While air quality has improved in the last three decades, half of all Americans live in counties where air pollution exceeds national health standards. Most of these places suffer from high levels of ozone and/or particle pollution. Ozone is the country's most pervasive air pollutant; particle pollution is the nation's deadliest air pollutant. Coal-fired power plants and motor vehicles are the largest sources of these pollutants. "Danger in the Air: Unhealthy Levels of Air Pollution in 2003," which is based on a comprehensive survey of environmental agencies from all 50 states and the District of Columbia, examines levels of ozone and fine particle pollution in cities and towns across the country in 2003 and finds that air pollution continues to pose a grave health threat to Americans.