Air pollution may directly increase the risk of Alzheimer’s by damaging the brain, according to a major new study involving nearly 28 million older people in the US.
Researchers found that long-term exposure to fine particulate air pollution, known as PM2.5, was linked to a higher risk of developing Alzheimer’s disease even after accounting for common health conditions such as high blood pressure, stroke and depression.
The study, published in PLOS Medicine, analysed data of 27.8 million US Medicare recipients aged 65 and above between 2000 and 2018, and compared exposure to PM2.5 – microscopic particles small enough to enter the bloodstream – with later diagnoses of Alzheimer’s.
While air pollution was already known to increase the risk of conditions such as hypertension and stroke, which in turn raise dementia risk, the researchers found that these illnesses explained only a small proportion of the association. The overall link between pollution exposure and Alzheimer’s remained largely unchanged after accounting for them.
“In this large national study of older adults, we found that long-term exposure to fine particulate air pollution was associated with a higher risk of Alzheimer’s disease, largely through direct effects on the brain rather than through common chronic conditions such as hypertension, stroke, or depression,” the researchers wrote.
The association was slightly stronger among people who had previously experienced a stroke, suggesting that pre-existing vascular damage could increase vulnerability.
“Our findings suggest that individuals with a history of stroke may be particularly vulnerable to the harmful effects of air pollution on brain health, highlighting an important intersection between environmental and vascular risk factors,” the researchers said.
Independent experts say the findings reinforce the growing evidence that polluted air likely affects the brain “in more direct ways, not just through these illnesses”.
“This forthcoming study adds weight to a growing idea: air pollution is not only a ‘heart-lung’ problem, but can act directly on the brain in ways that plausibly accelerate neurodegeneration,” Ryu Takechi, associate professor at the Curtin Medical Research Institute, said.
“The finding that the association with Alzheimer’s risk appears to be driven largely by direct brain effects, is consistent with what we and others have been seeing in mechanistic studies.”
Professor Ashley Bush, clinical lead of the mental health mission at The Florey, said the biological mechanism was plausible.
“Air pollutants contain chemicals that increase oxidative stress in the brain,” Dr Bush said. “These tiny pollutant particles can pass from the blood to the brain, causing chemical damage.”
Alzheimer’s is the most common form of dementia and affects around 57 million people worldwide. The researchers say improving air quality can play an important role in reducing future dementia risk, particularly as populations age and exposure to fine particle pollution from traffic, industry and wildfire smoke continues.
