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Ariana Zeka and Joel Schwartz

Harvard School of Public Health, Boston, Massachusetts, USA

Abstract
Misclassification of exposure usually leads to biased estimates of exposure-response associations. This is particularly an issue in cases with multiple correlated exposures, where the direction of bias is uncertain. It is necessary to address this problem when considering associations with important public health implications such as the one between mortality and air pollution, because biased exposure effects can result in biased risk assessments. The National Morbidity and Mortality Air Pollution Study (NMMAPS) recently reported results from an assessment of multiple pollutants and daily mortality in 90 U.S. cities. That study assessed the independent associations of the selected pollutants with daily mortality in two-pollutant models. Excess mortality was associated with particulate matter of aerodynamic diameter 10 µm/m³ (PM₁₀) , but not with other pollutants, in these two pollutant models. The extent of bias due to measurement error in these reported results is unclear. Schwartz and Coull recently proposed a method that deals with multiple exposures and, under certain conditions, is resistant to measurement error. We applied this method to reanalyze the data from NMMAPS. For PM₁₀, we found results similar to those reported previously from NMMAPS (0.24% increase in deaths per 10-µg/m³ increase in PM₁₀) . In addition, we report an important effect of carbon monoxide that had not been observed previously. Key words: air pollution, carbon monoxide, daily mortality, measurement error, particulate matter. Environ Health Perspect 112:1686-1690 (2004) . doi:10.1289/ehp.7286 available via http://dx.doi.org/ [Online 7 September 2004]

Address correspondence to A. Zeka, Exposure, Epidemiology, and Risk Program, Environmental Health Department, Harvard School of Public Health, 401 Park Dr., Suite 415 W, P.O. Box 15698, Boston, MA 02215 USA. Telephone: (617) 998-1001. Fax: (617) 384-8745. E-mail: azeka@hsph.harvard.edu

We thank B. Coull, M.S. O'Neill, and D.Q. Rich for their useful suggestions in the revision of the manuscript.

This work was supported by the Harvard Environmental Protection Agency Center, grant R827353. The views expressed are those of the authors and not the U.S. Environmental Protection Agency.

The authors declare they have no competing financial interests.

Received 25 May 2004 ; accepted 7 September 2004.