Long-term air pollution exposure and incident dementia: a systematic review and meta-analysis.
Level 3 - non-randomized controlled study
Systematic review and meta-analysis of observational cohort studies
PubMed 40716448 · doi:10.1016/S2542-5196(25)00118-4
What was done
The authors systematically searched eight electronic databases (MEDLINE, Embase, Cochrane Library, CINAHL, Global Health, PsycINFO, Scopus, Web of Science) through Oct 23, 2023. They included primary observational studies of adults (aged ≥18 years) assessing quantitative associations between long-term (≥1 year) outdoor air pollution exposure and subsequent physician-diagnosed dementia. Effect estimates were harmonized to prespecified exposure increments and combined using inverse-variance weighted random-effects meta-analyses when three or more studies evaluated an exposure-outcome pair. Risk of bias and evidence certainty were evaluated using the Office of Health Assessment and Translation tool.
What was found
From 15,619 records, 51 studies met inclusion criteria and 32 were included in meta-analyses. Long-term exposure was significantly associated with incident dementia for: - PM 2.5: pooled adjusted HR 1.08 (95% CI 1.02–1.14) per 5 μg/m³ increase (21 studies, n = 24,030,527; I² = 95%) - Nitrogen dioxide: pooled adjusted HR 1.03 (95% CI 1.01–1.05) per 10 μg/m³ increase (16 studies, n = 17,228,429; I² = 84%) - Black carbon/PM 2.5 absorbance: pooled adjusted HR 1.13 (95% CI 1.01–1.27) per 1 μg/m³ increase (6 studies, n = 19,421,865; I² = 97%) No significant association was found for: - Nitrogen oxides: pooled adjusted HR 1.05 (95% CI 0.97–1.13) per 10 μg/m³ increase (5 studies, n = 241,409; I² = 44%) - PM 10: pooled adjusted HR 1.52 (95% CI 0.80–2.87) per 15 μg/m³ increase (4 studies, n = 246,440; I² = 82%) - Annual ozone: pooled adjusted HR 0.82 (95% CI 0.35–1.92) per 45 μg/m³ increase (4 studies, n = 419,972; I² = 69%) Three of 32 studies (9%) had a probably high risk of bias in one domain; overall certainty was rated as moderate.
Why it matters
This review synthesizes recent evidence encompassing tens of millions of individuals, establishing that common ambient air pollutants (specifically PM 2.5, NO2, and black carbon) are modest but significant population-level risk factors for dementia.
Limits
All included studies were observational, which precludes causal determination and leaves potential for residual confounding. There was high between-study heterogeneity (I² = 84–97% for the significant pollutants). Analyses for PM 10, nitrogen oxides, and ozone included relatively few studies (4 to 5 each) with wide confidence intervals.
Cited by
- supports Epidemiological studies demonstrate that living in large cities with air pollution increases the risk of developing dementia.