Ann Arbor, Michigan – Make no mistake, obesity is a global problem. Globally, obesity rates have tripled since 1975. Expanding waist circumference is particularly prevalent in the United States, where more than four out of 10 adults meet the criteria for obesity. While there are a number of factors behind this worrying trend, a new study finds that air pollution may be another factor tipping the scales toward obesity — at least for some women.

Scientists at the University of Michigan report that middle-aged women exposed to smog may be more likely to be overweight and develop a higher body mass index, larger waist circumference, and increased body fat. The study authors said that a group of older, middle-aged women who had prolonged exposure to air pollution tended to gain more weight.

“Women in their late 40s and early 50s exposed to long-term air pollution — specifically, higher levels of fine particulate matter, nitrogen dioxide, and ozone — experienced increases in measures of body size and composition,” says study first author Shen Wang, a research investigator at Epidemiology at the UM School of Public Health, in a university statement.

These findings are based on a data set of 1,654 Caucasian, Black, Chinese, and Japanese women who participated in A study of women’s health across the nation. The researchers tracked all of these participants, whose average baseline age was close to 50, between 2000 and 2008.

Meanwhile, the study authors determined annual exposure to air pollution by correlating residential addresses with hybrid estimates of air pollutant concentrations. Next, the research team analyzed any and all associations between local air pollution levels and body size and composition metrics for each participant. One question in particular they wanted to clarify: Does exercise affect these associations?

The air you breathe can add more than 2 pounds

Ultimately, the team at UM discovered that exposure to air pollution showed an association with higher body fat percentage, higher body fat percentage, and lower lean mass among middle-aged women. For example, body fat increased by 4.5 percent (about 2.6 pounds).

The study authors were sure to explore the interactions between air pollution and physical activity on body composition. Sure enough, the researchers found that plenty of physical activity (based on frequency, duration, and perceived physical exertion for more than 60 exercises) was an effective way to mitigate and possibly even offset air pollution exposure.

However, Wang points out that because this project only included middle-aged women, its results should not be generalized to men or women of different age groups.

The study was published in the journal Diabetes care.