The Clean Air Act is given much of the credit. The act targets coal-fired power plants, diesel engines, and older SUVs, pick-up trucks, and vans.
Ozone levels and particle pollution are measured to test air quality. Both have gone down since 2000 even as economic activity is putting more vehicles on the road and factories are producing more goods.
Experts say air quality standards are still too low. More than 127 million Americans live in areas receiving an F grade for air quality. Los Angeles, Pittsburgh and Cincinnati remain among the most polluted cities in the nation.