People living near busy roads have a higher chance of developing dementia, according to researchers in Canada.

A study published in The Lancet medical journal on Thursday found that people who lived within 50 metres of high-traffic roads had a 7 percent higher chance of developing dementia than those who lived more than 300 metres away from busy roadways.

"Air pollutants can get into the bloodstream and lead to inflammation, which is linked with cardiovascular disease and possibly other conditions such as diabetes," said Ray Copes, an environmental and occupational health expert at Public Health Ontario who conducted the study with colleagues from Canada's Institute for Clinical Evaluative Sciences.

"This study suggests air pollutants that can get into the brain via the bloodstream can lead to neurological problems."

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The study, led by Hong Chen from Public Health Ontario, found that long-term exposure to two common pollutants, nitrogen dioxide (NO2) and fine particulates, were associated with dementia but did not account for the full effect.

Chen's team analysed records of more than 6.5 million Ontario residents aged 20 to 85 and found 243,611 cases of dementia between 2001 and 2012.

Then they mapped residents' proximity to major roadways using postal codes.

The increase in the risk of developing dementia went down to 4 percent if people lived 50 to 100 metres from major traffic, and to 2 percent if they lived within 101 to 200 metres.

At more than 200 metres, the elevated risk faded away.

The team also explored links between living near major roads and Parkinson's disease and multiple sclerosis, two other major neurological disorders, but the findings suggested no increased risk of these.

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The scientists said their results could be used to help town and city planners take traffic conditions and air pollution into account as urban areas become more densely populated.

Dementia is caused by brain diseases, most commonly Alzheimer's disease, which result in the loss of brain cells and affect memory, thinking, behaviour, navigational and spatial abilities and the ability to perform everyday activities.

The World Health Organization estimated the number of people with dementia in 2015 at 47.5 million, and that total is rising rapidly as life expectancy increases and societies age.

The incurable condition is a leading cause of disability and dependency, and is starting to overtake heart disease as a cause of death in some developed countries.