Alzheimer’s disease is the most common type of dementia. It is a progressive brain disease that causes a slow decline in memory, thinking and reasoning skills.

The causes of Alzheimer’s disease are very complicated, but one key part is the build-up of two substances inside the brain known as amyloid and tau.

These can then clump up and form tiny structures called plaques and tangles making it harder for the brain to work properly.

Now a study has revealed one potential contributor to higher amounts of amyloid and tau in the brain.

Research found that people exposed to higher levels of air pollution are more likely to display this sign of Alzheimer’s disease in their brain.

As part of the research, scientists from Emory University in Atlanta examined the brains of 224 people who donated their bodies to furthering dementia research after they died.

The team studied measures of Alzheimer’s in the participants’ brains – namely, the number of amyloid plaques and tau tangles.

They then compared this to the traffic-related air pollution exposure found at their homes.

It was discovered that those who had higher levels of air pollution exposure before death, which for this group, happened at an average age of 76, had higher levels of amyloid plaques in their brains.

More specifically, the results showed that people who had one microgram per cubic metre (µg/m3) higher PM2.5 exposure in the year before death were nearly twice as likely to have higher levels of plaques.

Those with 1 µg/m3 higher PM2.5 exposure in the three years before death were nearly nine times (87 percent) more likely to have higher levels of plaques.

The average level of exposure was 1.32 µg/m3 in the year before death and 1.35 µg/m3 in the three years before death.

This was measured via fine particulate matter (PM), which consists of pollutant particles of less than 2.5 microns in diameter (PM2.5).

Study author Dr Anke Huels said: “These results add to the evidence that fine particulate matter from traffic-related air pollution affects the amount of amyloid plaque in the brain.”

She stressed however that this does not mean air pollution causes more amyloid plaques in the brain – and by extension Alzheimer’s. It only proves an association between the two.

“More research is needed to investigate the mechanisms behind this link,” she explained.

The study, published in the journal Neurology, also investigated 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.

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

Dr Huels added: “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.”

She says further research will be needed to confirm the findings.