defined:

Threshold Limit Value - Time Weighted Average (TLV-TWA): average exposure on the basis of a 8h/day, 40h/week work schedule
Threshold Limit Value - Short Term Exposure Limit (TLV-STEL): spot exposure for a duration shorter than 15 minutes, that cannot be repeated more than 4 times per day
Threshold Limit Value - Ceiling (TLV-C): absolute exposure limit that should not be exceeded at any time

There are TLVs for physical agents as well as chemical substances. TLVs for physical agents are also defined in terms of what are considered safe levels (8 hours per day & 40 hours per work week). There are TLVs for noise exposure, vibration exposure, ionizing & non-ionizing radiation exposure and heat & cold stress.

The basic formula for TLV is: ppm = ((mg/m^3) * 24.45) / molecular weight