BACKGROUND: There is strong experimental evidence that physiologic stress from high temperatures is greater if humidity is higher. However, heat indices developed to allow for this have not consistently predicted mortality better than dry-bulb temperature. OBJECTIVES: We aimed to clarify the potential contribution of humidity an addition to temperature in predicting daily mortality in summer by using a large multicountry dataset. METHODS: In 445 cities in 24 countries, we fit a time-series regression model for summer mortality with a distributed lag nonlinear model (DLNM) for temperature (up to lag 3) and supplemented this with a range of terms for relative humidity (RH) and its interaction with temperature. City-specific associations were summarized using meta-analytic techniques. RESULTS: Adding a linear term for RH to the temperature term improved fit slightly, with an increase of 23% in RH (the 99th percentile anomaly) associated with a 1.1% [95% confidence interval (CI): 0.8, 1.3] decrease in mortality. Allowing curvature in the RH term or adding terms for interaction of RH with temperature did not improve the model fit. The humidity-related decreased risk was made up of a positive coefficient at lag 0 outweighed by negative coefficients at lags of 1-3 d. Key results were broadly robust to small model changes and replacing RH with absolute measures of humidity. Replacing temperature with apparent temperature, a metric combining humidity and temperature, reduced goodness of fit slightly. DISCUSSION: The absence of a positive association of humidity with mortality in summer in this large multinational study is counter to expectations from physiologic studies, though consistent with previous epidemiologic studies finding little evidence for improved prediction by heat indices. The result that there was a small negative average association of humidity with mortality should be interpreted cautiously; the lag structure has unclear interpretation and suggests the need for future work to clarify. https://doi.org/10.1289/EHP5430.

Air Health Science Division Health Canada Ottawa Canada

Biomedical Research Center Network of Epidemiology and Public Health Madrid Spain

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

Climate Air Quality Research Unit School of Public Health and Preventive Medicine Monash University Melbourne Australia

Department of Environmental and Occupational Medicine National Taiwan University Hospital Taipei Taiwan

Department of Environmental Engineering Graduate School of Engineering Kyoto University Kyoto Japan

Department of Environmental Health Faculty of Public Health University of Medicine and Pharmacy at Ho Chi Minh City Ho Chi Minh City Vietnam

Department of Environmental Health Harvard T H Chan School of Public Health Boston Massachusetts USA

Department of Environmental Health National Institute of Public Health Cuernavaca Morelos Mexico

Department of Environmental Health School of Public Health Fudan University Shanghai China

Department of Epidemiology and Preventive Medicine School of Public Health and Preventive Medicine Monash University Melbourne Australia

Department of Epidemiology Lazio Regional Health Service Rome Italy

Department of Family Medicine and Public Health University of Tartu Tartu Estonia

Department of Global Ecology Graduate School of Global Environmental Studies Kyoto University Kyoto Japan

Department of Pediatric Infectious Diseases Institute of Tropical Medicine Nagasaki University Nagasaki Japan

Department of Public Health and Clinical Medicine Umeå University Sweden

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

Department of Public Health Universidad de los Andes Santiago Chile

Department of Statistics and Computational Research University of València València Spain

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

Faculty of Health and Sport Sciences University of Tsukuba Tsukuba Japan

Graduate School of Public Health Seoul National University Seoul Republic of Korea

Institute for the Environment Brunel University London London UK

Institute of Advanced Studies University of São Paulo São Paulo Brazil

Institute of Atmospheric Physics Academy of Sciences of the Czech Republic Prague Czech Republic

Institute of Environmental Assessment and Water Research Barcelona Spain

Institute of Occupational Medicine and Industrial Hygiene NTU Hospital Taipei Taiwan

Institute of Research and Development Duy Tan University Da Nang Vietnam

National Institute of Environmental Health Science National Health Research Institutes Zhunan Taiwan

Santé Publique France Department of Environmental Health French National Public Health Agency Saint Maurice France

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

School of Forestry and Environmental Studies Yale University New Haven Connecticut USA

School of Public Health and Social Work Queensland University of Technology Brisbane Australia

School of Public Health Institute of Environment and Population Health Anhui Medical University Hefei China

Section of Sustainable Health Department of Occupational and Environmental Medicine Umeå University Umeå Sweden

Shanghai Children's Medical Center Shanghai Jiao Tong University School of Medicine Shanghai China

Swiss Tropical and Public Health Institute Basel Switzerland

Technological University Dublin Dublin Ireland

Universidad de Buenos Aires Facultad de Ciencias Sociales Instituto de Investigaciones Gino Germani Buenos Aires Argentina

University of Basel Basel Switzerland

Erratum In

Environ Health Perspect. 2019 Oct;127(10):109001. doi: 10.1289/EHP6302. PubMed

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