Mobile
sources account for more than half of all the air pollution in the United States. The primary
mobile source of air pollution is the automobile. EPA studies show that today's cars
emit 75 to 90 percent less pollution (for each mile driven) than their 1970
counterparts, thanks largely to advancements in vehicle and fuel technology. But today's
motor vehicles are still responsible for up to half of all the emissions released into
the air. The specific pollutant categories include 45 percent of the volatile
organic compound (VOC) emissions, 50 percent of the nitrogen oxides (NOX)
emissions, approximately 60 percent of the carbon monoxide (CO) emissions
and 50 percent of the hazardous air pollutants in urban areas.