For example, studies of the effects on animals of long-term exposure to ozone at concentrations similar to those seen in some US cities showed chronic lung damage that increased monotonically with cumulative dose; there was no evidence of a threshold (American Thoracic Society, 1996a). If similar effects also occur in humans, a substantial proportion of the population in the United States is exposed every summer to ozone concentrations that may produce chronic lung damage. Analyses of data in 8 cities have detected associations of airborne particles with small increases in the risk of mortality (Schwartz, 1991). In a community study, the closing of a steel mill for a year was associated with more than a 40% decrease in hospital admissions for asthma in children; the next year the mill reopened, and hospitalization rates rose to their previous level (Pope 1989, 1991). Another recent study found that rates of acute bronchitis in children were about twice as high in a town with particle concentrations at the US air-quality standard as they were in a community with near background concentrations; intermediate communities had intermediate rates (Dockery et al., 1989). Braun-Fahrlander et al. (1992) found major changes in respiratory symptoms among schoolchildren at concentrations commonly seen in the US urban population.

Acute Health Effects

Acute health effects of air pollution have long been established, although primarily by studies at concentrations of pollutants far higher than those now typical in developed countries. Recently, however, exposure to air pollution has been associated with acute outcomes—such as reversible reductions in lung function, increased respiratory symptoms and illness, emergency-room visits and increased hospitalization, and deaths from respiratory and cardiovascular diseases—at exposure levels far lower than those at which earlier data were collected. These are discussed briefly below in order of increasing severity. Several recent reports provide more-complete treatments of the evidence on acute health effects (Dockery and Pope, 1994; American Thoracic Society, 1996b).

Exercising volunteers exposed to ozone at concentrations below current air-quality standards show reversible decrements in lung function (McDonnell et al., 1985). Studies of ozone-exposed children in summer camps (Spektor et al., 1988; Lioy et al., 1985) have found that they have similar responses at even lower levels, at least in the summer months, when they are outdoors during much of the day. Smaller but still significant effects of ozone on lung function have been reported in schoolchildren during the school year, when outdoor activity is less (Kinney et al., 1989). Studies of lavage fluid from the lungs of volunteers have also provided evidence of inflammatory processes in the lung following ozone

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