By Daniel K. Benjamin
Anyone who has experienced the summer smog of a major city has some sense of the costs of air pollution. As I have reported before in this column (March 2004), more precise estimates of these costs are scarce, but are slowly accumulating. Recent research by Janet Currie and Matthew Neidell (2005) adds significantly to our stock of knowledge, showing that some of the costs of pollution can come in the form of elevated infant mortality.
Currie and Neidell use data from California for the 1990s to investigate the impact of three key pollutants on infant mortality rates: carbon monoxide (CO), particulate matter less than 10 microns in diameter (PM10), and ozone (O3). They find that although particulate matter and ozone have no discernible impact on infant deaths, exposure to higher levels of ambient carbon monoxide does elevate the infant mortality rate. Or, to consider the process in reverse: During the 1990s, emissions of CO in California were cut about 40 percent. According to the authors’ estimates, this reduction saved the lives of about 1,000 infants over the decade. Carbon monoxide is a colorless, odorless gas produced as a result of combustion. As an air pollutant, about 90 percent of it comes from automobiles. CO is highly poisonous in concentrated forms, and even in the dilute concentrations observed in polluted air, it reduces the body’s ability to deliver oxygen to organs and tissues. The potential for harm to infants is thought to be particularly great because of their immature respiratory systems.
In studying its effects on infants, the authors are able to control for an extraordinary array of other factors that might play a role in infant health outcomes. Most importantly, they control for the age and birth weight of infants, two factors known to play critical roles in influencing their mortality. But Currie and Neidell also control for racial, ethnic, and educational factors, as well as the age of the mother, pollution exposure before birth, and even the key weather features of the relevant geographic area. Moreover, they measure pollutants with far greater accuracy than is typically the case, which adds to the precision with which they are able to estimate its effects on infant mortality.
One particularly striking feature of the study is the authors’ finding of lethal effects of carbon monoxide at the relatively low levels to which infants were exposed. The data come from a period in which CO levels in California were on average about two-thirds below the national ambient air quality standards established by the Environmental Protection Agency. The hazards of CO in higher concentrations are well known, but this is the first time significant adverse effects of the pollutant have been observed at such low concentrations.
Although Currie and Neidell do not observe any impact of particulate matter or ozone on infant mortality, there may be simple explanations for this finding. For example, it is generally believed that PM10 has adverse effects arising chiefly from prolonged exposure, and it is in studies that measure such prolonged, chronic exposure to PM10 where such effects have been observed. In contrast, Currie and Neidell examine weekly mortality data, and so effectively measure the impact of changes in short run, or acute, exposure levels, minimizing the chances that PM10 will be observed to be harmful.
The failure to find any impact of ozone exposure may stem from the fact that the study focuses on infants, who spend much of their time indoors during the first year of life. It is well known that O3 reacts with indoor surfaces (and thus dissipates), so even high outdoor concentrations generally are not associated with high indoor levels. Thus, it is likely that the typical infant would have little effective exposure to ozone, regardless of the readings produced by outdoor pollution monitoring equipment.
Despite the study’s statistically robust findings, it is worth putting its results in some perspective. Over the period covered by the research, there were 4.6 million infants born in California, about 18,000 of whom died in their first year of life. The vast majority of these deaths were caused by such factors as inadequate pre- or post-natal health care, premature birth, and low birth weight. Overall, the estimated impact of the reduction in CO during this period—about 1,000 fewer infant fatalities—was a cut in the infant mortality rate of about 5 percent.
Achieving the reduced infant mortality observed in this study required the investment of considerable resources in pollution control. These resources could have been used to produce other important outcomes—including, for example, improvements in pre- and post-natal health care for infants and expanded educational programs to reduce smoking and alcohol consumption among pregnant women. Whether the redeployment of these resources from pollution control to health care or education would have saved the lives of even more California infants remains an open question. But it is surely a question worth asking.
Currie, Janet, and Matthew Neidell. 2005. Air Pollution and Infant Health: What Can We Learn from California’s Recent Experience? Quarterly Journal of Economics 120(3): 1003–30.
DANIEL K. BENJAMIN is a PERC senior fellow and Alumni Distinguished Professor at Clemson University. This column, "Tangents," investigates policy implications of recent academic research. He can be reached at firstname.lastname@example.org