In response to the Clean Air Act (CAA), the U.S. Environmental Protection Agency (EPA) established National Ambient Air Quality Standards (NAAQS) for various pollutants–known as criteria pollutants–that adversely affect human health and welfare. This brochure focuses on the four major transportation-related criteria pollutants:
Other criteria pollutants include sulfur dioxide (SO2) and lead. In the past, motor vehicles were a major source of lead emissions, however these were virtually eliminated when leaded gasoline was phased out.
Although not criteria air pollutants, toxic air pollutants, or air toxics, are pollutants that cause or may cause cancer or other serious health effects, such as reproductive effects or birth defects. Air toxics may also cause adverse environmental and ecological effects. The CAA identifies 188 air toxics. EPA has identified 21 pollutants as mobile source air toxics including diesel particulate matter, benzene, metals, and organic materials.
Fuel combustion by motor vehicles and other sources releases carbon dioxide (CO2), which is a greenhouse gas that traps heat within the earth's atmosphere.
Significant progress has been made in reducing criteria pollutant emissions from motor vehicles and improving air quality since the 1970s, even as vehicle travel has increased. The air is noticeably cleaner than in 1970, and total criteria-pollutant emissions from motor vehicles are less than they were in 1970 despite a near tripling of vehicle miles of travel. With the reduction in criteria pollutants, many air toxics have also been reduced.
Still, challenges remain. In recent years, EPA has issued revised standards for O3 and PM that reflect improved understanding of the health effects of these pollutants.
Based on monitoring data, more than 150 million people in the United States reside in counties that do not meet the air-quality standards for at least one NAAQS pollutant. For more detailed data on many of the subjects covered, refer to the sources and websites listed in this publication.