The 1-hour National Ambient Air Quality Standard (NAAQS) for ground-level ozone was set in 1979. The map above shows the original areas in New England that were designated "nonattainment" for the one-hour ozone standard of 0.12 parts per million (ppm) in 1991. Nonattainment areas for the one-hour ozone standard were classified as marginal, moderate, serious, severe, or extreme depending upon the severity of the air quality problem at the time of the Clean Air Act of 1990 passage. Nonattainment areas in existence before 1990 that had attainment data or no data were not classified.

Under EPA's new ozone implementation rule, the 1-hour ozone standard was revoked in June 2005

In New England, the states operate a network of approximately 60 ozone monitoring stations during the ozone season (i.e., April 1 through September 30). View the locations of the ozone monitors.