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 over 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.
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."
British experts described the findings as "important" and "provocative", but stressed that they did not demonstrate a causal link between exposure to traffic and dementia.