MRI Reveals Air Pollution Impact on Dementia Risk in Women

High levels of air pollution can put women in their 70s and 80s at a higher risk for developing a brain shrinkage that has been linked to Alzheimer’s disease, based on findings from their brain MRI scans.

In a study published in the Nov. 18 Neurology, investigators from the University of Southern California in Los Angeles outlined results that support putting more stringent policies in place to control air pollution levels and exposure.

“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 [Environmental Protection Agency] considers safe,” said group lead Diana Younan, Ph.D., USC senior research associate and Alzheimer’s Association Research Associate in a statement released by the American Academy of Neurology. “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.”

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In this study, the team set out to investigate whether fine particle air pollution – microscopic particles of chemicals, smoke, dust, or other pollutants suspended in the air – can cause structural changes in the brain, leading eventually to dementia or Alzheimer’s. These particles are smaller than 2.5 micrometers – 30 times smaller than the width of a human hair, the team said.

For their investigation, funded by the National Institute on Aging, the team enrolled 712 women with an average age of 78 who did not have dementia when the study began. All women underwent an MRI brain scan at the beginning of the study, as well as five years later. And, they all provided information about their health histories, race/ethnicity, education, employment, alcohol use, smoking, and physical activity.

By using residential addresses, the team was able to determine the average air pollution exposure for each participant for the three years prior to the first MRI scan. They, then, divided the study population into four equal groups based on their exposure ranges and compared those values to the average yearly exposure of up to 12 µg/m³ that the Environmental Protection Agency considers to be safe. The study group with the lowest exposure averaged 7-to-10 micrograms of fine particle pollution per cubic meter of air (µg/m³), and the highest group averaged 13-to-19 µg/m³.

At the five-year mark, the team applied a machine learning tool designed to identify patterns of brain shrinkage that are specific to an increased Alzheimer’s disease risk to their MRI scans and gave patients a score between 0 and 1 based on how similar their brain scans were to Alzheimer’s disease patterns. Higher scores indicated a patient had experienced more brain changes in areas that were vulnerable to the disease.

According to their analysis, women’s scores increased from 0.28 at the beginning of the study to 0.44 after five years. They also determined that for each 3 µg/m³ increase in air pollution exposure, patients experienced a 0.03 change. This finding indicated a greater extent of brain shrinking over the five years that was equivalent to a 24-percent increased Alzheimer’s disease risk. That increase remained constant, Younan pointed out, even after the team adjusted for age, education, employment, cardiovascular disease, high blood pressure, physical activity, and any other factors that could affect brain shrinkage.

Overall, Younan said, this study adds to the knowledge base around the effects of air pollution on the brain.

“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,” she said. “Our research suggests these toxins may disrupt brain structure or connections in the brain’s nerve cell network, contributing to the progression toward the disease.”

The study did have limitations, she said. Investigators only examined the brains of older women, and they only assessed regional fine particle pollution. They were also unable to assess participants’ air pollution exposure during middle-age and young adulthood. Consequently, additional research is needed.

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