Exposure to air pollution increases the levels of obesity-related hormone leptin in old adults, a new study has found.

The study, conducted by researchers at the Brown University and colleagues, found that traffic-related air pollutants are associated with an increase in blood leptin levels. Leptin is an inflammatory cytokine and is related to risk of heart disease, diabetes and obesity.

Data for the study came from 765 adults living in Boston area. The researchers found that people who were exposed to black carbon frequently were more likely to have high levels of leptin in their blood stream.

The researchers say that the study shows that increase in the hormone leptin is behind the rise in cardiovascular diseases in people living close to roadways. Previous research has shown that people exposed to air pollution have high risk of developing health complications. The International Agency for Research (IARC) on Cancer recently classified outdoor air pollution as a carcinogen.

The current study doesn't establish a cause-and-effect relationship between black carbon and leptin.

"If confirmed, these findings support the emerging evidence suggesting that certain sources of traffic pollution may be associated with adverse cardiometabolic effects," said Gregory A. Wellenius, ScD, of Brown University and colleagues.

The team also didn't find any link between leptin levels and the distance to major roads  from the participants' homes, Healthday reported.

The levels 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 wrote in a news release.

The study is published in the Journal of Occupational and Environmental Medicine.