People who live close to major roads may have a small increased risk of developing dementia, a new study suggests.

Precious research has suggested that air pollution and traffic noise may adversely affect the brain, so Canadian scientists set out to determine if there was any link between living close to heavy traffic and the onset of major brain diseases, such as dementia and Parkinson's.

They tracked all adults aged between 20 and 85 who were living in Ontario in Canada - that is around 6.6 million people - between 2001 and 2012.

Using postcodes, they assessed how close people lived to various roads. They also analysed medical records to see who developed dementia, Parkinson's and multiple sclerosis.

The vast majority of people lived within one kilometer of a major road, and half lived within 200 metres of one.

The study found no link between living close to a major road and Parkinson's disease or multiple sclerosis. However, it did find an increased risk of dementia among those living very close to a major road.

This risk reduced the further away from the road they lived.

Overall, those living within 50 metres of a major road had a 7% increased risk of developing dementia. This fell to 4% if they lived 50-100 metres away, and 2% if they lived 100-200 metres away. There was no increased risk if they lived more than 200 metres away.

The researchers also found a link between dementia and long-term exposure to two common pollutants (fine particulate matter and nitrogen dioxide), however this did not fully account for the findings. This suggests other factors, such as traffic noise, could also play a role.

"Despite the growing impact of these diseases, little is known about their causes and prevention. Our study suggests that busy roads could be a source of environmental stressors that could give rise to the onset of dementia.

"Increasing population growth and urbanisation has placed many people close to heavy traffic, and with widespread exposure to traffic and growing rates of dementia, even a modest effect from near-road exposure could pose a large public health burden," commented lead researcher, Dr Hong Chen, of Public Health Ontario, Canada.

The team called for more research in this area to understand this possible link.

Details of these findings are published in the medical journal, The Lancet.