People who live in close proximity to high-traffic roadways appear to have a higher risk of dementia than those who live farther away, say researchers, suggesting that air pollution from vehicles may be a factor in the development of the neurological disease.
In a study published in this week's Lancet, researchers found that Ontario residents who lived within 50 metres of a highway or major road had a seven per cent increased likelihood of developing dementia compared to those who lived more than 300 metres away from such busy transportation routes.
That increased risk dropped to four per cent for those who lived 50 to 100 metres from major traffic, and to two per cent if they lived between 100 and 200 metres. At more than 200 metres there was no elevated risk of dementia, the study found.
"We found that the closer you lived to a major source of traffic, the higher the risk of dementia became," co-author Dr. Ray Copes, chief of environmental and occupational health at Public Health Ontario, has said.
"And we also found that people who had always lived close to a roadway had an even higher risk of developing dementia than people who lived there, but not as long," said Copes, noting that the likelihood of having dementia rose to 12 per cent among people who lived for an extended period within 50 metres of a high-volume road.
However, researchers did not find any link between living near a highway or busy road and the risk of developing two other chronic neurological conditions -- Parkinson's disease and multiple sclerosis.
The study, led by scientists at Public Health Ontario (PHO) and the Institute for Clinical Evaluative Sciences (ICES), examined health records for more than 6.5 million Ontarians aged 20 to 85. To be included in the study, subjects had to be free of any neurological disease.
Researchers went back five years, to 1996, and mapped individuals' proximity to major roadways using their postal codes. Ontario Health card renewal records, which include any changes in address, allowed them to track how long each person had lived in a given location.
Nineteen per cent of Ontario's population lives within 50 metres of a major roadway and just under half live within 200 metres, said Copes, adding that in a large and high-density urban centre such as Toronto that percentage would be even higher.
During the 2001-2012 followup study period, researchers identified almost 244,000 people aged 55 and older with dementia across the province.
"Little is known in current research about how to reduce the risk of dementia," said lead author Dr. Hong Chen, an environmental and occupational health scientist at PHO and an adjunct scientist at ICES.
"Our findings show the closer you live to roads with heavy day-to-day traffic, the greater the risk of developing dementia," he said. "With our widespread exposure to traffic and the greater tendency for people to live in cities these days, this has serious public health implications."
While the study by its design cannot prove that vehicular pollution is a direct cause of dementia -- it can only show an association between the two --previous research has found that air pollutants can get into the bloodstream and lead to inflammation, which has been linked to cardiovascular disease and other chronic conditions.
Fine particulate matter from air pollution has been found in the brains of deceased dementia patients, noted Chen.
"It doesn't prove cause and effect," Copes said of the study's finding, "but we think it's strongly suggestive of a role for exposure to traffic pollution in the development of dementia."
Dr. Morris Freedman, head of neurology at Baycrest Health Sciences in Toronto, called the study "very interesting."
But Freedman, who was not involved in the research, said whether air pollution from vehicle emissions is a factor in the development of dementia remains a good question.
"The answer is not so clear ... As with any science, what this means is they have an important result, but that result needs replication," he said, referring to the need for other researchers to look at populations under similar conditions to see if the findings hold true.
Writing in an accompanying commentary, Dr. Lilian Calderon-Garciduenas of the University of Montana said it's been well-established that chronic exposure to air pollutants in urban environments is associated with oxidative stress, neuroinflammation and neurodegeneration in both humans and experimental animals.
"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 ... We must implement preventive measures now, rather than take reactive actions decades from now."
Copes said such measures could include stricter vehicle emission controls, land-use policies mandating that residential and institutional buildings be erected at a distance from high-volume roadways and building designs that provide greater protection from air-borne pollutants.