The study authors controlled for things like obesity and BMI, so it wasn’t the case that heavier people simply lived in more polluted neighborhoods and were also more likely to get diabetes.

The particles examined in this study are known as PM 2.5 , or particulate matter that’s 2.5 micrometers big—30 times smaller than a human hair. They are emitted by various types of industry and fuel burning, but in the United States, the biggest source of PM 2.5 is cars, says Ziyad Al-Aly, the study’s senior author and an assistant professor of medicine at Washington University at St. Louis. When there’s lots of PM 2.5 in the air, the air might look smoggy or hazy. In lighter concentrations, the particles are invisible.

Scientists are just beginning to understand what exactly makes PM 2.5 so harmful, but a major reason is that it’s so small and contains toxic metals. Its size allows it to penetrate the lungs and enter the bloodstream. There, it can circulate to different organs and cause inflammation. The inflammation increases insulin resistance. Eventually, this insulin resistance can become so severe the pancreas becomes unable to pump out enough insulin to compensate, and diabetes can set in.

Previous research has found that Latino children living in areas with more air pollution had a greater risk of developing type 2 diabetes. But other studies on the association between the two have generated mixed results.

This new study makes an even stronger case, suggesting that the current limits on air pollution in the United States might be too high. The EPA’s pollution threshold on particulate matter is 12 μg/m3, or micrograms per cubic meter of air, but this study says the risk of diabetes starts at about 2.4 μg/m3. Among people exposed to between five and 10 μg/m3 of particulate matter, about 21 percent developed diabetes. At the threshold of current “safe” levels, 24 percent do. For each 10 μg/m3 increase in particulate matter, the risk of developing diabetes goes up by 15 percent. This risk is present regardless of whether the individual becomes obese or not.

But this study and others might not lead to a tightening of the PM 2.5 standards because, under a rule proposed by the Trump administration in April, all studies used by the EPA to make air and water regulations must make their underlying data publicly available. As my colleague Robinson Meyer reported, studies like this and others, which show the detrimental health impacts of pollution, are based on health data that is confidential and cannot be de-anonymized.

That will greatly undermine regulations that have made for cleaner air, says Sanjay Rajagopalan, a cardiologist at University Hospitals Cleveland Medical Center, about the proposed rule. “Documents that have gone into EPA regulations go through rigorous peer review,” he says. “We have some of the cleanest air in the world, and there’s scientific data that this has protected millions of lives and contributed to the longevity of American citizens.”