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Growing Evidence Links Air Pollution to Alzheimer’s Disease

January 6, 2021

Two new studies tie air pollution to the brain changes of Alzheimer’s disease. The findings add to a growing body of evidence that living in areas with heavily polluted air, whether from car emissions, power plants, factories or forest fires, can accelerate the onset of Alzheimer’s in people who may already be vulnerable to developing the disease.

In one study, researchers at the University of California, San Francisco, looked at older Americans with dementia or milder memory and thinking problems, including mild cognitive impairment (or MCI), a cognitive state that often progresses to full-blown dementia. They found that the higher the levels of air pollution in their neighborhood, the greater the likelihood that their brains would contain high levels of amyloid plaques, a hallmark of Alzheimer’s disease.

Having amyloid plaques in your brain does not mean you will inevitably develop Alzheimer’s disease, but their presence increases your risk. Plaques can begin to develop decades before memory loss and other symptoms of Alzheimer’s disease become obvious.

The researchers, writing in JAMA Neurology, examined brain scans of 18,178 men and women, average age 75, living in various zip codes throughout the United States. Those who lived in the most polluted areas were 10 percent more likely to have amyloid plaques in their brains than those who lived in areas with the lowest levels of air pollution.

“Exposure in our daily lives to PM2.5” — the tiny particles, 30 times smaller than the width of a human hair, that are spewed out into the air by car and truck emissions, factory smoke and forest fires — “even at levels that would be considered normal, could contribute to induce a chronic inflammatory response,” said the study’s first author, Leonardo Iaccarino, of the UCSF Memory and Aging Center. “Over time, this could impact brain health in a number of ways, including contributing to an accumulation of amyloid plaques.”

While genetics, advancing age and other factors account for most cases of Alzheimer’s disease, another recent reported from an expert group, The Lancet Commission on Dementia Prevention, Intervention, and Care, reported that 40 percent of the cases of Alzheimer’s could be prevented or delayed by targeting 12 modifiable risk factors throughout life, including avoiding air pollution, as well as some more commonly known risks factors such as excessive alcohol intake and head injuries.

“I think it’s very appropriate that air pollution has been added to the modifiable risk factors highlighted by the Lancet Commission on dementia,” said Dr. Iaccarino. The group added air pollution to their established risk factors for Alzheimer’s include lack of formal schooling, hearing loss, smoking, high blood pressure, obesity, depression, lack of social contact and being sedentary.

Another study, published in the journal Neurology, found that older women who lived in neighborhoods with higher air pollution had more brain shrinkage than women who lived in less polluted areas. The researchers found that breathing in the tiny PM2.5 air pollutants was linked to shrinkage in areas of the brain vulnerable to Alzheimer’s disease. The analysis looked only at women, but presumably men would be similarly affected.

“Smaller brain volume is a known risk factor for dementia and Alzheimer’s disease, but whether air pollution alters brain structure is still being researched,” said study author Diana Younan, of the University of Southern California in Los Angeles. “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. Our research suggests these pollutants may disrupt brain structures or connections in the brain’s nerve cell network, contributing to the progression toward the disease.”

The increases in brain shrinkage remained after the researchers controlled for other factors that may raise Alzheimer’s risk, including age, education, employment, high blood pressure and physical activity.

“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 considers safe,” Dr. Younan added. “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.”

By ALZinfo.org, The Alzheimer’s Information Site. Reviewed by Marc Flajolet, Ph.D., Fisher Center for Alzheimer’s Research Foundation at The Rockefeller University.

Sources: Leonardo Iaccarino, PhD; Renaud La Joie, PhD; Orit H. Lesman-Segev, MD; et al: “Association Between Ambient Air Pollution and Amyloid Positron Emission Tomography Positivity in Older Adults with Cognitive Impairment.” JAMA Neurology, Nov. 30, 2020

Diana Younan, Xinhui Wang, Ramon Casanova, et al: “PM2.5 Associated With Gray Matter Atrophy Reflecting Increased Alzheimers Risk in Older Women.” Neurology, Nov. 18, 2020

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