During the past decade, a growing body of research has shown that air pollution harms older adults\u2019 brains, contributing to cognitive decline and dementia. What hasn\u2019t been clear is whether improving air quality would benefit brain health.<\/p>\n

Two studies published this year by researchers at six universities and the National Institute on Aging provide the first evidence of such benefits in an older population.<\/p>\n

One report, published in the Proceedings of the National Academy of Sciences,<\/a> found that the risk of dementia declined significantly in women 74 and older following a decadelong reduction in two types of air pollution: nitrogen dioxide<\/a>, a gaseous byproduct of emissions from motor vehicles, industrial sources, and natural events such as wildfires; and fine particulate matter<\/a>, a mix of extremely small solids and liquids arising from similar sources.<\/p>\n

A second report in PLOS Medicine<\/a>, relying 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 notable, the rate of cognitive decline was delayed by up to 1.6 years, depending on the test.<\/p>\n

Both studies are national in scope and account for other factors that could affect results, such as participants\u2019 socioeconomic status, neighborhood characteristics, preexisting medical conditions, and lifestyle choices such as smoking.<\/p>\n

What might explain their results? \u201cWe think that when air pollution levels are reduced, the brain is better able to recover\u201d from previous environmental insults, said Xinhui Wang, an assistant professor of research neurology at the University of Southern California\u2019s medical school. This hypothesis needs to be examined further in animal studies and through brain imaging, she suggested.<\/p>\n

There are several theories about how air pollution affects the brain. Extremely tiny particles \u2014 a human hair is at least 30 times as large as the largest particle \u2014 may travel from the nasal cavities to the brain via the olfactory (smell) system, putting the brain\u2019s immune system on high alert. Or, pollutants may lodge in the lungs, causing an inflammatory response that spreads and leads to the brain.<\/p>\n

Also, pollutants can damage the cardiovascular system, which is essential to brain health. (Links between air pollution, stroke, and heart disease are well established<\/a>.) Or tiny particles can cross the blood-brain barrier, wreaking direct damage. And oxidative stress may occur, releasing free radicals that damage cells and tissue.<\/p>\n

Older adults are especially vulnerable<\/a> to air pollution\u2019s harmful effects because of reduced lung capacity and pollutants\u2019 potential to exacerbate conditions such as respiratory illnesses and heart disease. Also, air pollution\u2019s effects accumulate over time<\/a>, and the longer people live the more risks they may encounter.<\/p>\n

Yet recognition of the potential cognitive consequences of air pollution is relatively recent. Following several small studies, the first national study<\/a> demonstrating a link between air pollution and cognition in a diverse sample of older men and women was published in 2014. It found that seniors living in areas with high levels of fine particulate matter were more likely to experience cognitive problems than people living in less polluted areas.<\/p>\n

Another study<\/a>, published a couple of years later, extended those findings by reporting that air pollution\u2019s cognitive effects are magnified in older adults living in disadvantaged neighborhoods where pollution levels tend to be highest. The chronic stress that residents of these neighborhoods experience may \u201cincrease the rate at which neurons are damaged by toxic challenges,\u201d the authors wrote.<\/p>\n

Air pollution is only one of many factors that influence cognitive decline and dementia, researchers agree, and results of this kind establish associations, not causation.<\/p>\n

Newer research suggests that older adults\u2019 cognition is affected even when exposures are below standards set by the Environmental Protection Agency<\/a>. \u201cWith older adults, there really is no level at which air pollution is safe,\u201d said Jennifer Ailshire<\/a>, an associate professor of gerontology and sociology at the University of Southern California.<\/p>\n

\u201cIt\u2019s important to keep on reducing the standards for these pollutants,\u201d said Antonella Zanobetti<\/a>, principal research scientist for environmental health at Harvard\u2019s T.H. Chan School of Public Health. With colleagues, she has a National Institute on Aging grant to study how air pollution affects the risk of Alzheimer\u2019s disease and related dementias among Medicare beneficiaries. In 2019, her work showed that higher levels of fine particulate matter are linked to more hospitalizations among older adults with dementia \u2014 a marker of disease progression.<\/p>\n