Haze, smoke and car exhaust may contribute to adult diabetes, even at pollution levels within federal limits, a recent large-scale study suggests.

The study, published in the journal Diabetes Care, was conducted by researchers from Children’s Hospital Boston. They evaluated adult diabetes rates and data from the Environmental Protection Agency on fine particulate air pollution in every county in the 48 contiguous states.

After adjusting for known diabetes risk factors, including obesity, exercise, ethnicity, and population density, the researchers found a strong correlation between higher air pollution and increased diabetes. Even in areas somewhat below EPA limits for air pollution, diabetes was more than 20 percent more prevalent than among people in areas with the cleanest air.

The findings square with smaller-scale population studies and animal laboratory studies suggesting a link between diabetes and small particulate air pollution, according to the report. The authors cautioned that the results don’t prove that air pollution causes diabetes. All the same, one of the authors, John Brownstein, told Science Daily that the findings suggest that EPA limits might not be tight enough to prevent air pollution-related health problems.

The number of Americans with diabetes has more than doubled over the past 15 years to roughly 24 million people, according to the report.