1 Institute of Statistical Science, Academia Sinica, Taipei 115, Taiwan.
2 Institute of Occupational Medicine and Industrial Hygiene, College of Public Health, National Taiwan University, Taipei 100, Taiwan.
Abbreviations:
PM10, particulate matter 10 µm in diameter
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INTRODUCTION |
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Dominici cited the findings of Katsouyanni et al. (3) and used the example of nitrogen dioxide, a traffic-derived pollutant, to explain how long-term pollution modifies the acute effects of particulate matter
10 µm in diameter (PM10) on mortality. As is shown in table 2 of our paper (2, p. 6), we found that the acute effect of pollution on PM10 was positively modified marginally significantly by carbon monoxide (coefficient = 0.23) but not significantly by nitrogen dioxide (coefficient = 0.03). Since our study sites included urban, rural, and industrial communities, we could better differentiate pollution sources in our ambient air monitoring data. The nitrogen dioxide concentrations in urban and industrial communities were both high, but the carbon monoxide concentrations were high in urban communities only. This indicated that the choice of suitable proxy indicators as effect modifiers was critical to fitting models and explaining results.
The findings shown in figures 2 and 3 of our paper (2, pp. 5 and 7) indicated that the two-stage modeling decreased the heterogeneity problem to some extent. We agree that heterogeneity is an important statistical and public health issue that deserves further study. As computerized data on community-related information become available, we will be able to use hierarchical models to model the origins of heterogeneity in environmental epidemiologic studies. Once the study sites become small enough, as in our design, a systematic way of collecting individual-related information may become feasible. Then we can apply either the subject-domain approach of Hwang et al. (4) or the case-crossover design of Maclure (5
) and Navidi (6
) to detect an association between exposure and response with time trends by adjusting for individual-related heterogeneity.
We think that studies on nonfatal health effects of air pollution are worthwhile and should be encouraged, even though major ongoing epidemiologic studies on air pollution are analyzing mortality. From a scientific viewpoint, studies of morbid effects can strengthen the consistency of findings regarding the biologic plausibility of mortality effects of air pollution. From a public health viewpoint, a minor health effect such as lower respiratory tract illness is an important issue, because it impacts many people in the population and can lead to the death of susceptible individuals if they are not appropriately treated.
Because more evidence of an association between low-level air pollution and various health effects has accumulated during the past two decades, we think the challenge to future epidemiologic studies on air pollution will be dealing with the following issues: Why does the exposure-response slope for individual air pollutants vary significantly among different study sites? Do pollution effects come from a single air pollutant or from mixtures of air pollutants? What is the relation between chronic exposure effects and acute exposure effects? Some forms of joint temporal-spatial models with attributes of hierarchical and small-area design may provide better approaches with which to address these challenges.
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NOTES |
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REFERENCES |
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