It’s no secret that air pollution can contribute to a number of health conditions, including heart disease and respiratory conditions. Less widely known is that it could play a role in declining cognition as well.
In a study published in BMJ, researchers at the Harvard T. H. Chan School of Public Health provide what they call the “best estimate” of how much air pollution may be contributing to dementia risk. Analyzing more than 50 studies that tracked air pollutants and dementia cases, the scientists found that the risk of dementia increased by 4% for every 2 microgram/cubic meter increase of particulates in the air annually. When they focused on more rigorous studies that actively followed people by regularly testing them for dementia, instead of retrospectively mining their electronic health records, that risk increased to 17%-42%.
“This is building on the evidence of air pollution’s role in contributing to dementia,” says Marc Weisskopf, professor of environmental health and epidemiology at the Harvard T. H. Chan School of Public Health and lead author of the study. A Lancet report published in 2020 was the first to mention air pollution as a contributing factor to dementia, Weisskopf says. “In that report, it was notable that the experts had a very hard time placing numbers on the risk, because that kind of data didn’t exist. That’s partly why we conducted this analysis. The more we can provide clinicians with data to make them aware of this connection, the more people will understand how pollution can contribute to dementia risk.”
Most of the studies Weisskopf analyzed focused on the concentration of tiny particulates less than 2.5 micrometers in diameter (3% of the diameter of a human hair), which are the most dangerous to human health. Currently, the U.S. Environmental Protection Agency sets 12 microg/m3 (micrograms per cubic meter) as the annual acceptable standard, but in Jan. announced that it plans to revise that standard downward to 9 to 10 microg/m3. (The proposal is currently open for public comment before the agency sets its final policy.)
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It’s unclear how much of a difference that will make when it comes to pollution’s contribution to dementia, says Weisskopf, since the data suggest there is no safe threshold below which the risk goes to zero. Even with lower concentrations of particulates, “it’s still an issue,” he says.
Exactly how airborne particulate matter can contribute to dementia isn’t known, but experts have some leading theories. One is related to how pollution affects the heart, and how heart health in turn affects the brain. One pathway connecting the two involves inflammation; particulates can trigger inflammatory reactions in tissues like the heart and lungs, and once the cascade is unleashed and remains uncontrolled, brain cells also become vulnerable to inflammation’s damage. There is also increasing evidence that particulates may directly affect brain cells and prompt inflammation of brain nerves. “Neuroinflammation is a good response and there to protect cells,” says Weisskopf. “But if it goes on too long, it can cause damage, and that’s one of the strong contenders for explaining how pollution affects dementia risk.”
Even if the risk of dementia isn’t entirely eliminated by reducing the concentration of pollutants in the air, it may still lower the risk, so it’s important for doctors to start raising awareness of the potential role that pollution can play with their patients. “Part of what we are trying to do is bring more prominence to this,” Weisskopf says. “Hopefully this risk is more likely to be incorporated in [discussions about dementia] in the future.”
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