Threshold Limit Value

Threshold Limit Value

The Threshold Limit Value (TLV) of a chemical substance is a level to which it is believed a worker can be exposed day after day for a working lifetime 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 1968 ACGIH TLVs, adopted into Federal regulation in that year under the Walsh-Healy Public Contract Act, and so are over 40 years old. Many are not considered by the industrial hygiene community to be sufficiently protective levels. 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.


The TLV for chemical substances is defined as a concentration in air, typically for inhalation or skin exposure. Its units are in parts per million parts of air (ppm) for gases and in milligrams per cubic meter (mg/m³) for particulates such as dust, smoke and mist.

The basic formula for converting between ppm and mg/m³ for gasses is ppm = (mg/m^3) * 24.45 / molecular weight. This formula is not applicable to airborn particles.

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 of 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 include those for noise exposure, vibration, ionizing & non-ionizing radiation exposure and heat & cold stress.

Similar concepts

The TLV is equivalent in spirit (although the materials covered, values recommended, and definitions used can differ) to the following concepts.

*WEEL ("Workplace Environmental Exposure Levels) created by a committee of the American Industrial Hygiene Association [ AIHA] Similar concepts used in other countries include:
**VME ("Valeur Moyenne d'Exposition")
**VLE ("Valeur Limite d'Exposition")
**AGW ("Arbeitsplatzgrenzwert")
**MAK ("Maximale Arbeitsplatz-Konzentration")
**MAC ("Maximaal Aanvaarde Concentratie")
**PEL ("Permissible Exposure Limits")
**NDN ("Najwyższe Dopuszczalne Natężenie")
**ПДК ("предельно допустимая концентрация")

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