Assessing Confounding, Effect Modification, and Thresholds in the Association between Ambient Particles and Daily Deaths

By Schwartz, Joel | Environmental Health Perspectives, June 2000 | Go to article overview

Assessing Confounding, Effect Modification, and Thresholds in the Association between Ambient Particles and Daily Deaths


Schwartz, Joel, Environmental Health Perspectives


I examined the relationship between daily deaths and airborne particles in 10 U.S. cities with varying climatic conditions and seasons in which particle concentrations were high. Airborne particles were associated with significant increases in daily deaths [0.67% increase for a 10 [micro]g/[m.sup.3] increase in particles; 95% confidence interval (CI), 0.52-0.81%]. This association was the same in summer and winter. To examine potential confounding by other pollutants, I regressed city- and season-specific effect sizes against the relationship between airborne particles and other pollutants. Controlling for other pollutants did not substantially (or significantly) change the estimated effect of airborne particles. Socioeconomic differences between cities likewise did not modify the effect. The increase in daily deaths that occurred out of hospitals (0.89% per 10 [micro]g/[m.sup.3]; CI, 0.67-1.10%) was substantially greater than the increase in deaths in hospitals (0.49%; CI, 0.31-0.68%). This is consistent with results previously reported in Philadelphia, Pennsylvania, and suggests that the particle-associated deaths are not just being brought forward by a few days. It is also consistent with recent animal and human studies of the mechanisms of particle toxicity. Key words: airborne particles, air pollution, climate mortality. Environ Health Perspect 108:563-568 (2000). [Online 3 May 2000]

http://ehpnet1.niehs.nih.gov/docs/2000/108p563-568schwartz /abstract.html

Studies on four continents have reported associations between daily concentrations of ambient particles and daily deaths (1,2). The magnitude of the regression coefficients varied, but were remarkably similar compared to epidemiologic studies of other exposures. Several arguments have been made to question the relevance of these findings for public health and preventive measures. It has been argued that the deaths are occurring in persons who were already seriously ill and who would have died in a few days anyway. It has been argued that air pollution is responsible for the deaths, but that airborne particles are not the responsible agent; rather, other pollutants confound the particle findings. It has also been argued that the particle associations only exist at higher concentrations, and therefore, most days are below a presumed threshold for effect; hence public health interventions to lower exposure would have no impact on most days.

Two recent papers addressed the first argument by showing that the association between daily deaths and airborne particles persisted after accounting for any short-term displacement of (reduced time until) deaths (3,4). In this paper, I address the latter two issues in a multiple-city analysis of particulate air pollution and daily deaths. I also indirectly address the first issue by an analysis stratified by location of death.

Recently, Sunyer et al. (5) reported that persons with a previous emergency room visit for chronic obstructive pulmonary disease (COPD) had a greater risk of air pollution-induced mortality. In general, there is interest in potential effect modifiers for particulate air pollution. Among these are social and economic factors that may represent differences in underlying risk. For example, income has been shown to be a potent predictor of life expectancy. These factors differ among cities in the United States, and these differences can be used to explore their role as effect modifiers for the impact of airborne particles.

Methods

Data. I selected 10 U.S. cities with approximately daily [PM.sub.10] (particulate matter [is less than or equal to] 10 [micro]m) monitoring to provide a reasonable number of locations for a combined analysis. The cities were New Haven, Connecticut; Pittsburgh, Pennsylvania; Birmingham, Alabama; Detroit, Michigan; Canton, Ohio; Chicago, Illinois; Minneapolis--St. Paul, Minnesota; Colorado Springs, Colorado; and Spokane and Seattle, Washington. …

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