Dementia is a long term, gradual disease in which the brain degenerates leading to impairment in a person’s memory and ability to think. This often results in severe disruptions in their day to day life. Alzheimer’s disease makes up 50-70% of all cases of dementia and a definitive cause or treatment are still not known. Alzheimer’s affects around 5 million Americans and is the 6th leading cause of death. This disease primarily affects the elderly with the incidence rate increasing as people get older. There is known to be a strong genetic component to the disease that likely combines with environmental factors to cause destruction of brain matter and progression of Alzheimer’s. Environmental risk factors include history of head injuries, hypertension, and depression. Recent studies have suggested that living close to major roads, and therefore exposure to road pollution, is associated with loss of cognition and a reduction in brain size possibly leading to dementia. The main components of road pollution are particulate matter (PM) that are on 5 to 20 times smaller than the width of a human hair. These PM’s are known to cause inflammation in the lung and cardiovascular system and could damage the brain thereby contributing to the development of dementia and Alzheimer’s. To test this hypothesis, a team of researchers from Ontario collected population data on all adults aged 20-50 years or 55-85 years currently living in Ontario and measured how their proximity to major roads affected their risk of developing dementia later in life.
In total, data from 4.4 million adults between 20-50 years old and 2.2 million adults between 55-85 years old was used in the study. All the subjects resided in Ontario on April 1, 2001 and were free of any neurological disease at the time of the study. Between the years 2001 and 2012 243,611 patients developed dementia, 31,577 developed Parkinson’s disease, and 9247 developed multiple sclerosis. Using each person’s postal code, the scientists estimated the distance from their house to the nearest major roadway. Feeding this into a statistical model, it was found that people who lived 50 meters or closer to a major roadway were more likely to develop dementia (7% greater risk) than if you lived more than 300 meters away. There was also significant, but smaller risks, associated with living 50-100 meters away and 101-200 meters away (4% and 2% respectively). Interestingly, this result was true even after the researchers accounted for other variables like smoking, education, and BMI that could contribute to the development of dementia. In this model, living closer to a main road was not associated with an increased risk of developing Parkinson’s disease or multiple sclerosis. Among people living closer to a major road, their risk of developing dementia was greater if they never moved during the study (12% increased risk), if they lived in 6 major cities in Ontario (12% increased risk), or if they lived in Urban areas during the study (9% increased risk).
While these risks may seem small (only a couple of percent) the data shows that of dementia cases in people living close to roads, 7-11% of them are attributable to traffic pollution exposure. We don’t yet know how exposure to high traffic roads can contribute to someone’s risk of neurodegenerative disease like Alzheimer’s but it likely has to do with particulate matter triggering inflammation in the blood stream and the brain. The study also did not take into account the location of each person’s work which may also influence their roadway pollution exposure. City planning to allow for houses to be built away from high pollution roads may reduce the incidence of not only dementia but potentially other chronic health concerns. Additionally, moving away from fossil fuel dependent transportation will alleviate most of the road associated pollution that is causing the problems in this study.
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