Living near a main road increases the risk of dementia, the first major research into air pollution and disease has shown.
A decade-long study of 6.6 million people, published in The Lancet, found that one in 10 dementia deaths in people living within 50 metres of a busy road was attributable to fumes and noise.
There was a linear decline in deaths the further people lived away from heavy traffic.
Air pollution is already known to contribute to the deaths of around 40,000 people in Britain each year by exacerbating respiratory and heart conditions, while previous research showed emissions can cause brain shrinkage.
But the new study by Canadian public health scientists is the first to find a link between living close to heavy traffic and the onset of dementia, a discovery described as "plausible" and "impressive" by British experts.
Dr Hong Chen, the lead author from Public Health Ontario, said:
"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."
"More research to understand this link is needed, particularly into the effects of different aspects of traffic, such as air pollutants and noise. Around 850,000 people suffer from dementia in Britain, and it is now the leading cause of death for both men and women."
There has been concern that air pollution could be driving neurodegenerative conditions for many years.
Last year British scientists discovered tiny magnetic particles produced by car engines and brakes in the brains of people diagnosed with Alzheimer's disease.
In the new study, the team tracked all adults aged between 20 and 85 living in Ontario for more than a decade from 2001 to 2012. They used postcodes to determine how close people lived to a main road and analysed medical records to see if they went on to develop dementia, Parkinson's disease or multiple sclerosis.
Over the study period, more than 243,000 people developed dementia, 31,500 people developed Parkinson's disease and 9,250 people developed multiple sclerosis.
While there was no association between living near a road and Parkinson's disease or multiple sclerosis, dementia risk reduced as people lived further from a main road, with a 7 percent higher risk in developing dementia among those living within 50 metres; a 4 percent higher risk at 50-100 metres and a 2 percent higher risk at 101-200 metres.
After 200 metres there was no increase.
Researchers believe that noise of traffic may also play a role in the raised risk as well as other urban pollution, which is often present near busy roads.