| The Impact of Secondary Particles on the Association between Ambient Ozone and Mortality Meredith Franklin and Joel Schwartz Department of Environmental Health, Harvard School of Public Health, Boston, Massachusetts, USA Abstract
Background:Although several previous studies have found a positive association between ambient ozone and mortality, the observed effect may be confounded by other secondary pollutants that are produced concurrently with ozone. Objectives:We addressed the question of whether the ozone–mortality relationship is entirely a reflection of the adverse effect of ozone, or whether it is, at least in part, an effect of other secondary pollutants. Methods: Separate time-series models were fit to 3–6 years of data between 2000 and 2005 from 18 U.S. communities. The association between nonaccidental mortality was examined with ozone alone and with ozone after adjustment for fine particle mass, sulfate, organic carbon, or nitrate concentrations. The effect estimates from each of these models were pooled using a random-effects meta-analysis to obtain an across-community average. Results: We found a 0.89% [95% confidence interval (CI) , 0.45–1.33%] increase in nonaccidental mortality with a 10-ppb increase in same-day 24-hr summertime ozone across the 18 communities. After adjustment for PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 µm) mass or nitrate, this estimate decreased slightly ; but when adjusted for particle sulfate, the estimate was substantially reduced to 0.58% (95% CI, –0.33 to 1.49%) . Conclusions: Our results demonstrate that the association between ozone and mortality is confounded by particle sulfate, suggesting that some secondary particle pollutants could be responsible for part of the observed ozone effect. Key words: confounding, meta-analysis, mortality, nitrate, organic carbon, ozone, PM2.5, secondary pollutants, sulfates, time series. Environ Health Perspect 116:453–458 (2008) . doi:10.1289/ehp.10777 available via http://dx.doi.org/ [Online 10 January 2008]
Address correspondence to M. Franklin, Department of Environmental Health, Harvard School of Public Health, Landmark Center West, Room 412-E, 401 Park Dr., Boston, MA 02215 USA. Telephone: (617) 384-8837. Fax: (617) 384-8859. E-mail: mfrankli@hsph.harvard.edu Funding was provided by grants from the National Institute of Environmental Health Sciences (ES00002) and the U.S. Environmental Protection Agency (R832416-010) . The authors declare they have no competing financial interests. Received 17 August 2007 ; accepted 10 January 2008.
Correction In Table 4, the righthand column ("Decrease in effect") , which appeared in the manuscript originally published online, has been removed.
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