A study on six million adults in Canada indicated that living next to a crowded motorway or highway raises dementia risk by 7 percent.
Researchers from the University of Toronto and Public Health Ontario 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.
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. 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.
“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.
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.
Independent experts say the findings are important but stress that they highlight associations and do not demonstrate a causal link between exposure to traffic and dementia.