Uncertainty associated with ambient ozone metrics in epidemiologic studies and risk assessments
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Epidemiologic studies relating ambient ozone concentrations to adverse health outcomes have typically relied on spatial averages of concentrations from nearby monitoring stations, referred to as “composite monitors.” This practice reflects the assumption that ambient ozone concentrations within an urban area are spatially homogenous. We tested the validity of this assumption by comparing ozone data measured at individual monitoring sites within selected US urban areas to their respective composite monitor time series. We first characterized the temporal correlation between the composite monitor and individual monitors in each area. Next, we analyzed the heteroskedasticity of each relationship. Finally, we compared the distribution of concentrations measured at individual monitors to the composite monitor distribution. Individual monitors showed high correlation with the composite monitor over much of the range of ambient ozone concentrations, though correlations were lower at higher concentrations. The variance between individual monitors and the composite monitor increased as a function of concentration in nearly all the urban areas. Finally, we observed statistical bias in the composite monitor concentrations at the high end of the distribution. The degree to which these results introduce uncertainty into studies that utilize composite monitors depends on the contributions of peak ozone concentrations to reported health effect associations.
KeywordsOzone Composite monitor Exposure surrogate Exposure measurement error
The authors would like to thank Pat Dolwick, Jennifer Richmond-Bryant, Elizabeth Naess, Richard Wayland, James Hemby, Jackie Ashley, and Michael Koerber at the US Environmental Protection Agency (EPA) for their thoughtful review and comment on this article. Although this paper has been reviewed by the US EPA and approved for publication, it does not necessarily reflect the US EPA’s policies or views. The authors declare no competing financial interests.
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