SierraScape February - March 2006
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by Ken Schechtman, PhD

Scientific reports from countries as diverse as China, the Czech Republic, Brazil, Mexico, and the United States have established that ambient air pollution is associated with adverse birth outcomes that include low birth weight, intrauterine growth retardation, and fetal mortality. To expand upon these observations, two papers in the American Journal of Epidemiology have investigated the association between air quality and cardiovascular birth defects.

Both investigations used a similar case-control study design. Case-control studies define a commonly used epidemiologic research methodology characterized by the identification of a set of individuals who have a particular disease (the cases) and a second set of individuals who do not have the disease (the controls). The goal is to select a control group that is as similar as possible to the cases with respect to demographic and clinical risk factors for the disease being studied. One then goes back in time to retrospectively assess exposure to environmental parameters of interest in order to determine if the cases had greater exposure to the environmental factors that may have caused the disease in question. If one identifies groups of cases and controls whose only difference is in the magnitude of exposure to measured environmental factors, one concludes that the increased exposure had a causative relationship with the disease being studied.

The two case-control studies of interest focused on pollution levels in a high pollution area of southern California and in seven lower pollution counties in Texas. Both studies relied on state birth and birth defect registries to identify participants. Both compared the known residence of mothers with environmental monitoring data to quantify the exposure of women to specific pollutants during their pregnancy. Both divided exposure into quartiles and used a standard statistical measure called the odds ratio to determine the odds of having a birth defect in higher exposure quartiles with that in the lowest exposure quartile.

The California study evaluated 1661 cardiovascular fetal anomalies broken down into six defined categories and 10,649 controls selected from a registry of 754.030 eligible infants and fetuses. The study found that there was a marked and highly significant relationship between the magnitude of exposure to carbon monoxide and four of the six anomaly categories. For example, infants whose mothers were in the highest quartile of exposure to carbon monoxide were 2.95 times as likely as those in the lowest quartile of exposure to have a ventricular septal defect, a hole between the right and left pumping chamber of the heart that can lead to congestive heart failure when the hole is large. Smaller and less conclusive associations were observed when ozone, nitrogen dioxide, and small particulate matter were observed.