The EPA Air Pollution Score is a measurement of tailpipe emissions for a vehicle. Pollutants include carbon-containing compounds, nitrogen oxides, particulate matter, carbon monoxide, and formaldehyde. Such emissions are key contributors to smog and haze, and also create health problems. The cleanest vehicles have an Air Pollution Score of 10, and do not emit these pollutants. For more information, see http://www.epa.gov/greenvehicles/Aboutratings.do#aboutairpollution