Air pollution harms billions of people worldwide. Pollutants are produced from all types of combustion, including motor vehicles, power plants, residential wood burning, and forest fires so they are found everywhere. One of the most dangerous of these, fine particulate matter, is 20 to 30 times smaller than a strand of human hair. Their tiny size allows them to be easily inhaled into the body, causing a number of adverse health effects. Over the past few decades, it has become widely recognized that outdoor air pollution is detrimental to respiratory and cardiovascular health, but recently scientists have come to acknowledge the damage it may cause on the brain as well.
A growing body of scientific evidence from around the world has shown that there may also be a link between fine particulate matter, cognitive performance, and Alzheimer’s disease and related dementias. The mechanisms by which fine particulate matter may lead to Alzheimer’s disease are unclear. Since animal studies have shown synaptic damage and neuronal loss resulting from exposure to ambient particles, could fine particulate matter be causing structural changes to the human brain, eventually leading to memory decline and Alzheimer’s disease?
Recently researchers used data from 998 women from the Women’s Health Initiative who were aged 73 to 87 and had up to two brain scans five years apart. Researchers scored women’s brain scans on the basis of their similarity to patterns of grey matter atrophy using a machine learning tool that had been trained to learn these patterns via brain scans of people with Alzheimer’s disease. Higher scores indicated that a woman’s brain structure was more similar to someone with Alzheimer’s disease. They also collected women’s addresses and built a mathematical model that allowed them to estimate the daily outdoor fine particulate matter levels where these women lived throughout the study period. When researchers combined all of the above information they found a significant association between higher exposure to fine particulate matter, physical brain changes, and memory problems, even before any symptoms of Alzheimer’s disease become apparent. More precisely, fine particulate matter was associated with gray matter atrophy in brain areas where Alzheimer’s disease neuropathologies are thought to first emerge, and those changes were connected to declines in episodic memory performance. These findings could not be explained by age, race, geographic region, income, education, employment status, smoking, alcohol use, physical activity, or cardiovascular risk factors.
It appears that fine particulate matter is associated with increased risk of Alzheimer’s disease and related outcomes. This identifies one way air pollution may be linked to memory decline.
While some risk factors for Alzheimer’s disease are non-modifiable (e.g., age; genetics), several risk factors, such as environmental exposures, may be modifiable through lifestyle changes. Since there are currently no treatments for Alzheimer’s disease, it is becoming increasingly important to identify modifiable risk factors of the disease. Air pollution appears to be one such risk.
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