PEOPLE living close to busy roads are more likely to develop dementia than those who live further away, according to a new study which identifies long-term exposure to traffic pollutants such as nitrogen dioxide as a potential trigger for the disease.

Research published by the Lancet found that around one in 10 cases of dementia among people living within 50 metres (55 yards) of a motorway could be linked to traffic exposure.

An accompanying editorial warns that the increased risk of dementia for urban residents is a "crucial global health concern for millions of people".

Dr Lilian Calderón-Garcidueñas, of the University of Montana, said: "We must implement preventive measures now, rather than take reactive actions decades from now."

The Canadian study tracked all adults aged between 20 and 85 living in Ontario – approximately 6.6 million people – for over a decade from 2001 to 2012. They used postcodes to determine how close people lived to a road and analysed medical records to check whether they went on to develop dementia, Parkinson’s disease or multiple sclerosis.

Friends of the Earth Scotland previously warned that Scotland is facing an "air pollution health crisis" from dangerous levels of nitrogen dioxide and tiny sooty particles emitted from cars, buses and lorries in major cities. In May last year, the World Health Organisation named Glasgow as one of 11 urban areas in the UK breaching safe limits for air particulates, increasing the risk of stroke, heart disease, lung cancer, and chronic and acute respiratory diseases such as asthma.

Previous research has suggested that air pollution and traffic noise may also contribute to neuro-degeneration, but the Lancet study is the first to investigate the link between living close to heavy traffic and the onset of major neurodegenerative diseases.

Almost everyone (95 per cent) in the Ontario study lived within one kilometre - just over half a mile - of a major road, and half lived within 200 metres (220 yards) of one.

The results showed that the risk of developing dementia reduced the further someone lived from a motorway. People living within 50 metres of a major road were 7 per cent more likely than average to develop dementia, with the increased risk gradually declining to 4 per cent for those living 50-100 metres away, and 2 per cent in the 101-200 metres threshold. There was no increased risk for those living more than 200 metres from a motorway.

The study found no similar correlation in risk for MS or Parkinson's disease.

Long-term exposure to two common traffic pollutants - nitrogen dioxide and fine particulate matter - was associated with dementia but did not fully explained the increased risk, suggesting that additional factors such as other hazardous air pollutants or traffic noise are also to blame.

However, the study controlled for socioeconomic status, education levels, BMI and smoking, meaning the link is unlikely to be explained by these factors

Lead author Dr Hong Chen, of Public Health Ontario, said: Our study suggests that busy roads could be a source of environmental stressors that could give rise to the onset of dementia.

"Increasing population growth and urbanisation has 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."

Meanwhile, scientists in Scotland have found that a Mediterranean-style diet can stave off age-related brain shrinkage.

The study, published in Neurology, scanned the brains of 401 people in their 70s. Those whose diet was rich in vegetables, fruits, olive oil, beans and cereal grains with moderate amounts of fish, dairy products and wine, but low in red meat and poultry, retained significantly greater brain volume after three years than those with different eating habits.

Lead researcher Dr Michelle Luciano, of Edinburgh University, said: "As we age, the brain shrinks and we lose brain cells, which can affect learning and memory.

"This study adds to the body of evidence that suggests the Mediterranean diet has a positive impact on brain health."

However, the researchers said further studies were needed to investigate whether the diet protects memory and thinking, in addition to brain size.