The Role of Greenness in Heat-Related Mortality: A Systematic Review and Meta-Analysis

Background: Rising temperatures and the frequency of heat waves are challenges that have an impact on the health and mortality of populations. A high level of greenness is emerging as a strategy to mitigate the effects of extreme heat. Objective: To assess whether higher levels of greenness are associated with reduced mortality compared with lower greenness during periods of extreme heat. Methods: In this systematic review and meta-analysis, we searched MEDLINE, SCOPUS, WEB OF SCIENCE, CINHAL, Environment Complete, and GreenFILE. Observational studies were included. Pooled risk ratios (RRs) with 95% confidence intervals (CIs) were calculated via a random-effects model. Results: The systematic review and meta-analysis included 11 articles. These cover data from China, France, Germany, Jordan, Switzerland, and the USA, as well as a global analysis covering 24 countries. The meta-analysis suggested that heat waves significantly increased mortality in areas with the lowest greenness (RR = 1.33; 95% CI: 1.15-1.55; I ² = 99.4%) and in areas with the highest greenness (RR = 1.18; 95% CI: 1.03-1.36; I ² = 97.9%). The pooled ratio of RR (RoR) indicated that the association was significantly stronger in low-greenness areas than in high-greenness areas (RoR = 1.11; 95% CI: 1.06-1.15; I ² = 90.5%), indicating that the heat-mortality association was 11% stronger in low-greenness areas. The pooled associations varied descriptively across subgroup analyses, although no statistically significant differences were observed between geographical regions or study settings. Conclusion: Higher temperatures were associated with increased mortality in both low- and high-greenness areas; however, the heat-mortality association was 11% stronger in low-greenness areas. These findings suggest that greater greenness may attenuate vulnerability to extreme heat, although the limited evidence base and considerable between-study heterogeneity warrant cautious interpretation.