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 three major transportation-related criteria pollutants:
Other criteria pollutants include sulfur dioxide (SO2), nitrogen dioxide (NO2), and lead (Pb). In the past, motor vehicles were a major source of lead emissions that were virtually eliminated as 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. The EPA has identified 21 pollutants as mobile source air toxics, including diesel particulate matter, benzene, and other organic materials and metals.
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. CO2 is not directly harmful to human health and is not regulated under the CAA.
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 rapidly. 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 1997, EPA issued revised standards for ozone and particulate matter that reflect improved understanding of the health effects of these pollutants. Based on monitoring data, approximately 146 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 Web sites listed in this publication.