Thanks to the Clean Air Act, we’ve made great progress in cleaning up air pollution from across the U.S. The State of the Air 2012 shows that the air quality in many places has improved, but that over 127 million people—41 percent of the nation—still suffer pollution levels that are too often dangerous to breathe. Unhealthy air remains a threat to the lives and health of millions of people in the United States, despite great progress. Air pollution lingers as a widespread and dangerous reality even as some seek to weaken the Clean Air Act, the public health law that has driven the cuts in pollution since 1970.

The State of the Air 2012 report looks at levels of ozone and particle pollution found in official monitoring sites across the United States in 2008, 2009, and 2010. The report uses the most current quality-assured nationwide data available for these analyses.

For particle pollution, the report examines fine particulate matter (PM_2.5) in two different ways: averaged year-round (annual average) and over short-term levels (24-hour). For both ozone and short-term particle pollution, the analysis uses a weighted average number of days that allows recognition of places with higher levels of pollution. For the year-round particle pollution rankings, the report uses averages calculated and reported by the U.S. Environmental Protection Agency. For comparison, the State of the Air 2011 report covered data from 2007, 2008 and 2009.