Association of air pollution with dementia: a systematic review with meta-analysis including new cohort data from China

It remains unclear whether a total exposure to air pollution (AP) is associated with an increased risk of dementia. Little is known on the association in low- and middle-income countries. Two cohort studies in China (in Anhui cohort 1402 older adults aged ≥ 60 followed up for 10 years; in Zhejiang cohort 6115 older adults followed up for 5 years) were conducted to examine particulate matter - PM_2.5 associated with all dementia and air quality index (AQI) with Alzheimer's disease, respectively. A systematic literature review and meta-analysis was performed following worldwide literature searched until May 20, 2020 to identify 15 population-based cohort studies examining the association of AP with dementia (or any specific type of dementia) through PubMed, MEDLINE, PsycINFO, SocINDEX, CINHAL, and CNKI. The cohort studies in China showed a significantly increased relative risk (RR) of dementia in relation to AP exposure; in Anhui cohort the adjusted RR was 2.14 (95% CI 1.00-4.56) in people with PM_2.5 exposure at ≥ 64.5 μg/m³ versus <63.5 μg/m³ and in Zhejiang cohort the adjusted RR was 2.28 (1.07-4.87) in AQI>90 versus ≤ 80. The systematic review revealed that all 15 studies were undertaken in high income countries/regions, with inconsistent findings. While they had reasonably good overall quality of studies, seven studies did not adjust smoking in analysis and 13 did not account for depression. Pooling all eligible data demonstrated that dementia risk increased with the total AP exposure (1.13, 1.08-1.19). Data analysis of air pollutants showed that the RR significantly increased with PM_2.5 (1.06, 1.03-1.10 in 2nd tertile exposure; 1.13, 1.07-1.19 in 3rd tertile versus 1st tertile), PM₁₀ (1.05, 0.86-1.29; 1.62, 0.60-4.36), carbon monoxide (1.69, 0.72-3.93; 1.52, 1.35-1.71), nitrogen dioxide (1.06, 1.03-1.09; 1.18, 1.10-1.28) and nitrogen oxides (1.09, 1.04-1.15; 1.26, 1.13-1.41), but not ozone. Controlling air pollution and targeting on specific pollutants would reduce dementia globally.