Outdoor air pollution has long been suspected of increasing the risk of lung cancer. 1 Although many pollutants have been linked with adverse health impacts, the component with the greatest public health impact is probably PM, a complex mixture of airborne solid and liquid particles including soot, organic material, sulfates, nitrates, other salts, metals and biologic material. Many carcinogens, including a multitude of PAHs, adsorb to particles and can be deposited throughout the respiratory tract. In most Western countries, motor vehicle exhaust represents the most widespread source of air pollution, including PM. 2 For research purposes, PM is usually subdivided into PM10 (inhalable particles), PM2.5 (fine particles) and PM0.1 (ultrafine particles). These PM size cuts generally represent different sources and display different physical and chemical properties. The physicochemical characteristics responsible for PM-associated toxicity are incompletely understood. For example, the relative contributions of particle size, number, mass, surface area and chemical composition have not been determined. For a given mass concentration (in g/m 3 ), the particle size cut will determine both particle number and surface area (e.g., for a mass concentration of 10 g/m, 3 a particle diameter of 2 corresponds to 1.2 particles/ml air with a surface area of 24 , 2 while a particle diameter of 0.02 corresponds to 2.4 million particles/ml with a surface area of 3,000 2 ). 3 A larger surface area potentially means greater exposure to carcinogenic agents, such as PAHs adsorbed to the particles. Long-term average concentrations of PM pollution are quite variable in Western countries, from Ο½10 g/m 3 of PM10 (e.g., in northern Sweden) to ΟΎ50 g/m 3 in southern Europe, with intermediate levels in the United States (for U.S. levels, see the website http://www.epa.gov/oar/aqtrnd97/brochure/pm10.html).
Gaseous constituents of outdoor air pollution include nitrogen compounds (e.g., NO 2 ), sulfur compounds (e.g., SO 2 ), CO, CO 2 , ozone and other chemicals. Unlike gaseous pollutants, such as ozone or SO 2 , the composition of PM varies considerably over space and time. In this minireview, we focus on the health impacts of particulate and gaseous emissions from diesel and gasoline exhaust. The contribution of other sources of outdoor air pollution, in particular industrial emissions, is beyond the scope of the present review.