If you live less than 50 meteres from a major road, you may be more likely to develop dementia.
That's what that results of a recent study looking at 6.6 million people has found, the first to investigate the link between living close to heavy traffic and the onset of major neurodegenerative diseases.
The observational study estimated that up to one in 10 of the cases of dementia among those who live within 50 metres of a major road "could be attributable to traffic exposure".
Research in the past has pointed the finger at air pollution and traffic noise as a contributor to neurodegeneration. One study even found living near a road was associated with reduced white matter and lower cognition.
The researchers in this latest study tracked all adults aged between 20 and 85 living in Ontario, Canada – 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 see if they went on to develop dementia, Parkinson's disease or multiple sclerosis.
Almost all people (95 per cent) 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 research found:
While there was no association between living near a road and Parkinson's disease or multiple sclerosis, dementia was more common the closer people lived to busy roads. The risk of developing dementia reduced as people lived further away from a main road – with a 7 per cent higher risk in developing dementia among those living within 50 metres, a 4 per cent higher risk at 50-100 metres, a 2 per cent higher risk at 101-200 metres and no increase in risk in those living more than 200 metres away.
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.
"Despite the growing impact of these diseases, little is known about their causes and prevention," said Dr Hong Chen from Public Health Ontario, Canada. "Our study suggests that busy roads could be a source of environmental stressors that could give rise to the onset of dementia".
Dr Chen says 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".
The study estimated air pollution exposure based on postcode, so does not account for each individual's exposure. Because the study is observational it cannot establish causality, but the study was designed to control for socioeconomic status, education levels, BMI and smoking meaning the link is unlikely to be explained by these factors.
Dr Chen says more research to understand the link is needed, especially looking at the effects of air pollutants and noise.