Air pollution emerges as a direct risk factor for Alzheimer’s disease: Study
Researchers tracked health data over nearly two decades.
In a study of nearly 28 million older Americans, long-term exposure to fine particle air pollution raised the risk of Alzheimer’s disease. That link held even after researchers accounted for common conditions like high blood pressure, stroke and depression.
Fine particle air pollution, known as PM2.5, consists of tiny particles in the air that come from car exhaust, power plants, wildfires, and burning fuels, according to the American Lung Association. They are small enough to travel deep into the lungs and even reach the bloodstream.
The research, conducted at Emory University and published in PLOS Medicine, tracked health data over nearly two decades to explore whether air pollution harms the brain indirectly by causing high blood pressure or heart disease, which, in turn, leads to dementia.
However, these "middleman" conditions accounted for less than 5% of the connection between pollution and Alzheimer’s, the research found.
The researchers say this suggests that over 95% of the Alzheimer’s risk comes from the direct impact of breathing in dirty air, likely through inflammation or damage to brain cells.
"The relationship between PM2.5 and AD [Alzheimer’s disease] has been shown to be pretty much linear," Kyle Steenland, a professor in the departments of environmental health and epidemiology at the Rollins School of Public Health at Emory University, and senior author of the study, told ABC News. “The reason this is particularly important is that PM2.5 is known to be associated with high blood pressure, stroke and depression – all of which are associated with AD. So, from a prevention standpoint, simply treating these diseases will not get rid of the problem. We have to address exposure to PM2.5.”
Even when conditions like high blood pressure are perfectly managed, the threat from air pollution remains, he added.
And, while the risk exists for everyone, the data show that those who have previously had a stroke are the most vulnerable -- though exactly why the brain is more susceptible to environmental toxins after a stroke remains an open question, according to Dr. Zaldy Tan, professor of neurology and director of the memory and aging program at Cedars-Sinai Medical Center in Los Angeles.
“There may be a direct effect in certain people whereby the harmful byproducts of particulate matter bypass the blood-brain barrier. There may also be an indirect effect where exposure to fine particulate matter increases the risk for conditions like high blood pressure, which then causes inflammation in the brain, " Tan told ABC News. “Or it may be both.”
Because this is such a large study, following nearly 3 million Alzheimer’s cases, Steenland said that it strengthens the case for improving air quality as a meaningful way to prevent dementia, especially for people in lower-income areas.
"The fact is disadvantaged communities tend to have more exposure to PM2.5," explained Steenland. “Though it is hard to say why.”
Stricter air quality standards at a community or national level could lead to meaningful reductions in Alzheimer’s cases, the researchers said. But they also noted that individual choices alone cannot offset the health effects of long-term exposure to polluted air.
Tan explained that there are still things people can do to protect themselves from the harms of dirty air.
His number one recommendation? Exercise.
"Because it addresses several of the other risk factors, including obesity, high blood pressure and diabetes,” he said. But he advised paying attention to air quality ratings and exercising indoors when air quality is poor.
He also advised investing in a high-quality air filter and checking your current air filter if you already have one. On high-pollution days, check your local air quality report and limit time near busy roads, avoid going outside during peak traffic hours, and keep windows closed during bad air days, he recommended.
Ultimately, there are numerous factors, both genetic and environmental, that contribute to the risk of Alzheimer’s disease, Steenland said. People can be vulnerable at both the individual and community levels.
“The best thing that people can do is educate themselves about these risks,” said Steenland.
Ishani D. Premaratne, MD, is an integrated plastic & reconstructive surgery resident and member of the ABC News Medical Unit.
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