35 Ill. Adm. Code 204.910
Ambient Air Ceilings
Section 204.910 Ambient Air Ceilings
No concentration of a pollutant shall exceed:
a) The
concentration permitted under the national secondary ambient air quality
standard; or
b) The
concentration permitted under the national primary ambient air quality
standard, whichever concentration is lowest for the pollutant for a period of
exposure.