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Air Pollution May Be Linked to Cognitive Decline in Older Adults

Article In Brief

Research suggests that air pollution, particularly nitrogen dioxide, may be associated with memory loss, cognitive deficits, and a faster rate of cognitive decline among the elderly.

Air pollution in the form of nitrogen dioxide and particulate matter may be linked to deficits in cognition and memory, as well as a steeper rate of cognitive decline in older populations, according to new findings published in the April 8 online issue of Neurology.

Lead author Erin Kulick, PhD, and colleagues analyzed data from two community samples of people living in the Northern Manhattan area of New York City who were enrolled in two larger long-term studies: 5330 people with an average age of 75 were enrolled in the Washington Heights-Inwood Community Aging Project (WHICAP); and 1093 people with an average age of 70 were enrolled in the Northern Manhattan Study (NOMAS). Dr. Kulick is a postdoctoral research fellow at Brown University School of Public Health.

The researchers analyzed data from WHICAP participants at baseline and at six time points collected every 18 to 24 months thereafter. In the NOMAS sample, they analyzed data from baseline and at the five-year follow-up point. Both groups were ethnically and racially diverse with black, white and Hispanic participants.

They also measured air pollution in the form of nitrogen dioxide (N02; parts per billion), fine particulate matter less than 2.5 μm in diameter (PM 2.5; μg/m3), and respirable particulate matter (PM 10; μg/m3) linked to participants' residential addresses.

Dr. Kulick and her colleagues found that WHICAP participants were exposed to a yearly average of 32 parts per billion of N02, 13 micrograms per cubic meter (μg/m3) of fine particulate matter and 21 μg/m3 of respirable particulate matter. The U.S. Environmental Protection Agency (EPA) considers up to 53 parts per billion to be a safe level of yearly average exposure to nitrogen dioxide, up to 12 μg/m3 for fine particulate matter and up to 50 μg/m3 for respirable particulate matter.

The team found that among 5330 participants in WHICAP, participants living in areas with higher concentrations of these pollutants, especially nitrogen dioxide and fine particulate matter, performed worse on several measures of cognition and experienced more rapid cognitive decline: a 1 IQR increase in nitrogen dioxide was associated with a 0.22 SD lower global cognitive score at enrollment (95% confidence interval [CI], −0.30, −0.14) and 0.06 SD (95% CI, −0.08, −0.04) more rapid decline in cognitive scores between visits.

Results were similar for fine and respirable particulate matter and across functional cognitive domains. However, the researchers found no evidence of an association between pollution and cognitive impairment in the smaller NOMAS sample.

For those in the WHICAP sample, the association between nitrogen dioxide and the accelerated rate of cognition decline was comparable to approximately one year of aging. “With the global prevalence of dementia expected to reach almost 90 million individuals within the next 20 years, even a small reduction in ambient air pollution could have a substantial effect on cognitive health,” the authors wrote.

“We saw that air pollution levels didn't differ much across the two cohorts,” Dr. Kulick told Neurology Today. “They are located in the same area within Northern Manhattan in New York City. We thought that these differences might be due to some differences in characteristics of the two cohorts, including stringent selection characteristics into the NOMAS cohort, selecting out individuals with dementia, prior stroke, and any cardiovascular events which may have biased the results.”

In addition, she noted, the NOMAS cohort only had two follow-ups over a five- year time period in contrast to WHICAP with up to six visits over more than a decade of follow-up.

“There are several biological mechanisms through which we believe air pollution impacts the brain, with the strongest evidence surrounding pathways of systematic inflammation and oxidative stress,” Dr. Kulick said. “Both have been investigated in a series of animal studies, and it's likely that they are working in concert with each other to cause damage to the brain leading to cognitive decline.”

The authors reported several study limitations: Many of the processes leading to cognitive decline begin much earlier in life, and risk factors at midlife are more important for the process of accelerated cognitive decline. In addition, the study only measured pollution levels at participants' home addresses, while participants could have been exposed to varying degrees of pollution elsewhere. In addition, there may have been selection bias in the samples.

On an editorial accompanying the article, author Jennifer Weuve, MPH, ScD noted that that levels of nitrogen dioxide measured near the WHICAP participant homes fell below the US regulatory annual standard of 53 parts per billion. While noting that these measured exposure levels could be representative of higher levels from an earlier period, “adverse associations observed at subregulatory standard levels raises questions about whether those standards are sufficiently low to protect population health.”

Dr. Kulick said she is currently examining the impact of air pollution on dementia incidence to be able to look at whether air pollution has a clinically important impact on cognitive function. “I'm also looking at this question in some national datasets to see if variation in air pollution levels across the country clarify these associations,” she said.

Expert Commentary

“While no study can definitely state that air pollution negatively impacts cognitive performance, this study surely adds to the evidence that there is a link,” Andrew Petkus, PhD, assistant professor in the department of neurology at the University of Southern California, told Neurology Today.

Dr. Petkus said he'd like to see more research conducted to examine factors contributing to the variability in the association between pollution and cognitive decline.

“There have been mixed findings in this association with some studies finding a link between exposure to air pollution while other studies fail to find an association. The current study also produces mixed findings as they find a significant association in the larger WHICAP cohort but not in the NOMAS cohort. This could be due to methodological differences including how the samples were drawn, variability in the amount of air pollution they were exposed to, and differing number of follow-up visit,” he said. “Identifying factors that may minimize or increase the adverse association between exposure to air pollution and cognitive performance is important to help identify individuals who are at greatest risk. Identifying these moderating factors may also serve as targets for intervention to promote good cognitive health during aging.”

Dr. Petkus said that he believes that it is important for neurologists to look for environmental factors that can impact cognition in older adult patients. While exposure to air pollution is a factor that individuals have little personal control over, he said, people can do certain things to ameliorate the risk, such as maintaining a health diet, managing stress, and managing risk factors for cardiovascular and cerebrovascular disease.

“This paper provides additional evidence that chronic exposure to air pollution...potentially increases the risk for accelerated cognitive decline in elderly,” said Masashi Kitazawa, PhD., associate professor in the Center for Occupational and Environmental Health at the University of California, Irvine. She speculated that the lack of the association between cognitive decline and air pollution could have been due to smaller cohort size, fewer assessments, or other covariants that they did not include such as genetic risk.

“We should be aware that air pollution can be an evolving environmental risk for dementia and Alzheimer's disease. These findings will encourage more research to elucidate cellular and molecular basis of air pollution-mediated neurotoxicity linking to Alzheimer's disease,” she said.


Drs. Kulik, Petkus, and Kitazawa had no disclosures.

Link Up for More Information

• Kulick ER, Wellenius GA, Boehme AK, et al. Long-term exposure to air pollution and trajectories of cognitive decline among older adults Neurology 2020; Epub 2020 Apr 8.
    • Weuve J. Editorial: Are we ready to call exposure to air pollution a risk factor for dementia Neurology 2020; Epub 2020 Apr 8.