Long-term air pollution exposure may raise Alzheimer’s risk mainly through direct effects on the brain, a study of US Medicare records suggests.
The research followed more than 27.8m US Medicare recipients aged 65 and over from 2000 to 2018, comparing estimated pollution exposure with later Alzheimer’s diagnoses.
Researchers focused on PM2.5, very fine particles smaller than 2.5 micrometres that can enter the bloodstream and travel to organs including the brain.
The team, led by Dr Yanling Deng of Emory University in Atlanta, Georgia, found higher PM2.5 exposure was linked to a greater risk of Alzheimer’s disease
The association was slightly stronger among people with a history of stroke, while high blood pressure and depression appeared to make little difference.
Dr Deng said: “Overall, the findings suggest that air pollution contributes to Alzheimer’s disease mostly through direct pathways rather than through other chronic health conditions.’
“However, people with a history of stroke may be especially susceptible to the harmful effects of air pollution on brain health.”
Dr Mark Dallas, associate professor in cellular neuroscience at the University of Reading, who was not involved in the study, said: “This long-term study, which tracked 28m adults, suggests that breathing tiny particles of air pollution may slightly increase the risk of Alzheimer’s disease.
“People who have had a stroke might be somewhat more sensitive to these effects, although the increase in risk remains small.”
“Because the research relies on broad pollution estimates and medical records, there are important limitations, but the findings align with growing evidence that air pollution is a modifiable risk factor for dementia.
“Overall, the study reinforces a simple idea: what we breathe over many years can shape how our brains age.”
Matt Loxham, professor of respiratory biology and toxicology at the University of Southampton, said stroke-related damage to blood vessels in the brain could increase their “leakiness”, potentially making it easier for particles or inflammatory molecules to enter.
The blood-brain barrier is a protective lining that normally helps keep harmful substances out.
“Following a stroke, damage to the blood vessels of the brain may increase the leakiness of the vessels (termed the blood-brain barrier), which could potentially facilitate entry of particles or inflammatory molecules into the brain,” he said.
“Alternatively, it is possible that the injury caused by the stroke may render the brain more susceptible to other effects of inhaled pollution.
“However, the mechanism through which any effect of PM2.5 on Alzheimer’s disease is caused is likely to be complex and multifactorial.
“Indeed, it is notable in this study here that the majority of the association of PM2.5 with Alzheimer’s disease was seen even in individuals without previous history of stroke.
“It might be that the mechanisms through which effects occur are in place in otherwise healthy individuals, but just heightened, or acting on already injured tissue, following stroke.”
Loxham said more work was needed to establish how pollution affects the brain, whether some pollutants are more harmful than others, and who is most at risk.