Health effects of outdoor air pollution

From both the clinical and public health perspective, air pollution remains a relevant topic for health professionals. Although dramatic air pollution episodes associated with readily evident excess morality now are unlikely in much of the world, ozone (O sub(3)), acid aerosols, and total particles may be responsible for adverse health effects at current levels of exposure. Many respiratory hazards have been recognized and controlled in the United States and other developed countries. Concern remains about the safety provided by existing standards for environmental exposures and the risks of new and unevaluated agents. Large segments of the population are exposed to ambient (outdoor) pollutants with adverse effects. Clinicians, toxicologists, and epidemiologists are asked to assess the effects of inhaled pollutants: the clinician to evaluate the health of exposed individuals, the toxicologist to define the injury resulting from the pollutant, and the epidemiologist to address the effects on exposed groups. Ambient air pollution resulting from the combustion of fossil fuels includes emissions from motor vehicle engines ("mobile" sources), from power plants ("stationary" sources), and from such "area" sources as fireplaces and barbecues. Tail-pipe emissions are a major contributor of carbon monoxide (CO), oxides of nitrogen (NO sub(x)), hydrocarbons, diesel particles, and other airborne particles. Before removal of tetraethyl lead from gasoline in the United States, lead oxide particles were another tail-pipe emission of public health concern. O sub(3) is a secondary pollutant produced in the atmosphere by photochemically driven reactions involving NO sub(x) and hydrocarbons. O sub(3) levels are typically higher outdoors than indoors. NO sub(x) also participate in the generation of atmospheric nitric acid, nitrate salts, and organic nitrates. Nitrogen dioxide (NO sub(2)) may reach much higher concentrations indoors than outdoors in the presence of unvented combustion sources. Power plant emissions are largely responsible for atmospheric sulfur dioxide (SO sub(2)) and for a complex array of respirable particulate matter. The latter includes nitrates, acid sulfates and other sulfate salts generated secondarily in the atmosphere, metal salts, and carbonaceous particles. Waste incinerators, smelters, and other industrial activities may be responsible for emissions of a variety of acidic, organic, and metallic compounds that may have significance for health effects. Additional hazardous pollutants may also be associated with point sources and are newly regulated under the "air toxics" provisions of the 1990 Clean Air Act amendments. These emissions have not been specifically addressed in this review. The health effects ascribed to exposure to air pollution are diverse. The evidence linking these effects to air pollution comes from a variety of disciplines, including epidemiology, human clinical exposure studies, field exposure studies, and animal toxicology. For some observed effects, the mechanisms and specific pollutants responsible remain unclear. The Clean Air Act does not provide a specific definition for an "adverse" health effect. In 1985, a committee of the American Thoracic Society offered guidelines concerning the definition of adverse health effects of pollution. Any definition of "adverse" reflects not only clinical and biologic significance but societal values as well. The health effects attributed to specific pollutants, as described in this review, are summarized. The focus on specific pollutants occurs because the regulatory structure and control technologies are pollutant-specific. It is important to recognize that mixtures of these individual pollutants commonly occur.*