maximum allowable concentration

in atmospheric chemistry
The maximum concentration of a pollutant which is considered harmless to healthy adults during their working hours, assuming they breathe uncontaminated air at all other times.
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2200 [Terms] [Paper]
See also:
PAC, 1993, 65, 2003. (Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)) on page 2066 [Terms] [Paper]