Combined heat and humidity is frequently described as the main driver of human heat-related mortality, more so than dry-bulb temperature alone. While based on physiological thinking, this assumption has not been robustly supported by epidemiological evidence. By performing the first systematic comparison of eight heat stress metrics (i.e., temperature combined with humidity and other climate variables) with warm-season mortality, in 604 locations over 39 countries, we find that the optimal metric for modelling mortality varies from country to country. Temperature metrics with no or little humidity modification associates best with mortality in ~40% of the studied countries. Apparent temperature (combined temperature, humidity and wind speed) dominates in another 40% of countries. There is no obvious climate grouping in these results. We recommend, where possible, that researchers use the optimal metric for each country. However, dry-bulb temperature performs similarly to humidity-based heat stress metrics in estimating heat-related mortality in present-day climate.

Air Health Science Division Heatlh Canada Ottawa Canada

Cabot Institute for the Environment University of Bristol Bristol UK

Centre for Statistical Methodology London School of Hygiene and Tropical Medicine London UK

Centre on Climate Change and Planetary Health London School of Hygiene and Tropical Medicine London UK

Climate and Environmental Physics Physics Institute University of Bern Bern Switzerland

Climate Research Foundation Madrid Spain

Department of Computational Hydrosystems Helmholtz Centre for Environmental Research GmbH UFZ Leipzig Germany

Department of Economics Ca' Foscari University of Venice Venice Italy

Department of Epidemiology Instituto Nacional de Saúde Dr Ricardo Jorge Lisbon Portugal

Department of Public Health Environments and Society London School of Hygiene and Tropical Medicine London UK

Faculty of Environmental Sciences Czech University of Life Sciences Prague Czech Republic

Forecast Department European Centre for Medium Range Weather Forecast Reading UK

Institute of Atmospheric Physics Czech Academy of Sciences Prague Czech Republic

Institute of Social and Preventive Medicine University of Bern Bern Switzerland

Oeschger Center for Climate Change Research University of Bern Bern Switzerland

School of Epidemiology and Public Health Faculty of Medicine University of Ottawa Ottawa Canada

School of Geographical Sciences University of Bristol Bristol UK

Spanish Consortium for Research on Epidemiology and Public Health Spain

Φ Lab European Space Agency Frascati Italy

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Regional variation in the role of humidity on city-level heat-related mortality