While dangerous pollutants still threaten the health of millions of Americans, the United States has made great strides in clearing the air, according to the American Lung Association.

In its annual State of the Air 2012 report, the organization said that between 2001 and 2010, ozone levels dropped 13%, year-round particle pollution declined 24% and short-term particle pollution 28% thanks to the Clean Air Act.

Particle pollution includes things like dust, metals, smoke, exhaust and acids, like nitrates and sulfates. Ozone, meanwhile, is created when a chemical or fossil fuel, like coal or gasoline, is partially burned and the unburned hydrocarbons, when combined with ultraviolet light, form a gas.