Over the past few years, research has been piling up that strongly suggests air pollution is one of the causes of dementia. Now, evidence from newer studies is so compelling that many leading researchers believe it’s conclusive.
“I have no hesitation whatsoever to say air pollution causes dementia,” says Caleb Finch, gerontologist and the leader of University of Southern California’s Air Pollution and Brain Disease research network, which has completed many of the newer studies. As to its effects on our health and welfare, Finch says, “air pollution is just as bad as cigarette smoke.”
What makes Finch — and other researchers — so sure? Of all the new research, three studies in particular paint a stark picture of the extent to which air quality can determine whether we'll 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 1.5 times more likely to develop Alzheimer’s across the study period than their neighbours exposed to the lowest levels — a replication of previous findings from Taiwan, where air pollution levels are much higher.
Another, a 2017 study published in The Lancet, followed all adults living in Ontario (roughly 6.5 million people) for over a decade and found those living closer to major high-traffic roads were significantly more likely to develop Alzheimer’s across the study period regardless of their health at baseline or socio-economic status. Both of these studies estimated that around 6–7 percent of all dementia cases in their samples could be attributed to air pollution exposures.
But the most compelling of the new studies, and least reported on, comes from the United States. Three economists at Arizona State University — Kelly Bishop, Nicolai Kuminoff and Jonathan Ketcham — have determined that an annual exposure to an average of just one more microgram of fine particle pollution per cubic metre of air (an amount well within the range of difference you could see if you moved from a clean neighbourhood to a more polluted one) raised the typical US elder’s risk of dementia as if they had aged a further 2.7 years. The authors estimated that the size of this elevated risk approached that of other well-known dementia drivers, including hypertension and heart disease.
Researchers now better understand what happens in the brain when we breathe polluted air — and how that can lead to neurodegeneration years later. When we inhale pollutants, the smallest particles — emitted by cars, power plants, and other places where fuel is burned — lodge in our lungs’ sensitive tissue or else pass into our blood stream. In those places, they trigger an immune response that seeks to trap, contain and remove the invading particles. In time, that response generalises to what is called “systemic inflammation”, or an overactive, overly excited immune response across the body.
Systemic inflammation appears to be the primary way air pollution harms the brain, says Finch. In early 2017, he and his colleagues showed that inflammation following air pollution exposure led to the formation of Alzheimer’s plaques in the brains of mice genetically altered to develop Alzheimer’s pathology. “That was impressive,” says George Martin, Director Emeritus of the University of Washington’s Alzheimer’s Disease Research Center, who was not involved in the study. Because of that study, and others like it, Martin now believes air pollution could be one potential cause of dementia, although he wants more evidence on the mechanisms, “and, ideally, on a specific component or components of air pollution.”
In the coming years, these new findings could shape scientists’ understanding of neurodegenerative disease. Because of these new studies, says George Perry, Chief Scientist of the Brain Health Consortium at the University of Texas at San Antonio and Editor-in-Chief of the Journal of Alzheimer’s Disease, “my view of Alzheimer’s is changing, and I think the field is changing with it.” Perry now believes air pollution is a potential risk factor for dementia. Motivated, in part, by the new evidence, Perry also increasingly sees dementia as a disease like cancer, where multiple factors could lead to pathology. “People develop cancer without smoking or being exposed to air pollution,” he says, “But each of those will raise your risk.”
Unlike with smoking, we can’t always know when we are being exposed to dirty air, and it's difficult to quit unless we move to a less polluted area. Yet Arizona State’s Kuminoff firmly believes we could avoid more dementia by strengthening our existing air pollution standards. If there is a safe level of exposure, he says, “We haven’t gotten there yet.”
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