Millions of people who live near a motorway are at greater risk of dementia, warns new research.
The study shows dementia is more common in people who live within 50 metres of a motorway than those who live further away.
But researchers found no link between traffic exposure and Parkinson's disease or multiple sclerosis (MS).
The observational study, involving 6.6 million people and published in The Lancet, estimated that up to one in nine cases of dementia could be attributed to living near a major road, and that the link is strongest for those living closest to heavy traffic.
Previous research has suggested that air pollution and traffic noise may contribute to neurodegeneration.
But the paper is the first to investigate the link between living close to heavy traffic and the onset of major neurodegenerative diseases.
Researchers tracked all adults aged between 20 and 85 living in Ontario, Canada - around 6.6 million people - for more than a decade from 2001 to 2012.
They used postcodes to determine how close people lived to a road, and analysed medical records to see if they went on to develop dementia, Parkinson's disease or MS.
Almost everyone in the study (95 per cent) lived within 1km of a major road and half lived within 200m of one.
Over the study period, more than 243,000 people developed dementia, 31,500 Parkinson's disease and 9,250 MS.
While there was no association between living near a road and Parkinson's or multiple sclerosis, dementia was more common the closer people lived to busy roads.
Lead author Doctor Hong Chen said: "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.
"More research to understand this link is needed, particularly into the effects of different aspects of traffic, such as air pollutants and noise."