1. Concentration of a substance in air, soil, water or other defined medium at which specified emergency counter-measures, such as the seizure and destruction of contaminated materials, evacuation of the local population or closing down the sources of [pollution], are to be taken.
2. Concentration of a [pollutant] in air, soil, water or other defined medium at which some kind of preventive action (not necessarily of an emergency nature) is to be taken. (IUPAC)