Air Pollution and Incidence of Dementia
Ehsan Abolhasani, Vladimir Hachinski, Nargess Ghazaleh, Mahmoud Reza Azarpazhooh, Naghmeh Mokhber, Janet Martin
Abstract
<h3>Background and Objectives</h3> Studies of association between air pollution and incidence of dementia have shown discrepant results. The aim of this study was to evaluate the association between air pollution and dementia. <h3>Methods</h3> In this systematic review and meta-analysis, PubMed, MEDLINE, EMBASE, PsycINFO, Scopus, and Web of Science were searched and updated in August 2021. Population-based cohort studies that reported on hazard ratio (HR) of dementia in association with exposure to fine particulate matter (PM<sub>2·5</sub>), nitrogen oxides (NO<sub>X</sub>), nitrogen dioxide (NO<sub>2</sub>), or ozone (O<sub>3</sub>) in those aged >40 years were included. Data were extracted by 2 independent investigators. The main outcome was the pooled HR for dementia per increment of pollutant, calculated using a random-effects model. Results were reported in accordance with PRISMA guidelines. The protocol was registered in PROSPERO (registration number: CRD42020219036). <h3>Results</h3> A total of 20 studies were included in the systematic review, and 17 provided data for the meta-analysis. The total included population was 91,391,296, with 5,521,111 (6%) being diagnosed with dementia. A total of 12, 5, 6, and 4 studies were included in the meta-analyses of PM<sub>2·5</sub>, NO<sub>X</sub>, NO<sub>2</sub>, and O<sub>3</sub>, respectively. The risk of dementia increased by 3% per 1 μg/m<sup>3</sup> increment in PM<sub>2·5</sub> (HR, 1.03; 95% CI [1.02–1.05]; I<sup>2</sup> = 100%). The association between dementia per 10 μg/m<sup>3</sup> increment in NO<sub>X</sub> (HR, 1.05; 95% CI [0.99–1.13]; I<sup>2</sup> = 61%), NO<sub>2</sub> (HR, 1.03; 95% CI [1.00–1.07]; I<sup>2</sup> = 94%), and O<sub>3</sub> levels (HR, 1.01; 95% CI [0.91–1.11]; I<sup>2</sup> = 82%) was less clear, although a significant association could not be ruled out, and there was high heterogeneity across studies. <h3>Discussion</h3> Existing evidence suggests a significant association between exposure to PM<sub>2·5</sub> and incidence of dementia and nonsignificant association between dementia and NO<sub>X</sub>, NO<sub>2</sub>, and O<sub>3</sub> exposure. However, results should be interpreted in light of the small number of studies and high heterogeneity of effects across studies.