Diabetes causes roughly 1.5 million deaths worldwide each year, making prevention a major focus of medical research. Most efforts concentrate on familiar risk factors such as diet, physical activity, and genetics.

In recent years, however, researchers have begun looking into a less obvious factor: air pollution. Diabetes is a metabolic disease involving the pancreas, blood sugar regulation, and insulin, while air pollution is something we associate with the lungs and breathing. On the surface, there is no obvious biological connection between the two.

Yet a growing amount of data suggests these two may be more closely correlated than they seem. In the last decade, several studies have found a link between air pollution and diabetes. What’s surprising is even areas with relatively clean air can have enough pollution to increase people’s risk.

The Denmark Study: Air Pollution and Diabetes

Researchers in Denmark studied a group of 28,731 female nurses from 1993 to 2013. They tracked a long list of health metrics among the nurses, including whether they got diagnosed with diabetes. They compared this to regional data on tiny particles in the air called “PM2.5.” These are particles smaller than 2.5 microns that come from things like car exhaust, coal-fired power plants, and wood-burning stoves.

In areas with more air pollution, more people developed diabetes over those 20 years. Diabetes risk went up 41% for each increase of 10 micrograms of PM2.5 (ug/m3).

But if you’re paying attention, you might be thinking about confounds. Areas with more air pollution might have other risk factors, like poverty, poor diet, and obesity. The researchers statistically controlled for a list of risk factors, such as smoking, diet, and BMI. After taking those factors into account, air pollution exposure remained significant.

Higher Risk for Healthy People

The researchers also broke out the effect of air pollution for different groups of people. This type of analysis can help us understand whether air pollution harms some people more than others. For example, does air pollution harm old people more? What about people who are overweight and therefore at higher risk of diabetes?

Weight did matter. Pollution had a stronger effect among people who were overweight.

But in the analysis, the factor that mattered most was smoking. The effect of air pollution was the strongest for people who didn’t smoke. For smokers, the effect of air pollution was effectively zero.

One possibility is that smoking is exposing people to so much particulate that it swamps the effect of outdoor air pollution. But for people who aren’t bombarding their lungs with cigarette smoke, outdoor air pollution matters a lot more.

Not Just One Study

A separate study in The Lancet Planetary Health found similar results. That study comparing regions in the US and found that people were about 30% more likely to be diagnosed with diabetes in places where PM2.5 averaged about 12 micrograms.

That increased prevalence is compared to places that average 6 micrograms. So we’re talking about a difference between 6 micrograms and 12 micrograms.

It’s worth thinking about just how low 12 micrograms is. Until 2023, 12 micrograms was in the green “healthy” zone for the US EPA air quality index. Now with the updated AQI standards, 12 micrograms is just barely in the yellow “moderate” zone.

In China, 12 micrograms is in the green. In fact, you’re still in the green in China if you’re breathing almost three times as much pollution.

Twelve micrograms would put you in the cleanest city in Thailand. And 12 is nowhere near the pollution of some of the cities with the world’s worst air. For example, Lahore, Pakistan averaged 150 micrograms in 2024.

The evidence for harm at such low levels of air pollution are why I use an air purifier in my home everyday, despite living in a city with very low levels of air pollution. In fact, increases in air pollution at lower levels actually seem to cause the most harm for human health. I explain that mind-bending idea here: why low levels of air pollution cause the most harm.

The Balance of The Evidence

Of course, we wouldn’t want to reach a firm conclusion from one or two studies. However, other studies have found an association too, such as studies in Los Angeles and Ontario. In fairness, a separate study in six US cities found an association, but it fell just short of statistical significance.

Studies can find different findings because of factors like levels of pollution exposure they are looking at, how fine-grained their pollution data is, or how long they’re tracking participants for. Despite the differences, the overall picture across studies is that particulate air pollution increases the risk of diabetes.

Why on Earth Would Breathing Air Pollution Cause Diabetes?

Why do researchers think there’s a plausible link when our lungs are not obviously connected to our pancreas? Researchers have some clues from animal studies. Researchers have exposed mice to more or less PM2.5 pollution in their cages.

What they found was that the mice experienced more inflammation throughout their bodies. That inflammation seemed to worsen insulin resistance and increase fat accumulation.

And believe it or not, researchers have intentionally exposed humans to air pollution in the lab. In those studies, air pollution increases markers of inflammation. In contrast, breathing air filtered with HEPA filters decreased markers of inflammation. This offers an explanation for why air pollution is linked to diabetes.