Cases of the illness could be reduced by cleaning up dirty air in cities and towns, researchers suggest

Living in a polluted area increases the risk of dementia by up to 40 per cent, the first British study of its kind has found.

Thousands of cases of the illness could be prevented every year by cutting traffic fumes, said researchers who have added to growing evidence that dirty urban air can damage the brain.

Polluted air is known to cause lung and heart problems as tiny soot particles and chemicals such as nitrogen dioxide (NO2) pass deep into the body.

Research is also increasingly linking traffic fumes to thinking problems. Last year a Canadian study of 2.2 million people concluded that those who lived continuously near a busy road were 12 per cent more likely to get dementia.

Scientists now say that Britain’s