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. PAC, 1990, 62, 2167 (Glossary of
atmospheric chemistry terms (Recommendations 1990)) on page 2173
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold
Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell
Scientific Publications, Oxford (1997). XML on-line corrected
version: http://goldbook.iupac.org (2006-) created by M. Nic, J.
Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN
0-9678550-9-8. doi:10.1351/goldbook