maximum permissible level (MPL)
Level, usually a combination of time and concentration, beyond which any exposure of humans to a chemical or physical agent in their immediate environment is unsafe.
See also:
maximum allowable concentration (in atmospheric chemistry)
PAC, 1993, 65, 2003. (Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)) on page 2067 [Terms] [Paper]