Exposure to air pollution as a potential contributor to cognitive function, cognitive decline, brain imaging, and dementia: A systematic review of epidemiologic research

Abstract

Background Dementia is a devastating condition typically preceded by a long prodromal phase characterized by accumulation of neuropathology and accelerated cognitive decline. A growing number of epidemiologic studies have explored the relation between air pollution exposure and dementia-related outcomes. Methods We undertook a systematic review, including quality assessment, to interpret the collective findings and describe methodological challenges that may limit study validity. Articles, which were identified according to a registered protocol, had to quantify the association of an air pollution exposure with cognitive function, cognitive decline, a dementia-related neuroimaging feature, or dementia. Results We identified 18 eligible published articles. The quality of most studies was adequate to exemplary. Almost all reported an adverse association between at least one pollutant and one dementia-related outcome. However, relatively few studies considered outcomes that provide the strongest evidence for a causal effect, such as within-person cognitive or pathologic changes. Reassuringly, differential selection would likely bias toward a protective association in most studies, making it unlikely to account for observed adverse associations. Likewise, using a formal sensitivity analysis, we found that unmeasured confounding is also unlikely to explain reported adverse associations. Discussion We also identified several common challenges. First, most studies of incident dementia identified cases from health system records. As dementia in the community is underdiagnosed, this could generate either non-differential or differential misclassification bias. Second, almost all studies used recent air pollution exposures as surrogate measures of long-term exposure. Although this approach may be reasonable if the measured and etiologic exposure windows are separated by a few years, its validity is unknown over longer intervals. Third, comparing the magnitude of associations may not clearly pinpoint which, if any, pollutants are the probable causal agents, because the degree of exposure misclassification differs across pollutants. The epidemiologic evidence, alongside evidence from other lines of research, provides support for a relation of air pollution exposure to dementia. Future studies with improved design, analysis and reporting would fill key evidentiary gaps and provide a solid foundation for recommendations and possible interventions.

Cognitive impacts of ambient air pollution in the National Social Health and Aging Project (NSHAP) cohort

“…Examples include an improved measure of individual socio-economic status in addition to our inclusion of education and median household income of the census tract. However, based on recent analysis of similar studies, further adjustment of using these improved measures of SES is unlikely to substantially affect our estimates of associations between PM 2.5 and cognitive decline (Power et al 2016). Lastly, there are only two waves of data with which to assess mediation and only one wave of data with the CCFM test to assess cognition.…”

The effects of air pollution on individual psychological distress

“…We used a one-year lagged measure of pollution to reflect the temporal ordering of our focal relationship and because we expected the effects of air pollution to be lagged or cumulative rather than instantaneous. Moreover, given the lack of evidence to support a specific lag period, we conceptualized a one-year lag as a proxy for longer-term exposure, as previous studies have also done 48 .…”

Neighborhood social stressors, fine particulate matter air pollution, and cognitive function among older U.S. adults

“…An increasing number of studies have also identified neighborhood-level physical and social stressors that may have an adverse impact on cognitive health in older adults. There is growing evidence, for instance, that older adults living in areas with higher concentrations of outdoor air pollution have worse cognitive function and are at greater risk of cognitive decline (Power et al 2016). Several studies have also linked neighborhood social stressors, such as poverty and disorder, to poor cognitive function (Wu, Prina, and Brayne 2014).…”