Researchers found that people living within 50 meters (164 feet) of such a road had a 7% greater risk of developing dementia.
The level of risk decreases proportionally, they say, with a 4% higher risk among people living 50 to 100 meters (328 feet) away and a 2% higher risk among people living 101 to 200 meters (656 feet, or about a 10th of a mile) away.
"There is a gradient of increased risk as you get closer to major roadways," said Ray Copes, chief of environmental and occupational health at Public Health Ontario, who co-led the study. "By the time you're 200 meters away, the risk is essentially down to baseline."
In the study, a road was defined as "major" based on daily traffic volume and was the equivalent of an interstate highway in the United States.
'Something different going on'
Previous research has suggested that exposure to air pollution and traffic noise could increase nerve degeneration within the brain. One recent study
found that certain particles common to air pollution could enter the brains of people who breathe them in.
With growing populations and greater urbanization taking place around the world, people are increasingly being forced to live in close proximity to major traffic arteries. Copes' team decided to investigate the effect this could have on the onset of major neurodegenerative diseases, namely dementia, multiple sclerosis and Parkinson's disease.
The team tracked adults between 20 and 85 years old, living in Ontario, for 10 years between 2002 and 2012. Every adult in the province was included, making up 6.6 million people, with their postcodes used to determine how close they lived to a major road and their medical records studied to find out whether they developed one of these three neurodegenerative conditions over the study period.
No association was seen between proximity to a major road and the development of Parkinson's disease or multiple sclerosis, but varying levels of increased risk -- determined by the level of proximity -- were discovered for dementia.
"There is something different going on with dementia," Copes said.
More than 47 million people have dementia worldwide, and 7.7 million new cases are estimated to occur each year, according to the World Health Organization
. There is no cure for dementia and no effective treatment, said Copes.
The researchers warn, however, that for now, they have seen only a link and cannot state that living near a major road causes an increased risk of dementia. Another limitation is the fact that although postcodes help determine distance from a major road, the level of exposure for each individual included in the trial could vary.
"A single study can never definitely prove cause and effect," Copes said, adding that the team adjusted for factors such as socioeconomic status, smoking, body mass index and education. "The link continues to exist."
Two common pollutants -- nitrogen dioxide and fine particulate matter -- were also seen to be associated with dementia risk, but the team believes that other factors are likely to be involved in this link, such as noise from traffic.
"There is no single cause for dementia ... but our study shows that one of the factors now appears to be exposure to traffic pollution," Copes said.
The team now plans to identify which pollutants are most responsible for the link and investigate the extent to which noise could explain the association.
"We want to understand what are the causes and how effective existing interventions are," said Hong Chen, associate professor in occupational and environmental health at the University of Toronto, who also co-led the study and wants to identify the potential impact of this newly identified risk.
A health concern for millions
"The robust observation of dementia involving predominantly urban versus rural residents opens up a crucial global health concern for millions of people," said professor Lilian Calderón-Garcidueñas
of the University of Montana, who wrote an accompanying article in the Lancet. "The health repercussions of living close to heavy traffic vary considerably among exposed populations, given that traffic includes exposures to complex mixtures of environmental insults," she said in a statement.
The findings add to the growing understanding of how air pollution affects human health, but others in the field stress the need for further insight.
"This study has identified major roads and air pollutants from traffic as possible risk factors for dementia, a finding which will need further investigation before any firm conclusions can be drawn about the relative risks of air pollutants for dementia versus other risks such as smoking, lack of exercise or being overweight," said David Reynolds, chief scientific officer at Alzheimer's Research UK.
"This research is interesting in its identification of an association between dementia and major roads, but if any causal link exists between these two factors, it can't be confirmed by this study," he added.
Though no link was identified between living near a major road and the development of multiple sclerosis or Parkinson's disease, some experts believe this may be due to these two conditions being less common.
It is "worth pointing out that the numbers of people developing MS and Parkinson's were much smaller than those developing dementia, so (numbers) may not have been large enough to show an effect if there was one," said professor Tom Dening, director of the Centre for Old Age and Dementia at the University of Nottingham.
But Dening added that attention should be given to the environments people are now being forced to live in. "This study does ask serious questions about the environments where many people live. Undoubtedly, living in conditions of severe air pollution is extremely unpleasant, and it is hard to suppose that it is good for anyone."
Rob Howard, professor of old age psychiatry at University College London in the UK, agreed. "Regardless of the route of causation, this study presents one more important reason why we must clean up the air in our cities," he said.
Copes believes that policies to reduce emissions in cities, better city planning that keeps residential areas away from major roadways, and designing buildings and ventilation systems to act as barriers to pollution are a few ways to alleviate the problem.
"There is a large burden associated with dementia," he said. "Prevention is really the key."