People who live near major roads have higher rates of dementia, research published in the Lancet suggests.
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.
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.
"More research to understand this link is needed, particularly into the effects of different aspects of traffic, such as air pollutants and noise."
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.
"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.