It should be fairly evident that living next to a busy road is not a great idea. In 2010, the Health Effects Institute examined over 700 studies and found sufficient evidence to link traffic pollution to childhood asthma, cardiovascular diseases and impaired lung function. Evidence linking traffic exposure to cancer was deemed inconclusive.
A new epidemiological study by Dr. Hong Chen and colleagues found that living near a major road is associated with an increased risk of dementia. The most common form of dementia is Alzheimer’s disease. The authors tracked all people between the ages of 20 and 85 living in Ontario, Canada (6.6 million people) between 2002 and 2012. Medical records were examined to determine whether they were diagnosed with dementia, Parkinson’s disease or multiple sclerosis. Their proximity to a major roadway was determined by postal code. The data analysis statistically controlled for alternative explanations such as socioeconomic status, education, smoking, diabetes and body mass index.
Living near a major highway was associated with dementia, but not Parkinson’s disease or MS. They identified 243,611 cases of dementia. Those who lived within 50 meters showed a 7% increase in the risk of dementia; those between 50 and 100 meters, a 4% increase; and those between 100 and 200 meters, a 2% increase. The greatest risk was found among those who had lived within 50 meters of the roadway for the entire decade, a 12% increase in the likelihood of dementia. According to their analysis, for those who lived within 50 meters, up to 1 in 10 cases (7-11%) of dementia were accounted for by traffic exposure.
Of course, this is a correlational study, and it is possible that other uncontrolled variables account for part of the effect. The fact that they measured diagnoses of dementia raises the possibility that the decision to seek treatment was a possible contaminant. However, it is not obvious why, in a country with universal healthcare, people living near major highways would be more likely to seek treatment.
The authors suggest that the effect may be due to a combination of air pollution and noise. They found that long-term exposure to two common pollutants, nitrogen dioxide and fine particulate matter, is related to dementia, but the effect is not large enough to explain their results. Studies have found toxic nanoparticles linked to Alzheimer’s disease in human brain tissue.
Noise is an environmental stressor that has been linked to declines in cognitive performance. Heat and crowding are two other stressors that may be plausibly linked to heavy traffic. A previous study found that children whose schools were located near major roadways in Barcelona showed smaller improvements in cognitive performance than other children from the same city.
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