Researchers tracked 6.6 million people in Ontario, Canada, over the decade to 2012. They found that the closer people lived to busy roads, the greater their risk of dementia.

There was a 7 per cent higher risk of developing dementia among those living within 50 metres of a main road, a 4 per cent higher risk at 50-100 metres, a 2 per cent higher risk at 101-200 metres, and no increase in risk among those living more than 200 metres away.

While the study only highlights an association between the two, air pollution experts said it opened up "a crucial global health concern for millions of people" and warranted further investigation to see if preventative measures could be found.

Dr Hong Chen, lead author of the research published in The Lancet on Thursday, said busy roads could be a source of environmental stressors that may give rise to 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," he said.

"More research to understand this link is needed, particularly into the effects of different aspects of traffic, such as air pollutants and noise."

The researchers said long-term exposure to two common pollutants, nitrogen dioxide and fine particulate matter, was associated with dementia. Other scientists have also linked air pollution and traffic noise to reduced brain matter and lower cognition, but this is the first study to investigate the connection between living near heavy traffic and the onset of major neurodegenerative diseases.

In good news, the researchers found no link between living close to major roads and Parkinson's disease or multiple sclerosis.

While the research was designed to control for people's wealth, education levels, body mass index and smoking (meaning the link is unlikely to be explained by these factors), the researchers acknowledged that they could not eliminate all potential confounding factors.

Professor Michael Woodward, director of aged care research at Austin Health and a chief medical advisor to Alzheimer's Australia, said dementia was already linked to lower socio-economic status, chronic stress and insomnia - factors that might be more common among people living near main roads.

He said the study did not prove that traffic pollution caused dementia, and that there might be a knock-on effect of respiratory and cardiovascular problems linked to living in built-up environments. Dementia is already linked to cardiovascular problems.

Professor Woodward said the study should trigger more research, though, because, if a particular toxin was found to cause dementia, it could provide a new target for a preventative drug treatment.

However, he said it was too early to issue public health warnings about living near heavy traffic.

"People should not be panicking if they live near a busy road."

Three in 10 Australians over the age of 85 and almost one in 10 over 65 have dementia.