Living close to a busy road increases the risk of Alzheimer's and other forms of dementia by up to 12%, a major study has found.
Scientists who tracked the progress of more than six million Canadian adults for 11 years found a clear trend with dementia incidence rising the nearer people lived to main roads.
Compared with those whose homes were more than 300 metres away, people living within 50 metres of heavy traffic had a 7% higher risk of developing dementia.
The increase in risk fell to 4% for residents living 50 to 100 metres from a busy road, and 2% at 101 to 200 metres. At more than 200 metres there was no evidence of a link with the condition.
Being a resident of a major city or never moving from an urban location close to a busy road appeared to boost the effect.
For people in both these categories, living less than 50 metres from a main road was associated with a 12% increase in dementia risk.
Although the differences are small, the findings add to recent evidence that long-term exposure to air pollution and traffic noise may contribute to brain shrinkage and mental impairment.
Other results from the study suggested a connection between dementia and exposure to two common traffic pollutants, nitrogen dioxide and fine particles of sooty material generated by diesel engines.
Lead scientist Dr Hong Chen, from Public Health Ontario, said: "Our findings show the closer you live to roads with heavy day-to-day traffic, the greater the risk of developing dementia. With our widespread exposure to traffic and the greater tendency for people to live in cities these days, this has serious public health implications.
"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."
The study, published in The Lancet medical journal, monitored the progress of every adult aged between 20 and 85 living in Ontario from 2001 to 2012. In total, around 6.6 million people took part in the research.
Post codes were used to determine how close people lived to a road, and rates of dementia, Parkinson's disease and multiple sclerosis (MS) obtained from medical records.
During the study period, the scientists identified 243,611 cases of dementia, 31,577 of Parkinson's and 9,247 of MS. No association was seen between proximity to busy roads and incidence of Parkinson's or multiple sclerosis.
British experts described the findings as "important" and "provocative", but stressed that they highlighted associations and did not demonstrate a causal link between exposure to traffic and dementia.
Dr David Reynolds, chief scientific officer at Alzheimer's Research UK, said: "There are 850,000 people in the UK living with dementia, and therefore interest in the risk factors driving the condition is high.
"This research is interesting in its identification of an association between dementia and major roads, but if any causal link exists between these two factors, it can't be confirmed by this study.
"Conditions like dementia have multiple risk factors including age and genetics, and other social factors relating to where people live in cities could also be playing a part here.
"This study has identified major roads and air pollutants from traffic as possible risk factors for dementia, a finding which will need further investigation before any firm conclusions can be drawn about the relative risks of air pollutants for dementia versus other risks such as smoking, lack of exercise or being overweight."
Professor Tom Dening, director of the Centre for Old Age and Dementia at the University of Nottingham, said: "Interesting and provocative findings .. It is certainly plausible that air pollution from motor exhaust fumes may contribute to brain pathology that over time may increase the risk of dementia, and this evidence will add to the unease of people who live in areas of high traffic concentration.
"It is unlikely that Ontario has the worst air quality in the world, so the risks might be even greater in cities that are habitually wrapped in smog.
"However, as the authors acknowledge here, the cause of these findings cannot be easily ascertained from a cross-sectional study. We simply can't tell if it's down to pollution or some other reason."
Rob Howard, Professor of Old Age Psychiatry at University College London, said: "Regardless of the route of causation, this study presents one more important reason why we must clean up the air in our cities."