10 and 2.5 microns (µm) in aerodynamic diameter, respectively; and nitrogen dioxide (NO₂).¹ The air toxics emitted by motor vehicles include aldehydes (acetaldehyde, formaldehyde, and others), benzene, 1,3-butadiene, and a large number of substances known as polycyclic organic matter (including polycyclic aromatic hydrocarbons, or PAHs).

All of the pollutants emitted from motor vehicles also are produced by other sources such as industrial processes, electric power generation, and home heating. The contribution of motor vehicles to ambient levels varies, depending on the pollutant and the location. In most cases, motor vehicles are a major contributor (between 25 and 40 percent of the ambient levels), and for some pollutants—for example, carbon monoxide, ultrafine particles (PM_0.1), and 1,3-butadiene—motor vehicles tend to be the dominant source.

Although motor vehicles contribute a significant portion, if not the largest part, of most air pollutants, in certain circumstances they can contribute a substantially higher amount to personal exposure. In particular, in urban centers, along roadsides, and especially in urban street canyons in crowded central business districts, mobile sources can contribute 2 to 10 times as much as in general background situations.² For example, in England urban background levels of PM₁₀ have been measured at 22–25 micrograms per cubic meter (µg/m³), and street-side levels have been measured at 24–38 µg/m³ (Department of the Environment, Transport, and Regions, 1999). Such a finding can have important implications for the potential acute health effects arising from exposure to these pollutants and for the chronic health effects on those people who spend a significant portion of their lives in these environments, especially the elderly, low-income, and other urban populations that may be especially sensitive to the effects of air pollution.

Research conducted over the past several decades has identified some of the effects that different pollutants have on human health, including those on the respiratory, neurological, and cardiac systems, and those that promote several types of cancer. One of the challenges