Living near a main road increases the risk of dementia, the first major research into air pollution and disease has shown.

A decade-long study of 6.6 million people, published in The Lancet, found that one in 10 dementia deaths in people living within 50 metres of a busy road was attributable to fumes and noise. There was a linear decline in deaths the further people lived away from heavy traffic.

Air pollution is already known to contribute to the deaths of around 40,000 people in Britain each year by exacerbating respiratory and heart conditions, while previous research showed emissions can cause brain shrinkage.

But the new study by Canadian public health scientists is the first to find a link between living close to heavy traffic and the onset of dementia, a discovery described as "plausible" and "impressive" by British experts.