Fossil fuel combustion for transportation contributes to air pollution, and air pollution degrades human health. However, the path from transportation to human health costs is anything but straightforward. Which pollutants are produced in which proportions depend on a number of factors, including the vehicle and fuel used and the driving conditions of a particular trip. These emissions are dispersed into the ambient air according to atmospheric conditions, which also influence the extent to which they react to form secondary pollutants. The degree to which people may be exposed to these primary or secondary pollutants depends on.
