Cleaner air, bigger lungs.

In the latter half of the 20th century, Los Angeles had, by many measures, higher levels of photochemical air pollutants than any other major city in the United States (Fig. 1). To address this problem, the California Air Resources Board and its partners became leaders in quantifying the health effects of air pollutants and in aggressively implementing pollution-control strategies. Even with these actions, air-pollution levels remained high. In 1993, “Health Advisories” were issued on 92 days.1 In that year, the prospective Children’s Health Study was launched to examine the effects of air pollution on lung growth in children. Fourth-grade children were recruited from 12 communities in southern California with varying exposures to the pollutants of concern (ozone, nitrogen dioxide, and particulate matter). Repeated lung-function measurements were taken for these children for 8 years, the period of life during which the greatest growth of lung function occurs. In this first cohort, children living in more polluted communities had lower cumulative lung growth during the follow-up period.2 These results were important clinically because even modest reductions in attained lung function at maturity are predictive of respiratory disease, coronary heart disease, and reduced life expectancy.3 Of course, such an association does not prove causality. However, the case for a causal relationship can be strengthened by consistent evidence from repeated studies. To that end, Gauderman and his colleagues enrolled two additional cohorts of children from the Children’s Health Study and found consistent associations between community air pollution and lung-function growth in the children recruited in 1993,2 1997,4 and 2003.5 The consistency of findings in the three separate cohorts is compelling. Moreover, the investigators sought to minimize the potential for confounding by controlling for known individual and community predictors of lung-function growth. Nevertheless, unmeasured or imperfectly measured characteristics of these communities, such as differences in ethnic background or socioeconomic status, may have confounded these analyses and produced a false positive association. Although lung-function growth and potential confounders were measured for each child, airpollution exposures were based on community means. Such studies have been described as “semi-individual”6 with respect to the exposure variables. Thus, these analyses could also have been influenced by differences in community characteristics not captured in the individual data. A community is more than the aggregate of individual characteristics.7 In this issue of the Journal, Gauderman et al.8 examine the association between improvements in air quality and changes in lung-function growth from 11 to 15 years of age across these three cohorts of children. They show that 4-year growth in forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC) improved as levels of air pollution (nitrogen dioxide and particulate matter with an aerodynamic diameter of <2.5 μm [PM2.5] and <10 μm [PM10]) declined in five of these communities. This study provides corroborating information because the analyses are based on comparisons within communities and thus are not con-