People who live near major roads have higher rates of dementia, research published in the Lancet suggests.
As many as 11% of dementia cases in people living within 50m of a major road could be down to traffic, the study suggests.
The researchers, who followed nearly 2m people in Canada over 11 years, say air pollution or noisy traffic could be contributing to the brain's decline.
UK dementia experts said the findings needed probing but were "plausible".
Nearly 50 million people around the world have dementia.
However, the causes of the disease, that robs people of their memories and brain power, are not understood.
The study in the Lancet followed nearly two million people in the Canadian province of Ontario, between 2001 and 2012.
There were 243,611 cases of dementia diagnosed during that time, but the risk was greatest in those living closest to major roads.
Compared with those living 300m away from a major road the risk was:
- 7% higher within 50m
- 4% higher between 50-100m
- 2% higher between 101-200m
The analysis suggests 7-11% of dementia cases within 50m of a major road could be caused by traffic.
The researchers adjusted the data to account for other risk factors like poverty, obesity, education levels and smoking so these are unlikely to explain the link.
Dr Hong Chen, from Public Health Ontario and one of the report authors, said: "Increasing population growth and urbanisation have 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."
Should I flee to the country?
There are already plenty of reasons to avoid the polluted air in our cities.
The World Health Organization says three million people die every year due to outdoor air pollution.
It increases the risk of stroke, heart disease, lung cancer and respiratory diseases.
However, it is not clear whether dementia should be added to the list.
This study "hints" there may be something going on, but does not definitively prove it.
So if you've not already been persuaded to up-sticks and move somewhere greener, then this study shouldn't change your mind.
But for Prof Rob Howard from UCL: "This study presents one more important reason why we must clean up the air in our cities".
The researchers suggest noise, ultrafine particles, nitrogen oxides and particles from tyre-wear may be involved.
However, the study looks only at where people diagnosed with dementia live. It cannot prove that the roads are causing the disease.
"This is an important paper," says Prof Martin Rossor, the UK's National Institute for Health Research director for dementia research.
He added: "The effects are small, but with a disorder with a high population prevalence, such effects can have important public health implications."
Risk factors for dementia
- Being female
- Type 2 diabetes
- Physical inactivity
- Unhealthy diet
Source: Alzheimer's Society
Prof Tom Dening, the director of the Centre for Dementia at the University of Nottingham, said the findings were "interesting and provocative".
He said: "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.
"Undoubtedly living in conditions of severe air pollution is extremely unpleasant and it is hard to suppose that it is good for anyone."
The best advice to reduce the risk of dementia is to do the things that we know are healthy for the rest of the body - stop smoking, exercise and eat healthily.
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