maximum allowable concentration

in atmospheric chemistry
https://doi.org/10.1351/goldbook.M03762
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.
Source:
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2200 [Terms] [Paper]