Over the past decade, a growing body of research has shown that air pollution harms the brains of older adults, contributing to cognitive decline and dementia. What is unclear is whether improving air quality would benefit brain health.
Two studies published this year by researchers from six universities and the National Institute on Aging provide the first evidence of such benefits in an older population.
A report, published in the Proceedings of the National Academy of Sciences, found that the risk of dementia dropped significantly among women aged 74 and older after a decade of reductions in two types of air pollution: carbon dioxide nitrogen, a gaseous byproduct of motor vehicle emissions, industrial sources and natural events such as forest fires; and fine particles, a mixture of extremely small solids and liquids from similar sources.
A second report published in PLOS Medicine, based on the same sample of more than 2,200 older women, found that lower levels of these pollutants were associated with a slower rate of cognitive decline. In areas where air quality improvement was most noticeable, the rate of cognitive decline was delayed by up to 1.6 years, according to the test.
Both studies are national in scope and take into account other factors that could affect results, such as participants’ socioeconomic status, neighborhood characteristics, pre-existing medical conditions, and lifestyle choices such as smoking. .
What could explain their results? “We believe that when air pollution levels are reduced, the brain is better able to recover” from previous environmental insults, said Xinhui Wang, assistant research professor of neurology at the University’s medical school. of Southern California. This hypothesis needs to be further investigated in animal studies and by brain imaging, she suggested.
There are several theories about how air pollution affects the brain. Extremely tiny particles – a human hair is at least 30 times larger than the largest particle – can travel from the nasal cavities to the brain via the olfactory (smell) system, putting the brain’s immune system on high alert. Or, pollutants can lodge in the lungs, causing an inflammatory response that spreads and leads to the brain.
Additionally, pollutants can damage the cardiovascular system, which is essential for brain health. (The links between air pollution, stroke, and heart disease are well established.) Or tiny particles can cross the blood-brain barrier, causing direct damage. And oxidative stress can occur, releasing free radicals that damage cells and tissues.
The elderly are particularly vulnerable to the harmful effects of air pollution due to reduced lung capacity and the potential of pollutants to worsen conditions such as respiratory disease and heart disease. Also, the effects of air pollution accumulate over time, and the longer people live, the more risk they may be at.
Yet recognition of the potential cognitive consequences of air pollution is relatively recent. Following several small studies, the first national study demonstrating a link between air pollution and cognition in a diverse sample of older men and women was published in 2014. It found that older adults living in areas with high particulate matter content were more likely to experience cognitive problems than people living in less polluted areas.
Another study, published a few years later, extended these findings by reporting that the cognitive effects of air pollution are amplified in older adults living in poor neighborhoods where pollution levels tend to be highest. . The chronic stress experienced by residents of these neighborhoods can “increase the rate at which neurons are damaged by toxic challenges,” the authors wrote.
Air pollution is just one of many factors that influence cognitive decline and dementia, researchers agree, and findings like these establish associations, not causation.
More recent research suggests that older people’s cognition is affected even when exposures are below the standards set by the Environmental Protection Agency. “With older adults, there’s really no level at which air pollution is safe,” said Jennifer Ailshire, associate professor of gerontology and sociology at the University of Southern California.
“It’s important to continue to lower standards for these pollutants,” said Antonella Zanobetti, senior environmental health researcher at Harvard’s TH Chan School of Public Health. Along with her colleagues, she has a grant from the National Institute on Aging to study how air pollution affects the risk of Alzheimer’s disease and related dementias in Medicare beneficiaries. In 2019, his work showed that higher levels of fine particles are linked to more hospitalizations in older people with dementia – a marker of disease progression.
Last year, in one of the largest US studies to date, a different group of researchers looked at the link between long-term exposure to fine particulate matter and nitrogen dioxide in 12 million Medicare beneficiaries. with Alzheimer’s disease and other forms of dementia. Exposure to high levels of these pollutants appears to accelerate already relatively advanced cognitive decline, leading to increased diagnoses, the researchers concluded.
In addition to population-scale studies, nearly 20 science labs around the world are studying how air pollution contributes to dementia in animals. At USC, Caleb Finch, a professor who studies the neurobiology of aging, is co-principal investigator for a five-year, $11.5 million grant from the National Institute on Aging to study how air pollution in urban areas informs the risk of dementia and accelerates brain aging.
Among the questions that Finch says need to be addressed include: Which areas of the brain seem most vulnerable to airborne pollutants? When are people most at risk? How long does the damage last? Is recovery possible? And are lifestyle interventions such as diet and exercise helpful?
“The main point is that we now realize that Alzheimer’s disease is very sensitive to environmental effects, including air pollution,” Finch said.
Recognizing this, the Lancet Commission on Dementia Prevention, Intervention and Care in 2020 added air pollution to a list of modifiable risk factors for dementia and estimated that up to 40% of Worldwide cases of dementia could be prevented or delayed if these risk factors were addressed. .
For his part, Ailshire is optimistic that public policy can make a difference. From 2000 to 2019, she noted, average annual fine particulate pollution decreased by 43% nationwide due to efforts to improve air quality. “I’m hopeful that these efforts will continue,” she told me.
What can seniors concerned about air pollution do for themselves?
On very hot days, go for a walk in the morning instead of the afternoon, when ozone levels are higher, said Dr. Anthony Gerber, a pulmonologist at National Jewish Health, a Denver medical center specializing in respiratory diseases. Ozone, a toxic gas, forms when various chemicals interact with sunlight and heat.
If you live in the western United States, where wildfires scatter fine particles have become more common, “wear a KN95 mask” on days when fires affect air quality in your area, said Gerber. Also, if you can afford it, consider buying air purifiers for your home, he advised, noting that fine particles can enter homes that aren’t properly sealed.
To check the air quality levels in your area, go to AirNow.gov, recommended by Ailshire. “If it’s a high-risk day, it might not be the day to go out and do some heavy yard work,” she said.
But don’t stay indoors all the time and become overprotective. “Being outdoors and exercising is really important for older people,” Gerber said. “We don’t want older people to end up sick because they breathe in a lot of particles, but we also don’t want them to become inactive and stuck at home.”
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