Objectively measured environmental features and their association with cognition and dementia: A systematic review and meta-analysis.

Dementia affects millions of people worldwide. Since effective treatments are still lacking, it is important to identify factors that may help prevent dementia. Recent studies suggest environmental factors may affect dementia risk, but findings are inconsistent and often rely on subjective measures. This study evaluated the association between objectively measured environmental factors, such as air pollution and built environment features, and the risk of dementia and cognitive decline. We systematically reviewed studies that employed objective measures of environmental factors and reported their association with dementia risk and cognitive decline. Meta-analysis was performed to synthesize data on environmental exposures and the onset of dementia. Air pollution exposure was linked to higher dementia risk (PMx risk ratio 1.09; 95 % CI 1.06,1.12) (NOx risk ratio 1.10; 95 % CI 1.01,1.20) and cognitive decline, while exposure to park areas or green/blue spaces was generally associated with reduced dementia risk (risk ratio 0.94; 95 % CI 0.92,0.96) and slower cognitive decline. Living closer to major roads increased the risk of dementia (risk ratio 1.10; 95 % CI 1.06,1.13), and cognitive impairment. Street layouts with better connectivity and walkability are associated with a reduced risk of cognitive impairment. Access to local amenities, such as food stores, community centers, and healthcare amenities, supports cognitive health. These findings underscore the importance of considering environmental factors in dementia prevention and highlight the need for further research to clarify the role of urban design in supporting cognitive health.