The U.S. Environmental Protection Agency (EPA) has identified these cars as those with the best gas mileage and the lowest emissions in their categories. For a wider selection of vehicles, see www.epa.gov/emissweb/howto.htm.
The Air Pollution Score is figured according to the amount of pollution that causes smog and health problems. The levels of pollution for each vehicle are determined by the EPA or the state of California’s exhaust-emission standards for the major pollutants from vehicle exhaust. These include nitrogen oxides (NOx) and non-methane organic gases (NMOG), which, combined with sunlight, create smog. Also included are other pollutants that affect health, like particulate matter (PM), a lung irritant, and carbon monoxide (CO), a poisonous gas.
The Greenhouse Gas Score ranks emissions of carbon dioxide (CO2), one of the biggest byproducts of vehicle engines. The lower the fuel-economy ranking of a vehicle, the more carbon dioxide is released. The amount of carbon dioxide produced per gallon of fuel burned varies by fuel type, since each type contains a different amount of carbon per gallon. The average Greenhouse Gas Score for 2006 vehicles is 5.
Both scores are calculated on a scale of zero to 10, with 10 as the best rating.