Is Air Pollution Associated with Increased Risk of Dementia? A Meta-Analysis of Epidemiological Research

There are prevailing inconsistencies in epidemiological research about air pollution being a risk factor for dementia.

We performed meta-analyses to calculate the pooled estimates of the risk of developing dementia due to air pollution exposure.

We performed a systematic search in PubMed, Google Scholar, The Cochrane Library, and J-GATEPLUS databases for peer-reviewed epidemiological studies reporting the risk of developing all-cause dementia, cognitive decline, Alzheimer's disease (AD), and vascular dementia (VaD) due to exposure to particulate matter with an aerodynamic diameter less than 2.5 μm (PM_2.5) and less than 10 μm (PM₁₀), ozone (O₃), carbon monoxide (CO), nitrogen dioxide (NO₂), nitrogen oxides (NO_X) and sulfur dioxide (SO₂) from the beginning until December 2020. Meta-analysis was performed by adopting the random-effects model using Meta-XL.

In all-cause dementia, the pooled hazard ratio (HR) for PM_2.5 and NO₂ exposure was 1.03 [(95%CI: 1.01-1.06; I² = 99% (P < 0.001)] and 1.00 [(95%CI: 1.00-1.01; I² = 96% (P < 0.001)], respectively. The pooled HR for NO_X was 1.00 [(95%CI: 1.00-1.01; I² = 61% (P = 0.05)]. In AD, the pooled HRs for PM_2.5 and O₃ was 1.08 (95%CI: 1.01-1.15; I² = 99% (P < 0.001)]) and 1.02 (95%CI: 0.96-1.08; I² = 100% (P < 0.001)], respectively. In VaD, the pooled HR for PM_2.5 exposure was 1.03 (95%CI: 1.01-1.06; I² = 91% (P < 0.001)]. In NO₂/NO_X, the results were found to be equivocal. Meta-analysis could not be performed in cognitive decline because of wide variations in assessments methods.

The present study showed exposure to PM_2.5 as a risk factor for all-cause dementia, AD, and VaD and exposure to O₃ as a possible risk factor for AD. These findings can be used for policy measures and further research.