Dementia Risk Rises With Proximity to Major Roads

January 05, 2017

Living close to heavy traffic was associated with a higher incidence of dementia in a new large population-based cohort study. But no association was seen between proximity to heavy traffic and Parkinson's disease or multiple sclerosis.

The study, published online in The Lancet on January 4, showed a clear dose response in that the risk for dementia increased with the proximity to a major road, with a 7% increased risk when the distance from the road was less than 50 meters, a 4% increase at 50 to 100 meters, and a 2% increase at 100 to 200 meters. No increased risk was seen at over 200 meters.

"Our study suggests a role for traffic pollution in contributing to dementia," coauthor Ray Copes, MD, Public Health Ontario, Toronto, Canada, commented to Medscape Medical News. "This is not the first study to show a link between pollution and dementia, but it is the largest to date and it used the best methods to assess such associations."

The highest risk of all was seen in people who lived in an urban environment, close to a major road, and had lived there for a long time. "This was the most heavily exposed group and they showed a 12% increased risk of developing dementia," Dr Copes said.

"We believe this data, along with previous studies showing air pollution contributes to cardiovascular and respiratory disease, is enough to warrant action," he added.

This could include further controls on traffic-related emissions, planning future land use so that major roads are positioned away from residential areas, and improvements to buildings that are near major roads to protect against air pollution, he said. "It also has implications for people taking exercise outside — if you are running, walking, or cycling — try to avoid areas with heavy traffic."

"It would be foolish if we didn't consider such accumulating evidence on the health risk of air pollution when going forward in land use planning," Dr Copes added. "This is particularly important in the large cities in Asia where urban development is expanding quickly."

Lead author, Hong Chen, PhD, Public Health Ontario, added: "Increasing population growth and urbanization 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."  

Coauthor of an accompanying "Comment," Lilian Calderón-Garcidueñas, MD, University of Montana, Missoula, commented on the study for Medscape Medical News.

"This is a very important study because of its size — with the entire population of Ontario included," she said. "It shows a very straightforward result — the closer you live to a major highway the higher your risk of dementia.

"This has massive implications for some cities — such as Mexico City and New York — where almost everybody lives near a major road," she added. "In the US there are many schools have been built close to major highways for logistical reasons but we have to rethink this idea."

For the study, the researchers analyzed data on all adults aged 20 to 50 years (about 4.4 million; multiple sclerosis cohort) and all adults aged 55 to 85 years (about 2.2 million; dementia or Parkinson's disease cohort) who resided in Ontario, Canada, in 2001. Eligible patients were free of these neurologic diseases, had been Ontario residents for 5 years or longer, and were Canadian-born.

Their proximity to major roadways was based on their residential postal-code address in 1996. Incident diagnoses of dementia, Parkinson's disease, and multiple sclerosis were ascertained from provincial health administrative databases.

Almost all people (95%) in the study lived within 1 kilometer of a major road and half lived within 200 meters of one. Between 2001 and 2012, 243,611 incident cases of dementia were identified.

After adjustment for individual and contextual factors, such as diabetes, brain injury, socioeconomic status, education levels, and body mass index, the adjusted hazard ratio (HR) of incident dementia was 1.07 for people living less than 50 meters from a major traffic road (95% confidence interval [CI], 1.06 - 1.08), 1.04 (95% CI, 1.02 - 1.05) for 50 to 100 meters, 1.02 (95% CI, 1.01 - 1.03) for 101 to 200 meters, and 1.00 (95% CI, 0.99 - 1.01) for 201 to 300 meters vs farther than 300 meters (P for trend = .0349).

The associations were robust to sensitivity analyses and seemed stronger among urban residents, especially those who lived in major cities (HR, 1.12 for people living <50 meters from a major traffic road) and those who had never moved (HR, 1.12 for people living <50 meters from a major traffic road).

The study also identified 31,577 cases of Parkinson's disease and 9247 cases of multiple sclerosis, but no association between these conditions and proximity to a major road was seen.

Dr Copes pointed out that using records from the Canadian single-payor health system meant that patients with dementia could be identified with good accuracy. "We were also able to accurately assess exposure from information on postal addresses and we were able to ascertain that the dementia developed after exposure to pollution."

On the possible mechanism involved, he said: "We know the fine particles in exhaust fumes get into the lungs and the circulation and move around the body. We also know they cause oxidative stress and inflammation throughout the body. This is possibly the mechanism that contributes to dementia, which has a chronic inflammatory vascular component." He added that traffic-related noise may also play a role.

In the "Comment" piece, Dr Calderón-Garcidueñas and coauthor Rodolfo Villarreal-Ríos, Universidad del Valle de México, Mexico City, note that this study might also provide new insights into the mechanisms for the early development of oxidative stress, neuroinflammation, and neurodegeneration, and opportunities to prevent and ameliorate such harmful brain effects.

They write: "We have made little progress in understanding the pathogenesis of neurodegenerative disorders such as Alzheimer's disease. We desperately need novel integrative paradigms that cut across the molecular and cellular level to the individual and societal level. In light of these new research directions, we need to prioritise research funding, including understanding brain health from conception, the impact of epigenetic changes, and the early interaction between environment and genetics.

"We must implement preventive measures now, rather than take reactive actions decades from now."

The study was funded by Health Canada. The authors and editorialists have disclosed no relevant financial relationships.

Lancet. Published online January 4, 2017.  Abstract, Comment

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