The EPA and conventional air pollution regulations are back in the news. NPR reported that the seven-member Clean Air Scientific Advisory Committee (CASAC), which provides the EPA with technical advice for National Ambient Air Quality Standards, is “considering guidelines that upend basic air pollution science.” But NPR’s oversimplified depiction of a settled scientific debate ignores real misgivings about the science that has justified the regulations and provides an opportunity to ask questions about the proper role of science in public policy.
The pollutant in question is particulate matter (PM), tiny particles or droplets emitted from power plants, factories, and cars. The EPA contends that PM with diameters smaller than 2.5 micrometers, about 3 percent of the size of a human hair, is the most harmful because the particles can be inhaled deep into the lungs. Along with five other criteria pollutants, the Clean Air Act requires that the EPA periodically prepare an analysis that “accurately reflects the latest scientific knowledge” on the health effects of PM exposure. It must then set air quality standards “requisite to protect the public health…allowing an adequate margin of safety.”
Whether one favors leaning towards caution and setting stringent pollutant standards or is skeptical of the efficacy of air quality rules and worries about the costs of the regulations, PM is important. On the one hand, the supposed harms of PM are high. One (contested) study claimed that 2005 levels of PM caused about 130,000 premature deaths per year, which would put PM as the sixth leading cause of death in the United States after strokes. On the other hand, the regulations are expensive. Between 2003 and 2013, EPA regulations accounted for 63–82 percent of the estimated monetized benefits and 46–56 percent of the costs of all federal regulations. The benefits of reducing PM specifically are 90 percent of the monetized benefits of EPA air regulations, meaning PM rules play an outsized role in the justification for many of the costliest federal regulations.
No matter which side of the debate one is on, it would seem important that the EPA have a rational standard-setting process that properly weighs both the possible reduction in the harms of PM and the potential costs. Unfortunately, that is not the case.
The scientific evidence of the harms of PM is much more uncertain than many observers claim and the conflict over what we do and do not know about the effects of PM has existed for decades. The evidence of negative health effects of PM is primarily two studies published in the 1990s, the Harvard Six Cities Study (SCS) and the American Cancer Society Study (ACS). As I have previously noted,
The SCS has been the subject of intense scientific scrutiny and much criticism because of results that are biologically puzzling. The increased mortality was found in men but not women, in those with less than high school education but not more, and those who were moderately active but not sedentary or very active. Among those who migrated away from the six cities, the PM effect disappeared. Cities that lost population in the 1980s were rust belt cities that had higher PM levels and those who migrated away were younger and better educated. Thus, had the migrants stayed in place it is possible that the observed PM effect would have been attenuated.
Furthermore, a survey of 12 experts (including 3 authors of the ACS and SCS) asked whether concentration-response functions between PM and mortality were causal. Four of the 12 experts attached nontrivial probabilities to the relationship between PM concentration and mortality not being causal (65 percent to 10 percent). Three experts said there is a 5 percent probability of noncausality. Five said a 0-2 percent probability of noncausality. Thus 7 out of the 12 experts would not reject the hypothesis that there is no causality between PM levels and mortality.