Original articleHas reducing fine particulate matter and ozone caused reduced mortality rates in the United States?
Section snippets
Introduction: using data from natural experiments to understand causality
An aim of applied science in general and of epidemiology in particular is to draw sound causal inferences from observations. Students are taught to develop hypotheses about causal relations, devise testable implications of these causal hypotheses, carry out the tests, and objectively report and learn from the results to refute or refine the initial hypotheses. For at least the past two decades, however, epidemiologists and commentators on scientific methods and results have raised concerns that
Data and methods
Cause-specific mortality rates, by county and age group, were downloaded from the Centers for Disease Control and Prevention Wonder “Compressed Mortality, 1999 to 2010” database [43]. To create a geographically diverse sample, mortality rates were extracted at the county level for the 15 largest states in the United States (California, Texas, New York, Florida, Illinois, Pennsylvania, Ohio, Georgia, Michigan, North Carolina, New Jersey, Virginia, Washington, Massachusetts, and Arizona)
Descriptive statistics
Figure 1 shows trends in average pollution levels, population, and mortality rates for all counties from 2000 to 2010. For each time series, values are normalized by dividing by the value in 2000, so that all time series values in 2000 are defined as 1.0. PM2.5 and CVD mortality rates declined most steeply over this interval (two lowest curves), whereas population levels and external-cause mortality rates (e.g., from accidents) increased, perhaps reflecting a longer-lived aging population.
Discussion and conclusions: caveats for causal interpretations of regression coefficients
The epidemiologic and risk assessment literature on human health effects of air pollution contains dozens of studies that attribute reductions in mortality risks to reductions in air pollution levels and that estimate the slope of the concentration-response association between exposures to pollutants and corresponding mortality rates [12], [20], [22], [47], [51], [52]. The work reported here contributes a new data set to this literature. It supports previous findings of positive PM2.5-mortality
Acknowledgments
The research described here was supported in part by the American Petroleum Institute and by Cox Associates, LLC. The research questions asked, methods used, and conclusions reached are solely those of those the authors. The authors thank reviewers for their comments and suggestions, which improved the clarity of the exposition.
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