Air pollution from traffic is linked to some of the more severe forms of dementia, and could be a significant cause of the condition among those who are not already genetically predisposed to it, research suggests.

Research carried out in Atlanta, Georgia, found that people with higher exposure to traffic-related fine particulate matter air pollution were more likely to have high amounts of the amyloid plaques in their brains that are associated with Alzheimer’s.

The findings, which will alarm anyone living in a town or city, but particularly those living near busy roads, add to the harms already known to be caused by road traffic pollution, ranging from climate change to respiratory diseases.

A team of researchers from Atlanta’s Emory University set out to specifically investigate the effects on people’s brains of exposure the type of fine particulate matter known as PM2.5.

This consists of particles of less than 2.5 microns in diameter – about a hundredth the thickness of a human hair – suspended in the air, and is known to penetrate deep into living tissue, including crossing the blood-brain barrier. Traffic-related PM2.5 concentrations are a major source of ambient pollution in the metro-Atlanta area, and also in urban centres across the planet.

The Emory researchers examined the brain tissue of 224 people from Atlanta, 90% of whom had a diagnosis of some form of dementia, who had agreed to donate their brains to medical science after their deaths. They also investigated the traffic-related PM2.5 pollution exposure at subjects’ homes in the years leading up to their deaths.

The average level of exposure in the year before death was 1.32 micrograms per cubic metre (µg/m3) and 1.35 µg/m3 in the three years before death.

“We found that donors who lived in areas with high concentrations of traffic-related air pollution exposure, in particular PM2.5 exposure, had higher levels of Alzheimer’s disease neuropathology in their brain,” said Anke Huels, an assistant professor at Emory University in Atlanta, who was the lead author on the study.

“In particular, we looked at … a score that is used to evaluate evaluate amyloid plaques in the brain, in autopsy samples, and we showed that donors who live in areas with higher levels of air pollution, and also higher levels of amyloid plaques in their brain.”

There was a positive relationship between exposure to high levels of PM2.5 and levels of amyloid plaques in the brains of the subjects the team examined. They found that people with a 1 µg/m3 higher PM2.5 exposure in the year before death were nearly twice as likely to have higher levels of amyloid plaques in their brains, while those with higher exposure in the three years before death were 87% more likely to have higher levels of plaques.

Huels and her team also investigated whether having the main gene variant associated with Alzheimer’s disease, ApoE4, had any effect on the relationship between air pollution and signs of Alzheimer’s in the brain.

“We found that the association between In air pollution and severity of Alzheimer’s disease was stronger among those who did not carry an ApoE4 allele, those who did not have that strong genetic risk for Alzheimer disease,” Huels said. “Which kind of suggests that environmental exposures like air pollution may explain some of the Alzheimer’s risk in people whose risk cannot be explained by genetic risk factor.”

The findings are published in the 21 February 2024, online issue of Neurology®, the medical journal of the American Academy of Neurology.