For the first time, the report includes trends analysis for the 25 most-polluted cities. The key trends message for all regions is: When states make a commitment to clean the air, it works—but progress stalls if controls are not tightened.
Ten-year ozone trends indicate that although ozone levels remain lower than in the 1990s, they appear to be leveling off. Particle pollution trend analysis showed a clear improvement from 2000 to 2006, although most of that decrease occurred between 2000 and 2002.
State of the Air: 2008 
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