alert levels

in atmospheric chemistry
https://doi.org/10.1351/goldbook.A00215
Designated concentrations of certain key pollutants at which some degree of danger to public health is expected. In many areas in which a relatively high level of pollution is often encountered, several levels of alert are often established. For example, a first alert may signify a potential problem exists; a second alert becomes a signal for the curtailment of certain significant emission sources; the third alert signifies the need for some specified emergency action which must be taken to ensure the public safety.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2173 (https://doi.org/10.1351/pac199062112167)