People in areas of the United States with high levels of a certain kind of air pollution have a greater risk of dementia, a new study found.
The study, published Monday in the journal JAMA Internal Medicine, looked at data from 27,857 survey participants between 1998 to 2016. About 15%, or 4,105, developed dementia during the study period, and all lived in areas of the US with higher concentrations of particle pollution than those who didn’t develop dementia, the researchers found. The study authors said it’s the first nationally representative study of the potential effects of particle pollution on dementia in the US, and the link to dementia was most robust in areas with pollution from agriculture and wildfires.
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It’s important to note, the study authors said, that these associations were observed even at pollution levels lower than the current national ambient air quality standards.
Particle pollution, also called PM2.5 or particulate matter, is the mix of solid and liquid droplets floating in the air, according to the US Environmental Protection Agency. It can come in the form of dirt, dust, soot or smoke. Particulate matter can come from coal- and natural gas-fired plants, cars, agriculture, unpaved roads, construction sites and wildfires.
Most previous research on the issue has looked largely at particle pollution from fossil fuels. But in the new study, the connection to dementia seemed most robust with pollution connected to agriculture and wildfires, although it could also come from other sources like traffic and coal combustion.
“At first, when agriculture and wildfires were the two that popped out, Boya and I were both surprised,” said Dr. Sara Dubowsky Adar, associate chair of epidemiology at the University of Michigan School of Public Health, who worked on the study with a team that included Dr. Boya Zhang, a research fellow in the department.
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”In hindsight, it really makes a lot of sense, mostly because of the fact that we’re looking at impacts on the brain, and agriculture we know is using a lot of pesticides,” Adar said.
Pesticides are neurotoxins to animals, she said, so those may be the particles in agriculture pollution that are affecting human brains, as well. As for wildfires, the smoke doesn’t just come from burning trees; things like homes and gas stations burn too, becoming the particle pollution that people breathe in.
Particle pollution is particularly deadly because PM2.5 is so tiny — 1/20th of a width of a human hair — that it can travel past your body’s usual defenses. Instead of being breathed out when you exhale, it can get stuck deep in your lungs or go into your bloodstream.
The particles cause irritation and inflammation and can lead to respiratory problems. Research has found that long-term exposure to particle pollution can also cause cancer, depression, breathing problems and a variety of heart problems.
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“Just like cigarettes, there’s no such thing as good inhaled particles,” said Dr. Caleb Finch, a professor and the ARCO/William F. Kieschnick Chair in the Neurobiology of Aging at the University of Southern California, who was not involved with the new study. “Almost everything that air pollution does, cigarette smoke also does.”
More than 55 million people worldwide have dementia, and 10 million more develop it each year, according to the World Health Organization. Because of the aging population and other health issues like obesity, smoking and high blood pressure, that number is expected to grow significantly. In 2021, the Alzheimer’s Association said that increasing levels of air pollution and increasing cases of dementia worldwide should be treated as serious public health crises.
The new study cannot determine the exact mechanism connecting particle pollution and dementia, but scientists have some theories.
These tiny pollution particles could be getting into the brain through the nose and causing neuronal cell death that’s connected to dementia. It’s also possible that particle pollution is modifying inflammatory proteins that act on the brain.
Dr. Masashi Kitazawa, an associate professor of environmental and occupational health at the University of California, Irvine, speculated that the pollution may be having an indirect effect. Scientists know that exposure to particle pollution causes heart conditions and vascular problems, for example, and both can be a risk for Alzheimer’s disease and dementia.
“Is that causing the cardiovascular failure that leads to the less oxygen supply to the brain, and then that caused the accelerating dementia, or the PM is getting into the brain and causing some neurotoxic reaction? We still don’t know yet,” said Kitazawa, who was not involved with the new study.
It’s also important to keep in mind that this research points to a correlation, he said, but it does not show that air pollution directly causes dementia.
“I don’t want the general public to panic,” Kitazawa said. Rather, more research on this connection will be needed.
Kitazawa and Finch’s labs are among those working to understand the connection. Finch said his research has shown that air particles from fossil fuels can increase the brain’s level of amyloid protein, which is associated with Alzheimer’s disease.
“Overall, I think that there’s reason to take the association seriously,” he said.
Other studies have detected a similar connection between particular pollution and dementia.
A 2016 study of 6.6 million people from Canada found that people who lived within 164 feet of a major road were 7% more likely to develop dementia than those who lived 984 feet away, where fine particulate matter levels were up to 10 times lower.
A study in England found that adults living with the highest annual concentration of air pollution had 1.4 times the dementia risk as those living with the lowest annual concentration.
A study in California found that older women exposed to higher levels of air pollution did worse on cognitive tests than those exposed to lower levels of pollution. A scan also showed shrinkage in areas of the brain that typically are affected by Alzheimer’s.
Even without a definite link between dementia and particle pollution, study co-author Zhang believes people should take action now to limit their exposure because of all the other health problems that air pollution can cause.
Many countries have created laws and incentives to reduce air pollution, but almost the entire global population breathes air that exceeds World Health Organization air quality limits, and the number of “very unhealthy” and “hazardous” air quality days has grown over the years, in large part because of the climate crisis. In 2011 in the US alone, exposure to this kind of pollution resulted in 107,000 extra premature deaths for all causes, according to a recent study.
At an individual level, steps to reduce exposure include using air purifiers in the home and wearing masks if going outside amid wildfire smoke, Zhang said.
At a policy level, if it turns out that pesticides are the problem, governments could restrict use. “That’s good for the welfare, making it a global action to reduce people’s exposure,” Zhang said.
Study co-author Adar hopes the research will prompt broader changes.
“Hopefully, this is also one more reason that might motivate people to act on climate change and think about ways in which we can slow the progression of climate change,” she said. “We see so many tragic examples with what’s happening right now.”
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