Just a small increase in the pollution people breathe can raise their risk of developing dementia, according to a new study that lays the groundwork for stricter air quality regulations. 

The analysis, conducted by researchers at Harvard’s medical school, was released on Wednesday in the BMJ, a peer-reviewed medical journal. It’s the most comprehensive look yet at the link between the neurological condition and exposure to PM2.5 — fine particles that are 2.5 microns wide or less released by wildfires, traffic, power plants, and other sources. Dementia, an umbrella term for the loss of mental functioning that includes Alzheimer’s disease, afflicts more than 7 million people in the United States and 57 million worldwide.

The study found that the risk of dementia rose by 17 percent for every two micrograms per cubic meter increase in people’s annual exposure to PM2.5. For context, the average American is exposed to an average of 10 micrograms per cubic meter every year, much of it from burning fossil fuels; during Beijing’s most polluted years a decade ago, the city hovered around 100 micrograms.

“Two micrograms per cubic meter is not that much,” said Marc Weisskopf, the lead author of the study and a professor of environmental epidemiology and physiology at Harvard University. “You know, that could easily be the difference between being in Boston versus a rural part of Massachusetts.”

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That even small increases can raise dementia risks suggests that governments need to revamp their rules. The Environmental Protection Agency places the limit at 12 micrograms per cubic meter, and the European Union puts the threshold at a comparatively lax 25 micrograms.

The Harvard study is an “alarm” the EPA should pay attention to, said Afif El-Hasan, a pediatrician and volunteer spokesperson for the American Lung Association who was not involved with the new research. He called for the agency to “get very aggressive” on reducing particulate matter with new guidelines that account for the dementia risks laid out in this new report.

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“It is devastating to think that it’s, once again, another penalty that’s being paid by people who live in areas with poor air,” El-Hasan said. “It’s another penalty they have to pay, risk not only for their lungs, not only increased cancer risk or heart risks for heart problems, but mental problems as well. And it’s sad, as a society, that that has to be the case.”

In light of the thousands of scientific studies showing how particulate matter hurts people’s health, the EPA recently proposed tightening its limits for PM2.5 to nine or 10 micrograms. The agency said that these stricter standards could prevent more than 4,000 premature deaths each year and save $43 billion in health costs in 2032. But health advocates have argued that the EPA’s proposal still falls short of what’s needed. It also doesn’t take the risk of dementia into account, unlike more established research on heart and lung conditions.

“The literature has been growing rapidly recently, but it’s a little bit maybe too new for the EPA,” Weisskopf said. 

For the most recent report, the Harvard researchers looked at more than 50 studies that assessed the link between dementia and air pollution, then narrowed the batch down to 16 using a new tool that can detect bias in studies. For example, many epidemiological studies rely on large stores of medical data that don’t include people who aren’t able to afford medical care. Despite concerns that scientists might have been overestimating the link between dementia and PM2.5 exposure, the study showed that, if anything, the effect was underestimated, Weisskopf said.

It doesn’t bode well, especially as climate change threatens to undo decades of progress on air pollution. The number of Americans exposed to wildfire smoke, for instance, has increased 27-fold over the last decade, with fires amped up by hotter temperatures routinely blanketing cities in the western U.S. in plumes of smoke.

It’s worth noting that pollution isn’t the only factor behind the rise in dementia, much of which can be attributed to an aging population. Previous research suggests that about 40 percent of dementia cases are preventable, as smoking, education, and cardiovascular health also play roles. Air pollution doesn’t appear to be as big a risk factor as smoking, Weisskopf said, but because it touches basically everyone, it can have a huge effect across the population.

Scientists are not sure when exposure to PM2.5 is the most harmful — when people are young, old, or throughout their entire life. Most studies only look at exposure in the years directly preceding the onset of dementia. “Until we understand that better, there’s going to be still some fuzziness,” Weisskopf said.

The study’s findings could be used to calculate the cost-benefit analyses that are used to develop environmental regulations. Establishing the link between dementia and PM2.5 has “huge societal and financial implications,” Weisskopf said, “because the amount of money that gets spent on dementia care and caring for people and treating people is enormous.” Last year, medical costs for dementia, which affects roughly one in nine Americans who are 65 and older, added up to about $592 billion in the U.S.

“Doing the right things in terms of air quality doesn’t just improve everyone’s life, make our lives longer and more productive, but it also costs society less,” said El-Hasan.