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.