on, motor vehicles account for at least half of the impurities added to the air. Such pollution generally grows in direct proportion to the size of a given city. Typically, the rate of contaminant production begins to overwhelm the atmosphere's natural dissipating forces more and more frequently (Dansereau 131-139). Relatively healthy people really only pay attention to air pollution either during or immediately after a severe pollution episode. Thus, atmospheric contamination is a low-visibility public health problem. Nonetheless, various scientific investigations indicate that such pollution can have a significant low-level, long-range effect on urban populations (Schachter 87-91). Researchers have estimated, for example, that Americans spend more than ten billion dollars a year on medical problems which are caused by outdoor pollutants (Grove, Spiegel, & Bond 503-536). Some of these costs can be attributed to past air pollution problems. Prior to 1975, for example, automobiles did not have catalytic converters and lead was a significant component of air pollution. When this soft metal is inhaled or ingested, it accumulates in bones and other tissues. Such buildups can result in irreversible brain and kidney damage. These pathologic effects are generally most severe in young children. The developing nervous system is vulnerable to toxic exposures. Pathologic effects may include decreased intelligence, shortened attention span, learning disabilities, and also possibly hyperactivity. In addition, elevated blood lead levels can have adverse effects in adults. Adult exposures have been correlated with high blood pressure. During the late 1980s, one Federal Environmental Protection Agency (EPA) researcher estimated that between "60,000 and 70,000 heart attacks per decade could be prevented if the mean blood lead level of all Americans was reduced by a third (Grove, Spiegel, & Bond 503-536)."
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