To the list of human hazards associated with bad air—heart attacks, decreased lung function, asthma, and early death—we can add one more: Alzheimer’s.

In a wryly titled working paper put out last month, “Hazed and Confused,” researchers from Arizona State University found that increased, incremental exposure to fine particle pollution over the course of a decade increases the chances of a dementia diagnoses by 1.3 percentage points. The pollutants they studied are known as PM 2.5 for their small size—invisible fragments thrown off by fields, factories, and fires that easily make their way into our lungs and correlate more closely with health effects than any other common pollutant.

It’s the second recent study to make the connection between cognition and pollution. In August, a study published in the Proceedings of the National Academy of Sciences determined that in Chinese cities, where smog can hide the sun for weeks at a time, long-term exposure to pollution was linked to declining test scores—especially in older men.

There’s been little study of particle pollution and dementia, of which Alzheimer’s, which affects more than 5 million Americans, is the most common form. The authors of the working paper, led by ASU economist Kelly Bishop, used 15 years of Medicare records for nearly 7 million adults to figure out where older Americans lived, when they were diagnosed with dementia, and what kind of air they were breathing. Their conclusion—that a diagnosis probability goes up more than 1 percentage point for every additional 1 microgram-per-cubic-meter (μg/m3) exposure over 10 years—might be more alarming than it sounds, because the variance between U.S. counties ranges as high as 13 μg/m3. It’s worth noting that both studies assess the correlation of air pollution and cognitive problems—it’s unclear if the first causes the second, or if some third factor is undergirding both.

Fine particle levels have fallen consistently in the United States since the millennium, thanks in part to Environmental Protection Agency regulation and the long-term shift away from burning coal and heating oil. The average concentration in the U.S. now hovers around 9 μg/m3. (The World Health Organization recommends staying below 10, a standard for clean air that fewer than 10 percent of people on Earth enjoy.)

The worst place to breathe in the United States is Kern County, California, home of Bakersfield, where agriculture, transportation, and power combine with still desert air to create small-particle levels that can run three times as high as they do over the mountains in smoggy Los Angeles. Kern County’s weighted annual average PM 2.5 count was 18.2 in 2017—twice that of Manhattan. The Central Valley sits at the top of the EPA’s list of U.S. counties sorted by particle pollution, but a handful of car-dependent areas are up there too, including Detroit; Oakland, California; and Los Angeles.

But combustion—whether from forest fires, power plants, or stoves—continues to be one of the biggest factors. That’s why little Fairbanks, Alaska, has some of the worst air in the United States: Alaskans like their fireplaces. In the Western U.S., recent wildfires are reversing national trends, sending fine particle pollution back up in 2017. In Britain, according to the Economist, wood fires at home now account for 34 percent of PM 2.5 emissions—nearly three times more than cars and trucks.

The good news, I suppose, is that one reason PM 2.5 has gone down is that the EPA started directing states to police counties’ pollution levels in the late ’90s, with the maximum average concentration set at 15.5 μg/m3. At the time, 27 percent of the U.S. population was breathing air above that mark. Today, just a handful of counties are up there, and all but one (San Bernardino, California) are rural. But the Trump administration is threatening to undo that progress. Under acting administrator Andrew Wheeler, the EPA is considering changing its classification of PM 2.5 to a “threshold pollutant,” arguing that any exposure below a threshold of 12 μg/m3 is safe. “If Wheeler changes the official designation of PM 2.5, the EPA’s position would be that breathing in small amounts of soot has the same impact as breathing in none,” Emily Atkin writes at the New Republic, arguing that that change—which flies against most evidence on PM 2.5—would make it easier to repeal the Clean Power Plan.

Being an economics paper, the ASU study focuses more on money than science. America spends upward of a quarter-trillion dollars on Alzheimer’s-related care every year, but that’s probably a big underestimate, because it doesn’t include what’s thought to be billions of labor-hours put in by unpaid caregivers (mostly the children and spouses of those afflicted). Some researchers think dementia may be the reason older Americans start making worse financial decisions as they age.

Put it all together and it means that the cleanup is significantly reducing the incidence of Alzheimer’s. But again, these are economists we’re talking about, so they put a number on the additional dementia-related savings from the EPA’s action since 2000: $150 billion.