Ambient air pollution and clinical dementia: systematic review and meta-analysis

Abstract

ObjectiveTo investigate the role of air pollutants in risk of dementia, considering differences by study factors that could influence findings.DesignSystematic review and meta-analysis.Data sourcesEMBASE, PubMed, Web of Science, Psycinfo, and OVID Medline from database inception through July 2022.Eligibility criteria for selecting studiesStudies that included adults (≥18 years), a longitudinal follow-up, considered US Environmental Protection Agency criteria air pollutants and proxies of traffic pollution, averaged exposure over a year or more, and reported associations between ambient pollutants and clinical dementia. Two authors independently extracted data using a predefined data extraction form and assessed risk of bias using the Risk of Bias In Non-randomised Studies of Exposures (ROBINS-E) tool. A meta-analysis with Knapp-Hartung standard errors was done when at least three studies for a given pollutant used comparable approaches.Results2080 records identified 51 studies for inclusion. Most studies were at high risk of bias, although in many cases bias was towards the null. 14 studies could be meta-analysed for particulate matter <2.5 µm in diameter (PM2.5). The overall hazard ratio per 2 μg/m3PM2.5was 1.04 (95% confidence interval 0.99 to 1.09). The hazard ratio among seven studies that used active case ascertainment was 1.42 (1.00 to 2.02) and among seven studies that used passive case ascertainment was 1.03 (0.98 to 1.07). The overall hazard ratio per 10 μg/m3nitrogen dioxide was 1.02 ((0.98 to 1.06); nine studies) and per 10 μg/m3nitrogen oxide was 1.05 ((0.98 to 1.13); five studies). Ozone had no clear association with dementia (hazard ratio per 5 μg/m3was 1.00 (0.98 to 1.05); four studies).ConclusionPM2.5might be a risk factor for dementia, as well as nitrogen dioxide and nitrogen oxide, although with more limited data. The meta-analysed hazard ratios are subject to limitations that require interpretation with caution. Outcome ascertainment approaches differ across studies and each exposure assessment approach likely is only a proxy for causally relevant exposure in relation to clinical dementia outcomes. Studies that evaluate critical periods of exposure and pollutants other than PM2.5, and studies that actively assess all participants for outcomes are needed. Nonetheless, our results can provide current best estimates for use in burden of disease and regulatory setting efforts.Systematic review registrationPROSPERO CRD42021277083.

Comparison of Particulate Air Pollution From Different Emission Sources and Incident Dementia in the US

“…Overall, our findings agree with the accumulated evidence on the association between total PM 2.5 and incident dementia . When results were scaled to 1 μg/m 3 , we observed a 2% increased rate of dementia for total PM 2.5 , which is consistent with the overall HR of dementia reported by a recently published meta-analysis among 51 studies identified from 2080 records . Our results are also comparable with 2 other studies of national cohorts in the US, including 1 of more than 12 million US Medicare beneficiaries (aged ≥65 years) .…”

“…[34][35][36] When results were scaled to 1 μg/m 3 , we observed a 2% increased rate of dementia for total PM 2.5 , which is consistent with the overall HR of dementia reported by a recently published meta-analysis among 51 studies identified from 2080 records. 37 Our results are also comparable with 2 other studies of national cohorts in the US, 34,36 including 1 of more than 12 million US Medicare beneficiaries (aged ≥65 years). 36 This finding is particularly notable given that our individual-level exposure estimates and standardized dementia classifications should have less measurement error than the zip code-level exposure estimates and administrative outcomes used in the Medicare beneficiaries study.…”

Air quality and mental health: evidence, challenges and future directions

“…23 There is emerging evidence of air pollution in cognitive function and dementia. 25 Brain imaging and animal studies could help to further elucidate relevant mechanisms. A recent systematic review suggests that depression, suicide and neurodevelopmental disorders (such as autism for pregnancy-related exposures) may be more common among those exposed to air pollution.…”

Air pollution and dementia

“…The plausible effect of ambient air pollutants on brain pathology, via cerebrovascular disease or inflammation,3 has contributed to an increasing volume of research exploring whether air pollution is a risk factor for dementia. In a linked BMJ paper,4 Wilker and colleagues (doi:10.1136/bmj-2022-071620) examined the association between ambient air pollution and risk of all-cause dementia in a systematic review of 51 longitudinal studies from North America, Europe, Asia, and Australia. They found that higher environmental exposure to fine particulate pollution—particulate matter of less than 2.5 microns in diameter (PM 2.5 )—was associated with an increased risk of dementia: for every 2 µg/m 3 increase in average annual PM 2.5 concentration, overall risk of dementia rose by 4% (hazard ratio 1.04 (95% confidence interval 0.99 to 1.09)).…”