Greater air quality improvement linked to slower cognitive decline in older women

Greater improvement in air quality was associated with slower cognitive decline in older, community-dwelling women, according to a longitudinal study in PLoS Medicine.

The findings were initially presented at the Alzheimer’s Association International Conference last year.

“We know that breathing in high levels of outdoor air pollution is bad for our brains and may actually increase the likelihood of developing dementia,” Diana Younan, PhD, MPH, previously a senior research assistant in the department of population and public health sciences at the University of Southern California, told Healio. “However, the studies looking at air pollution and cognitive decline haven't been as consistent.”

“Scientists have shown that improved long-term air quality extends life expectancy and saves lives in adults and can decrease asthma and improve lung function in kids,” she added. “However, no studies have examined whether improving air quality could also slow cognitive decline in older adults.”

Measuring cognitive decline, air quality

Using data from the Women’s Health Initiative Memory Study-Epidemiology of Cognitive Health Outcomes (WHIMS-ECHO) study, Younan and colleagues analyzed cognitive decline in 2,232 community-dwelling, postmenopausal women without dementia at enrollment in 2008 to 2012 over a median follow-up period of 6.2 years. Most women were non-Hispanic white (n = 2,042), and more than half were aged older than 80 years (n = 1,353). The researchers recorded sociodemographic information such as medical conditions and education.

The researchers assessed the cognitive status of all the women using a 16-item modified Telephone Interview for Cognitive Status (TICSm), and a subset of 1,721 patients who participated in the phone-based California Verbal Learning Test (CVLT) were assessed for episodic memory. The TICSm was administered a median of seven times to each participant during follow-up, and the CVLT was administered a median of six times.

Younan and colleagues geographically matched residential addresses to Environmental Protection Agency data on air quality. They assessed air quality using estimated annual concentrations of fine particulate matter (PM_2.5) and nitrogen dioxide (NO₂). These were combined to create 3-year averages, which were calculated immediately before and 10 years before enrollment in WHIMS-ECHO. A decrease between these time points constituted individual-level air quality improvement.

Improved air quality has 'universal' benefits for older women According to the researchers, there was significant mean air quality improvement over 10 years, with reductions of ambient PM_2.5 (mean ± standard deviation: 13.3 ± 2.7 to 10.6 ± 2 g/m³; P < .001) and NO₂ (15.7 ± 7.2 to 10.4 ± 4.9 parts per billion [ppb]; P < .001). Areas with high levels of air pollution 10 years before enrollment tended to have greater improvements in PM_2.5 (correlation, 0.67; P < .001) and NO₂ (correlation, 0.8; P < .001).

TICSm and CVLT scores were higher for women living in the Midwest and who had higher education or income, were currently employed, currently drinking alcohol, did not have hypertension and did not carry the apolipoprotein E (ApoE) e4 genotype, the researchers wrote.

Though mean TICSm and CVLT scores did not change much, results showed that greater air quality improvement was associated with slower cognitive decline (0.026 per year for improved PM_2.5 by each interquartile range [IQR] = 1.79 g/m³ reduction; 95% CI, 0.001-0.05; and 0.034 per year for improved NO₂ by each IQR = 3.92 ppb reduction; 95% CI, 0.01-0.06) and slower decline in episodic memory (0.07 per year for improved PM_2.5 by each IQR = 1.79 g/m³ reduction; 95% CI, 0.02-0.12; and 0.06 per year for improved NO₂ by each IQR = 3.97 ppb reduction; 95% CI, 0.005-0.12).

These TICSm and CVLT data showed women living in places with greater air quality improvement had slower cognitive decline equivalent to being 0.9 to 1.2 and 1.4 to 1.6 years younger, respectively.

“As seniors, people with lower levels of education, people living in certain areas across the U.S. and people with preexisting heart disease are affected more by air pollution; what surprised us was that these benefits were seen in older women of all ages, levels of education, geographic regions of residence and cardiovascular histories,” Younan said. “The Clean Air Act mandates that the Environmental Protection Agency sets air quality standards to provide a safe margin for sensitive populations and our results showed that the benefits may be universal in older women.”

Moving forward, the researchers suggested future studies examine the neuropathological processes that could explain the link between improved air quality and slower cognitive decline late in life.

Other research could focus on “whether air quality improvement could help older people preserve the structure of healthy brains and maintain their cognitive function, even before the onset of symptoms of dementia,” which the University of Southern California team will continue investigating, Younan said.