People with higher exposure to traffic-related air pollution are more likely to have high amounts of amyloid plaques in their brains associated with Alzheimer's disease, according to a study.

The study, published in the online issue of Neurology, does not prove that air pollution causes more amyloid plaques in the brain. It only shows an association.

Researchers from Emory University in Georgia, US, examined the brain tissue of 224 people who agreed to donate their brains at death to advance dementia research. The people had died at an average age of 76.

They looked at the traffic-related air pollution exposure based on the people's home addresses in the Atlanta area at the time of death.

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

Researchers then compared pollution exposures to measures of the signs of Alzheimer's disease in the brain: amyloid plaques and tau tangles.

They found that people with higher exposures to air pollution one and three years before death were more likely to have higher levels of amyloid plaques in their brains.

People with 1 microgram per cubic metre higher PM 2.5 exposure in the year before death were nearly twice as likely to have higher levels of plaques, while those with higher exposure in the three years before death were 87 per cent more likely to have higher levels of plaques.

"These results add to the evidence that fine particulate matter from traffic-related air pollution affects the amount of amyloid plaque in the brain," said Anke Huels, from Emory University.

"More research is needed to investigate the mechanisms behind this link."

Further, the researchers also looked at whether having the main gene variant associated with Alzheimer's disease, APOE e4, had any effect on the relationship between air pollution and signs of Alzheimer's in the brain.

They found that the strongest relationship between air pollution and signs of Alzheimer's was among those without the gene variant.

"This suggests that environmental factors such as air pollution could be a contributing factor to Alzheimer's in patients in which the disease cannot be explained by genetics," Huels said.