Nearly one half of all Americans—an estimated 150 million—live in areas that don’t meetfederal air quality standards. Passenger vehicles and heavy-duty trucks are the main sources of this pollution, which includes ozone, particulate matter, and other smog-forming emissions.The health risks of air pollution are extremely serious. Poor air quality increases respiratory ailments like asthma and bronchitis, heightens the risk of life-threatening conditions like cancer, and burdens our health care system with substantial medical costs. Particulate matter is singlehandedly responsible for up to 30,000 premature deaths each year.Passenger vehicles are a major pollution contributor, producing significant amounts of nitrogen oxides, carbon monoxide, and other pollution. In 2013, transportation contributed more than half of the carbon monoxide and nitrogen oxides, and almost a quarter of the hydrocarbons emitted into our air.