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Improved air quality was tied to lower dementia risk and slower cognitive decline, studies presented at the Alzheimer's Association International Conference (AAIC) showed.

Dementia risk dropped by 26% in older women who lived in U.S. locations with greater reductions in traffic-related nitrogen dioxide (NO₂) and by 14% in areas with a greater decline in fine particulate matter (PM_2.5), said Xinhui Wang, PhD, of University of Southern California. These relationships occurred regardless of age, education, geographic region, APOE4 genotype, or cardiovascular risk.

Similar findings emerged in France, where reduced fine particulate matter concentrations were tied to a lower risk of all-cause dementia and Alzheimer's disease, reported Noémie Letellier, PhD, of University of California San Diego.

Moreover, long-term air pollution exposure was linked to higher plasma beta-amyloid levels in U.S. adults, according to research presented by Christina Park, MPH, of University of Washington in Seattle.

Last year, the Lancet Commission added late-life air pollution exposure to its list of key modifiable risk factors for dementia. "We've known for some time that air pollution is bad for our brains and overall health, including a connection to amyloid buildup in the brain," noted Claire Sexton, DPhil, of the Alzheimer's Association in Chicago, who wasn't involved with studies presented at AAIC.

"But, what's exciting is we're now seeing data showing that improving air quality may actually reduce the risk of dementia," Sexton said in a statement. "These data demonstrate the importance of policies and action by federal and local governments, and businesses, that address reducing air pollutants."

In her study, Wang followed 2,239 American women ages 74 to 92 who did not have dementia at enrollment in the Women's Health Initiative Memory Study-Epidemiology of Cognitive Health Outcomes (). was a study designed to assess the effects of hormone therapy in postmenopausal women; WHIMS ECHO was an extension that started in 2008.

Wang and her team included estimates of annual air pollutant exposures from 1996 to 2012 at participants' residential locations in their analyses. Age-equivalent effect sizes were calculated using a scale of 1.2 μg/m³ for PM_2.5 and 5.3 ppb for NO₂ (10% of EPA standards), Wang told MedPage Today.

From 2008 to 2018, participants had annual evaluations that included global cognitive function (assessed by the [TICSm]) and episodic memory (assessed by the telephone-based [CVLT]).

Over a median 6.1 years of follow-up, 398 incident cases of dementia were diagnosed. Women living in areas with reduced levels of NO₂ and PM_2.5 had significantly lower dementia risk, equivalent to levels seen in women 2.4 years younger.

During follow-up, general cognitive status and episodic memory declined significantly as women aged. TICSm and CVLT scores indicated that participants who lived in areas with greater air quality improvement experienced slower trajectories of decline, equivalent to women about 1 and 1.5 years younger, respectively. These associations didn't significantly differ by age, region, education, APOE4 genotype, or cardiovascular risk factors.

"Our findings are important because they strengthen the evidence that high levels of outdoor air pollution in later life harm our brains and also provide new evidence that, by improving air quality, we may be able to significantly reduce risk of cognitive decline and dementia," Wang said.

"The possible benefits found in our studies extended across a variety of cognitive abilities, suggesting a positive impact on multiple underlying brain regions," she added.

In the cohort of Bordeaux, Dijon, and Montpellier residents, Letellier and colleagues analyzed data from 7,051 people with a median age of 73. Overall, PM_2.5 concentrations declined from 1990 to 2000. All-cause dementia risk dropped by 15% (HR 0.85, 95% CI 0.76-0.95) and Alzheimer's disease risk fell by 17% (HR 0.83, 95% CI 0.72-0.94) for each μg/m³-decrease in fine particulate matter.

In the (GEMS), Park and co-authors evaluated 3,029 adults 75 and older who were dementia-free at baseline. The researchers averaged air pollution levels of NO₂, PM_2.5, and coarse particles (PM₁₀) at participants' residential addresses for up to 20 years before plasma beta-amyloid levels (Aβ1-40) were tested. Analyses were adjusted for age, race, sex, site, education, treatment, neighborhood deprivation index, alcohol intake, smoking, and other variables.

Higher levels of PM₁₀ over 10 years were associated with higher levels of plasma beta-amyloid; similar findings were seen for other contaminants. "These findings provide some of the first epidemiologic evidence suggesting long-term concentrations of PM_2.5, PM₁₀, and NO₂ are associated with increased Aβ1-40," the researchers noted.

All studies were based on observational data, and unmeasured confounders may have influenced results. Still, the findings may have wide-ranging implications: "In the context of climate change, massive urbanization, and worldwide population aging, it is crucial to accurately evaluate the influence of air pollution change on incident dementia to identify and recommend effective prevention strategies," Letellier said.