Research indicates that large portions of the population may be at-risk due to exposure at high pollution concentrations, and under certain circumstances, health impacts can be significant. Due to concern about the health effects of air pollutants on people and the proportion of pollutants that come from cars, trucks, and buses, the provisions of the CAA relating to transportation projects, plans, and programs were made substantially more stringent than in earlier Clean Air Acts. The principal transportation related pollutants addressed in the CAA are listed below. VOCs come from vehicle exhaust, paint thinners, solvents and other petroleum-based products. VOCs and nitrogen oxides react in the presence of sunlight to form ozone.
Ozone irritates the eyes, impairs the lungs, and aggravates respiratory problems.
Ozone can cause chest pain, coughing, nausea, pulmonary congestion, and possible long term lung damage. A number of exhaust VOCs are also toxic, with the potential to cause cancer. Under the high pressure and temperature conditions in an engine, nitrogen and oxygen atoms in the air react to form various nitrogen oxides, collectively known as NOx. NOx, like hydrocarbons, is a precursor to the formation of ozone and also contributes to the formation of acid rain. NOx impacts the respiratory system, causing a high incidence of acute respiratory diseases.