The rising humid heat is regarded as a severe threat to human survivability, but the proper integration of humid heat into heat-health alerts is still being explored. Using state-of-the-art epidemiological and climatological datasets, we examined the association between multiple heat stress indicators (HSIs) and daily human mortality in 739 cities worldwide. Notable differences were observed in the long-term trends and timing of heat events detected by HSIs. Air temperature (Tair) predicts heat-related mortality well in cities with a robust negative Tair-relative humidity correlation (CT-RH). However, in cities with near-zero or weak positive CT-RH, HSIs considering humidity provide enhanced predictive power compared to Tair. Furthermore, the magnitude and timing of heat-related mortality measured by HSIs could differ largely from those associated with Tair in many cities. Our findings provide important insights into specific regions where humans are vulnerable to humid heat and can facilitate the further enhancement of heat-health alert systems.

Barcelona Institute for Global Health ISGLOBAL Doctor Aiguader 88 08003 Barcelona Spain

Chair of Epidemiology Institute for Medical Information Processing Biometry and Epidemiology Faculty of Medicine LMU Munich Marchioninistr 15 Munich 81377 Germany

Climate Air Quality Research Unit School of Public Health and Preventive Medicine Monash University Level 2 553 St Kilda Road Melbourne VIC 3004 Australia

Department of Civil Engineering Graduate School of Engineering The University of Tokyo 7 3 1 Hongo Bunkyo ku Tokyo 113 0033 Japan

Department of Economics Ca' Foscari University of Venice San Giobbe Cannaregio 873 Venice 30121 Italy

Department of Global Environmental Health Graduate School of Medicine The University of Tokyo 7 3 1 Hongo Bunkyo ku Tokyo 113 0033 Japan

Department of Global Health Policy Graduate School of Medicine The University of Tokyo 7 3 1 Hongo Bunkyo ku Tokyo 113 0033 Japan

Department of Hygiene Epidemiology and Medical Statistics National and Kapodistrian University of Athens 75 Mikras Asias Athens 11527 Greece

Department of Public Health Environments and Society London School of Hygiene and Tropical Medicine Keppel Street London WC1E 7HT United Kingdom

Department of Statistics Computer Science and Applications G Parenti University of Florence Viale Morgagni 59 Florence 50134 Italy

Environment and Health Modelling Lab Department of Public Health Environments and Society London School of Hygiene and Tropical Medicine 15 17 Tavistock Place London WC1H 9SH United Kingdom

Environmental Research Group School of Public Health Imperial College London White City Campus Wood Lane London W12 0BZ United Kingdom

Faculty of Environmental Sciences Czech University of Life Sciences Kamýcká 129 Prague 165 00 Czech Republic

Institute of Atmospheric Physics Czech Academy of Sciences Boční 2 1401 Prague 141 31 Czech Republic

Institute of Epidemiology Helmholtz Zentrum München German Research Center for Environmental Health Ingolstädter Landstraße 1 Neuherberg 85764 Germany

Institute of Hydraulics and Ocean Engineering Ningbo University 818 Fenghua Road Ningbo 315211 China

Institute of Industrial Science The University of Tokyo 4 6 1 Komaba Meguro ku Tokyo 153 8505 Japan

Institute of Social and Preventive Medicine University of Bern Mittelstrasse 43 Bern 3012 Switzerland

Moon Soul Graduate School of Future Strategy Korea Advanced Institute of Science and Technology 291 Daehak ro Yuseong gu Daejeon 34141 Republic of Korea

Oeschger Center for Climate Change Research University of Bern Hochschulstrasse 4 Bern 3012 Switzerland

School of Health Policy and Management College of Health Sciences Korea University B dong Hana Science Building 145 Anam ro Seongbuk gu Seoul 02841 Republic of Korea

School of Public Health and Institute of Environment and Human Health Anhui Medical University No 81 Meishan Road Hefei 230032 China

School of Public Health and Social Work Queensland University of Technology Kelvin Grove Campus Victoria Park Rd Kelvin Grove Brisbane QLD 4059 Australia

School of the Environment Yale University 195 Prospect Street New Haven CT 06511 USA

Shanghai Children's Medical Centre Shanghai Jiao Tong University 1678 East Road Shanghai 200127 China

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