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Bart Ostro,¹ Rachel Broadwin,¹ Shelley Green,¹ Wen-Ying Feng,² and Michael Lipsett³

¹California Office of Environmental Health Hazard Assessment, Oakland, California, USA; ²University of California Davis, Davis, California, USA; ³University of California San Francisco, San Francisco, California, USA

Abstract
Many epidemiologic studies provide evidence of an association between daily counts of mortality and ambient particulate matter < 10 µm in diameter (PM₁₀) . Relatively few studies, however, have investigated the relationship of mortality with fine particles [PM < 2.5 µm in diameter (PM_2.5) ], especially in a multicity setting. We examined associations between PM_2.5 and daily mortality in nine heavily populated California counties using data from 1999 through 2002. We considered daily counts of all-cause mortality and several cause-specific subcategories (respiratory, cardiovascular, ischemic heart disease, and diabetes) . We also examined these associations among several subpopulations, including the elderly (> 65 years of age) , males, females, non-high school graduates, whites, and Hispanics. We used Poisson multiple regression models incorporating natural or penalized splines to control for covariates that could affect daily counts of mortality, including time, seasonality, temperature, humidity, and day of the week. We used meta-analyses using random-effects models to pool the observations in all nine counties. The analysis revealed associations of PM_2.5 levels with several mortality categories. Specifically, a 10-µg/m³ change in 2-day average PM_2.5 concentration corresponded to a 0.6% (95% confidence interval, 0.2-1.0%) increase in all-cause mortality, with similar or greater effect estimates for several other subpopulations and mortality subcategories, including respiratory disease, cardiovascular disease, diabetes, age > 65 years, females, deaths out of the hospital, and non-high school graduates. Results were generally insensitive to model specification and the type of spline model used. This analysis adds to the growing body of evidence linking PM_2.5 with daily mortality. Key words: air pollution, California, fine particles, mortality, particulate matter, PM_2.5. Environ Health Perspect 114:29-33 (2006) . doi:10.1289/ehp.8335 available via http://dx.doi.org/ [Online 1 September 2005]

Address correspondence to B. Ostro, Air Pollution Epidemiology Section, California Office of Environmental Health Hazard Assessment, 1515 Clay St., 16th Floor, Oakland, CA 94612 USA. Telephone: (510) 622-3157. Fax: (510) 622-3210. E-mail: bostro@oehha.ca.gov

We thank F. Forastiere and M. Stafoggia for their technical assistance.

The opinions expressed in this article are solely those of the authors and do not represent the policy or position of the State of California or the California Environmental Protection Agency.

The authors declare they have no competing financial interests.

Received 18 May 2005 ; accepted 1 September 2005.

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