Dementia is more common in people who live within 50 metres of a major road than those who live further away, according to a study looking at 6.6 million people published in The Lancet. However, the study found no link between traffic exposure and Parkinson's disease or multiple sclerosis.

Previous research has suggested that air pollution and traffic noise may contribute to neurodegeneration, with one study finding that living near a road was associated with reduced white matter and lower cognition.

Almost all people (95%) in the study lived within one kilometre of a major road and half lived within 200 metres of one. Over the study period, more than 243,000 people developed dementia, 31,500 people developed Parkinson's disease and 9,250 people developed multiple sclerosis.

The researchers also found that long-term exposure to two common pollutants (nitrogen dioxide and fine particulate matter) was associated with dementia but this did not account for the full effect, meaning other factors are also likely to be involved. These could include other air pollutants or noise from traffic.