In
most US cities, warnings are issued when monitors for ozone
reach or exceed 125 ppb, averaged over 1 hour. The EPA color
codes ozone pollution levels. An orange warning is issued if 1-
hour ozone levels are between 125 and 164 ppb, a red warning
between 165 and 204 ppb, and a purple warning between 205
and 404 ppb. Efforts by the EPA to control airborne volatile
organic carbon and particle pollution have reduced ground level
ozone in the US for the past 20 years,