Fast food and American food deserts may not be the only things to blame for the childhood obesity epidemic. A new study shows that exposure to tobacco smoke as well as road air pollution leads to a higher incidence of childhood obesity. The study, published in Environmental Health Perspectives, was led by researchers from Keck Medicine of the University of Southern California (USC).

Children who lived close to highway or roadway air pollution, as well as secondhand tobacco smoke, were more likely to see an increase in weight gain during adolescence, the researchers found. Children exposed to both air pollution and secondhand smoke were even more likely to experience weight gain than those who were exposed to only one — or to neither.

“Vehicle miles traveled, exposure to some components of the near-roadway air pollutant mixture, and near roadway residential development have increased across the United States over the last several decades corresponding to the epidemic of childhood obesity,” Dr. Rob McConnell, a professor of preventive medicine at USC and a lead author of the study, said in the press release. “The potential for near-roadway air pollution to be among several factors contributing to the epidemic of obesity merits further investigation.”

In the study, researchers analyzed 3,000 children who had been exposed to tobacco smoke during their time in the womb or after birth. They also measured the effects of air pollution from busy roads or highways. The children in the study had been enrolled at age 10 in the Southern California Children’s Health Study, which began in 1992 with the goal of analyzing long-term effects of air pollution on kids. These kids were examined annually for eight years, until they turned 18 years old.

While it’s been known for some time that tobacco smoke can exacerbate medical conditions and cause serious disorders like heart disease and lung cancer, it’s not until recently that researchers have begun connecting obesity to secondhand smoke. A recent study out of Brigham Young University, for example, found that secondhand smoke increased the risk for obesity. The researchers examined mice who were exposed to secondhand smoke, and found that tobacco caused the body to develop insulin resistance, which is of course the first step toward type 2 diabetes, a chronic disorder often linked to obesity.

“For people who are in a home with a smoker, particularly children, the increased risk of cardiovascular or metabolic problems is massive,” Benjamin Bikman, professor of physiology and developmental biology at Brigham Young University, said in a press release. “The idea that there might be some therapy we could give to innocent bystanders to help protect them from the consequences of being raised in a home with a smoker is quite gratifying.”

The USC study is one of the first to link childhood obesity to both air pollution and exposure to tobacco. More research will be needed to examine just how much we can curb the childhood obesity epidemic in the U.S. by also reducing smoking and air pollution.

“Further research is needed to determine if our findings can be replicated in other populations,” McConnell said in the press release, “and to assess both the potential contribution of combustion sources to the epidemic of obesity and the potential impact of interventions to reduce exposure.”