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Explanation: Если стандарт - бритиш инглиш, то тогда WEL TWA.
WEL = Workplace Exposure Limit, а TWA - то же самое Time Weighted Average concentration, которое для долгосрочной экспозиции (8 час) является эквивалентом среднесменного ПДК.
Вот вам ссылка:
The MEL (Maximum Exposure Limit) is the maximum permitted concentration of a chemical to which a worker may be exposed over an extended period of time. Typically, MELs are quoted in ppm for an 8-hour reference period, though shorter periods may be quoted for some materials. MELs are, in many countries, enforceable by law. ***In the UK, MELs have now been replaced by Workplace Exposure Limits (WELs) *** http://msds.chem.ox.ac.uk/glossary/mel.html