The results underline the need to enforce strict air pollution exposure protocols, a group led by Diana Younan, PhD, of the University of Southern California in Los Angeles said in a statement released by the American Academy of Neurology (AAN), publisher of the journal.
"Our findings have important public health implications, because not only did we find brain shrinkage in women exposed to higher levels of air pollution, we also found it in women exposed to air pollution levels lower than those the EPA [Environmental Protection Agency] considers safe," Younan said. "While more research is needed, federal efforts to tighten air pollution exposure standards in the future may help reduce the risk of Alzheimer's disease in our older populations."
Previous research has shown that air pollution is a risk factor for brain aging and ensuing dementia and Alzheimer's disease, the team noted. In the current paper, researchers sought to further explore the connection between air pollution exposure and neurological disease.
The investigation included 712 women (average age, 78) who did not have dementia at the start of the study. The women submitted information on their race/ethnicity, education, employment, alcohol use, smoking, and physical activity; each underwent brain MRI exams at the beginning of the study and at five-year follow-up.
Younan's team used participants' home addresses to estimate average air pollution exposure three years before the baseline MRI scans and created four groups based on pollution exposure ranges, with the lowest range being an average of seven to 10 micrograms of fine particle pollution per cubic meter of air (µg/m3) and the highest an average of 13 to 19 µg/m3. (The EPA states that average yearly exposures up to 12 µg/m3 are safe.)
At the five-year follow-up MRI exam, Younan and colleagues assessed signs of Alzheimer's disease by identifying brain shrinkage patterns on MRI using a machine-learning algorithm. Baseline MRI exams and follow-up exams were assigned scores to reflect any brain changes, with higher scores indicating more changes.
Overall, study participants' brain shrinkage scores changed from 0.28 at baseline MRI to 0.44 at five-year follow-up. Each 3 µg/m3 increase in air pollution exposure levels translated to an average increase of 0.03, equivalent to a 24% increase in Alzheimer's disease risk over five years -- a trend that persisted even when Younan and colleagues adjusted for age, education, employment, cardiovascular disease, high blood pressure, and physical activity.
"Our study found that women in their 70s and 80s who were exposed to the higher levels of air pollution had an increased risk of brain changes linked to Alzheimer's disease over five years," Younan said. "[This] suggests these toxins may disrupt brain structure or connections in the brain's nerve cell network, contributing to the progression toward the disease."
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