Air quality is one of the great Michigan success stories. Michigan cities monitored by the EPA are below the health-based thresholds set by the Clean Air Act for all six "criteria" pollutants (lead, carbon monoxide, ozone, particulates, sulfur dioxide, and nitrogen dioxide, described beginning next page) and are experiencing downward trends. Most Michigan cities not only meet the national standard, but are below the national average. The one exception to this is Detroit, which is slightly above the national average for particulates and sulfur dioxide. On the other hand, the Foundation for Clean Air Progress in Washington, D.C. lists Detroit as one of the nation's 10 best cities in terms of ozone reductions over the last decade, which is especially notable because ozone is the most stubborn pollutant to control.13

Charts 2-7 display trend data for ambient air pollution levels in Michigan cities, since it is ambient levels, rather than emissions, that are the important factor in determining actual exposure and health risk to humans. These charts include measurements for each city the EPA monitors; the EPA does not monitor all six criteria pollutants in each location.