Association between all-cause mortality and locally-defined extreme heat events: A global systematic review and...

Extreme heat events are a growing health threat, but their impact is heterogeneous because different settings have different levels of heat adaptation. Previous reviews have assessed morbidity and mortality as a function of meteorological conditions such as air temperature. We aimed to conduct the first systematic review and meta-analysis of the association between meeting local definitions of extreme heat events and risks of hospitalization and mortality overall and by population segment. We searched PubMed, Web of Science, Scopus, ScienceDirect, and Google Scholar on February 05, 2025, with no restrictions on language or publication date. Data extraction and risk of bias assessment were conducted by multiple reviewers. We estimated the risk ratio (RR) for hospitalization and mortality when meeting local definitions of extreme heat events using a random effects model to account for heterogeneity. In subgroup analysis, we examined variations by date, global region, country income group, extreme heat event definition type (absolute thresholds, percentile-based, composite indices), and population characteristics including age, socioeconomic context, and urban versus rural settings. In sensitivity analysis, we assessed the robustness of results by excluding lower-quality studies and applying alternative regression models. From 6015 initial records, 21 studies (n = 126,930,288 individuals) met inclusion criteria. The pooled RR for mortality associated with extreme heat events was 1.24 (95 % CI: 1.06-1.46) with substantial heterogeneity across studies (I² = 99.8 %), which was explored through subgroup, meta-regression, and sensitivity analysis. The meta-regression showed a significant increase in RR over time (p < 0.05). No significant publication bias was detected (Begg's test, p = 0.458). In subgroup analysis, higher RRs were observed in studies from Europe (RR range: 1.16-4.24) and low- and middle-income countries, in older adults (RR range: 1.16-2.24), in urban populations, in older populations, and in studies using absolute temperature thresholds or composite indices. Findings were similar when excluding lower-quality studies and applying alternative regression models. Extreme heat events were significantly associated with mortality, with risks increasing over time and elevated among older adults and urban populations. Municipal and health authorities should prioritize extreme heat adaptation, as mortality is likely to increase as the climate warms.