Chen H, Kwong JC, Copes R, Tu K, Villeneuve PJ, van Donkelaar A, Hystad P, Martin RV, Murray BJ, Jessiman B, Wilton AS, Kopp A, Burnett RT. Living near major roads and the incidence of dementia, Parkinson's disease, and multiple sclerosis: a population-based cohort study. Lancet. 2017 Feb 18;389(10070):718-726. Epub 2017 Jan 5 PubMed.

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  1. The paper reports the findings of a study conducted in Ontario, Canada, using linkage techniques across multiple databases. The study was possible because of the unique availability of routinely collected medical and environmental data for millions of residents. The study is an excellent example of the power of linking passively collected data over large populations or regions (records-linkage systems).

    The authors reported that living near major highways is associated with an increased risk of dementia, but not of Parkinson’s disease or multiple sclerosis, in a 10-year cohort study. The authors reported a "dose-response" risk for dementia: the closer the person lived to a major roadway, the higher their risk. The mechanism remains unclear, but the authors suggest it might relate to air pollution or to the effects of noise or stress on the brain. Even though the magnitude of the relative risk was moderate (7 to 12 percent increased hazard ratios for living less than 50 meters from a major road), up to 7-11 percent of the cases of dementia that developed during the study in persons who lived less than 50 meters from a major road was attributable to (or explained by) the traffic exposure. Therefore, if the findings are not due to some unrecognized confounding mechanisms, and can be replicated in additional studies, then they create a major global health concern for millions of people, as suggested by the editorial accompanying the paper.

    The authors of the accompanying editorial suggested that factors such as sex, body-mass index, and genetic variants may modify the response of specific individuals to air pollutants or to noise. In particular, they mentioned that women may be more sensitive than men.

    The findings from the Ontario study may relate to findings of decreased dementia incidence in recent years. A growing body of epidemiologic evidence indicates a decline in the incidence or prevalence of dementia in high income countries in the past 25 years. This decline is in part explained by the reduction of known cardiovascular risk factors or events, by improved education, and by increased wealth. However, part of the decline remains unexplained. It is conceivable that improved air quality in high income countries may have contributed to the decline. The changes in type of vehicles, availability of less polluting fuels, emission control technologies, and emission control policies may have impacted the level or air pollution for people living near major roads and in major urban centers in the more recent decades. If the hypothesis is correct, further emission control policies and traffic reduction interventions may impact the future risk of dementia. 

    View all comments by Walter A. Rocca

  2. The culprit may indeed be air pollutants. But there's a potential confounder that, I would suggest, is quite plausible. Who wants to live near a major highway? Not me, nor most people I would think. Money buys privilege, so real property values near highways are lower than those in the hinterlands. Ergo, lower-income people live nearer highways. Lower-income people have poorer access to healthcare and poorer health maintenance practices They smoke more and eat less heart-healthy diets, etc., etc. Could these factors alone not account for the increased incidence of dementia in such folks? I'm glad to hear that my friends Gail Li and Eric Larson are investigating this topic in the ACT cohort. The health records and exquisite personal data available in that cohort should help "clear the air" on whether the highway proximity finding is really attributable to pollutants. We all know that association does not equate to causality!

    View all comments by John Breitner

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  1. Dementia Risk Ticks Up Near Major Roadways