Environmental factors and risks of cognitive impairment and dementia: A systematic review and meta-analysis

Dementia is a challenging neurodegenerative disease. This systematic review aimed to summarize natural, physical, and social environmental factors that are associated with age-related cognitive impairment and dementia. We systematically searched PubMed, EMBASE, Web of Science, and PsychINFO till January 11, 2021 for observational studies. The hazard ratio (HR), relative risk (RR), and odds ratio (OR) with 95% confidence interval (CI) were aggregated using random-effects methods. The quality of evidence for each association was evaluated. Of the 48,399 publications identified, there were 185 suitable for review across 44 environmental factors. Meta-analyses were performed for 22 factors. With high-to-moderate quality of evidence, risks were suggested in exposure to PM2.5 (HR=1.24, 95%CI: 1.17–1.31), NO2 (HR=1.07, 95%CI: 1.02–1.12), aluminum (OR=1.35, 95%CI: 1.14–1.59), solvents (OR=1.14, 95%CI: 1.07–1.22), road proximity (OR=1.08, 95%CI: 1.04–1.12) and other air pollutions, yet more frequent social contact (HR=0.82, 95%CI: 0.76–0.90) and more greenness (OR=0.97, 95%CI: 0.95–0.995) were protective. With low-to-very low quality, electromagnetic fields, pesticides, SO2, neighborhood socioeconomic status, and rural living were suggested risks, but more community cultural engagement might be protective. No significant associations were observed in exposure to PM10, NOx, noise, silicon, community group, and temperature. For the remaining 22 factors, only a descriptive analysis was undertaken as too few studies or lack of information. This review highlights that air pollutions, especially PM2.5 and NO2 play important role in the risk for age-related cognitive impairment and dementia. •Air pollutions, tobacco smoke and pesticides could elevate the risks of dementia.•Exposure to electromagnetic fields was found positive associated with dementia.•Solvents and aluminum were found hazard for dementia, while silicon might be benefit.•Living in rural area and more deprived neighborhood were adverse for dementia.•Residential greenness and social contacts were proposed as beneficial factors.