Gregory A. Wellenius of Brown University and colleagues analyzed the relationship between measures of traffic-related air pollution and blood leptin levels in 765 Boston-area older adults. Higher levels of leptin, an "inflammatory cytokine," have been linked to increased rates of heart disease, obesity, and diabetes.

The results showed a significant association between exposure to black carbon—a measure of fine-particle air pollution from traffic sources—and leptin levels.

Participants with higher exposure to black carbon were less likely to be white, had lower incomes, and had higher rates of high blood pressure and diabetes.

But even after adjustment for these differences, average leptin levels were 27 percent higher for older adults in the highest category of black carbon exposure.

An alternative measure of exposure to traffic-related pollution—residential distance to the nearest major roadway—was unrelated to leptin levels. The estimates of black carbon exposure "likely reflect contributions from traffic on a wider range of roadways in the immediate vicinity of each participant's home," the researchers write.

While the study can't prove any causal link, the link between black carbon exposure and leptin levels may help to explain observed increases in cardiovascular disease risk associated with air pollution—especially from traffic. Dr Wellenius and colleagues conclude, "If confirmed, these findings support the emerging evidence suggesting that certain sources of traffic pollution may be associated with adverse cardiometabolic effects."