, new research reveals. The findings have implications for individual behaviors, such as avoiding areas with poor air quality, but they also have implications for public policy, said study investigator, Xinhui Wang, PhD, assistant professor of research neurology, department of neurology, University of Southern California, Los Angeles.
“Controlling air quality has great benefits not only for the short-term, for example for pulmonary function or very broadly mortality, but can impact brain function and slow memory function decline and in the long run may reduce dementia cases.”
The findings were presented at the 2021 Alzheimer’s Association International Conference.
New approach
Previous research examining the impact of reducing air pollution, which has primarily examined respiratory illnesses and mortality, showed it is beneficial. However, no previous studies have examined the impact of improved air quality on cognitive function.
The current study used a subset of participants from the Women’s Health Initiative Memory Study-Epidemiology of Cognitive Health Outcomes (WHIMS-ECHO), which evaluated whether postmenopausal women derive cognitive benefit from hormone therapy.
The analysis included 2,232 community-dwelling older women aged 74-92 (mean age, 81.5 years) who did not have dementia at study enrollment.
Researchers obtained measures of participants’ annual cognitive function from 2008 to 2018. These measures included general cognitive status assessed using the Telephone Interview for Cognitive Status-modified (TICSm) and episodic memory assessed by the telephone-based California Verbal Learning Test (CVLT).
The investigators used complex geographical covariates to estimate exposure to fine particulate matter (PM2.5) and nitrogen dioxide (NO2), in areas where individual participants lived from 1996 to 2012. The investigators averaged measures over 3-year periods immediately preceding (recent exposure) and 10 years prior to (remote exposure) enrollment, then calculated individual-level improvements in air quality as the reduction from remote to recent exposures.
The researchers examined pollution exposure and cognitive outcomes at different times to determine causation.
“Maybe the relationship isn’t causal and is just an association, so we tried to separate the timeframe for exposure and outcome and make sure the exposure was before we measured the outcome,” said Dr. Wang.
The investigators adjusted for multiple sociodemographic, lifestyle, and clinical characteristics.
Reduced dementia risk
The analysis showed air quality improved significantly for both PM2.5 and NO2 before study enrollment. “For almost 95% of the subjects in our study, air quality improved over the 10 years,” said Dr. Wang.
During a median follow-up of 6.2 years, there was a significant decline in cognitive status and episodic memory in study participants, which makes sense, said Dr. Wang, because cognitive function naturally declines with age.
However, a 10% improvement in air quality PM2.5 and NO2 resulted in a respective 14% and 26% decreased risk for dementia. This translates into a level of risk seen in women 2 to 3 years younger.
Greater air quality improvement was associated with slower decline in both general cognitive status and episodic memory.
“Participants all declined in cognitive function, but living in areas with the greatest air quality improvement slowed this decline,” said Dr. Wang.
“Whether you look at global cognitive function or memory-specific function, and whether you look at PM2.5 or NO2, slower decline was in the range of someone who is 1-2 years younger.”
The associations did not significantly differ by age, region, education, APOE ε4 genotypes, or cardiovascular risk factors.
Patients concerned about cognitive decline can take steps to avoid exposure to pollution by wearing a mask; avoiding heavy traffic, fires, and smoke; or moving to an area with better air quality, said Dr. Wang.
“But our study mainly tried to provide some evidence for policymakers and regulators,” she added.
Another study carried out by the same investigators suggests pollution may affect various cognitive functions differently. This analysis used the same cohort, timeframe, and air quality improvement indicators as the first study but examined the association with specific cognitive domains, including episodic memory, working memory, attention/executive function, and language.
The investigators found women living in locations with greater PM2.5 improvement performed better on tests of episodic memory (P = .002), working memory (P = .01) and attention/executive function (P = .01), but not language. Findings were similar for improved NO2.
When looking at air quality improvement and trajectory slopes of decline across cognitive functions, only the association between improved NO2 and slower episodic memory decline was statistically significant (P < 0.001). “The other domains were marginal or not significant,” said Dr. Wang.
“This suggests that brain regions are impacted differently,” she said, adding that various brain areas oversee different cognitive functions.
Important policy implications
Commenting on the research, Rebecca Edelmayer, PhD, senior director of scientific engagement, Alzheimer’s Association, said she welcomes new research on environmental factors that affect Alzheimer’s disease.
Whereas previous studies have linked longterm air pollution exposure to accumulation of Alzheimer’s disease-related brain plaques and increased risk of dementia, “these newer studies provide some of the first evidence to suggest that actually reducing pollution is associated with lower risk of all-cause dementia,” said Dr. Edelmayer.
Individuals can control some factors that contribute to dementia risk, such as exercise, diet, and physical activity, but it’s more difficult for them to control exposure to smog and pollution, she said.
“This is probably going to require changes to policy from federal and local governments and businesses, to start addressing the need to improve air quality to help reduce risk for dementia.”
A version of this article first appeared on Medscape.com.