This story originally appeared on Mother Jones and is part of the Climate Desk collaboration.

A few years ago I stood in a cramped trailer beside the busy 110 freeway in Los Angeles as researchers at the University of Southern California gathered soot thrown off by vehicles pounding by just a few yards from their instruments, which rattled whenever a heavy truck passed. I was there to learn about how scientists were beginning to link air pollution—from power plants, motor vehicles, forest fires, you name it—to one of the least understood and most frightening of illnesses: dementia.

At that time, as I reported in Mother Jones, the research implicating air pollution as one factor that can contribute to dementia was alarming, consistent, and, ultimately, “suggestive.” Since then scientists have published a wave of studies that reveal that air pollution is much worse for us than we had previously imagined. The evidence is so compelling, in fact, that many leading researchers now believe it’s conclusive. “I have no hesitation whatsoever to say that air pollution causes dementia,” says Caleb Finch, gerontologist and the leader of USC’s Air Pollution and Brain Disease research network, which has completed many of these new studies. In terms of its effects on our health and welfare, Finch says, “air pollution is just as bad as cigarette smoke.” This evidence arrives alongside the alarming news that air quality is actually worsening for many cities in the United States, while the Trump administration continues its effort to delay or roll back environmental safeguards.

What makes Finch—and the half dozen other researchers I talked to—so sure? Of all the new research, three studies in particular paint a stark picture of the extent to which the quality of our air can determine whether we will age with our minds intact. In one from 2018, researchers followed 130,000 older adults living in London for several years. Those exposed to higher levels of air pollutants, particularly nitrogen dioxide and fine particulate matter released by fossil fuel combustion, were significantly more likely to develop Alzheimer’s disease—the most common kind of dementia—than their otherwise demographically matched peers. In total, Londoners exposed to the highest levels of air pollution were about one and a half times more likely to develop Alzheimer’s across the study period than their neighbors exposed to the lowest levels—a replication of previous findings from Taiwan, where air pollution levels are much higher.

A 2017 study published in The Lancet followed all adults living in Ontario (roughly 6.5 million people) for over a decade and found that those who lived closer to major high-traffic roads were significantly more likely to develop Alzheimer’s disease across the study period, regardless of their health at baseline or socioeconomic status. Both of these studies estimated that around 6 to 7 percent of all dementia cases in their samples could be attributed to air pollution exposures.

Those studies from the UK and Canada are certainly intriguing, but the most compelling, and least reported on, study comes from the United States. It was also, incidentally, inspired by our previous reporting.

Following our early report on the link between air pollution and dementia, three economists at Arizona State University—Kelly Bishop, Nicolai Kuminoff, and Jonathan Ketcham—decided to pursue a large-scale investigation of the issue. “We found the Mother Jones article compelling,” Ketcham says. “It was informative about the plausible pathways and the need for more rigorous studies that could test causality.”