by the Environmental Protection Agency

About the author: The Environmental Protection Agency (EPA) is a department of the federal government that establishes and enforces the nation’s environmental laws.

EPA has set national air quality standards for six principal air pollutants (also referred to as criteria pollutants): carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate matter (PM), and sulfur dioxide (SO2). Four of these pollutants (CO, Pb, NO2, and SO2) result solely from direct emissions from a variety of sources. PM can result from direct emissions also, but is commonly formed when emissions of nitrogen oxides (NOx), SO2, ammonia, and other gases react in the atmosphere. Ozone is not directly emitted, but is formed when NOx and volatile organic compounds (VOCs) react in the presence of sunlight.

Each year EPA examines changes in levels...