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)
The term "action level" was found in the following pages: