The Role of Humidity in Associations of High Temperature with Mortality: A Multiauthor, Multicity Study

Ben Armstrong, Francesco Sera, Ana Maria Vicedo-Cabrera, Rosana Abrutzky, Daniel Oudin Åström, Michelle L. Bell, Bing Yu Chen, Micheline de Sousa Zanotti Stagliorio Coelho, Patricia Matus Correa, Tran Ngoc Dang, Magali Hurtado Diaz, Do Van Dung, Bertil Forsberg, Patrick Goodman, Yue Liang Leon Guo, Yuming Guo, Masahiro Hashizume, Yasushi Honda, Ene Indermitte, Carmen ÍñiguezHaidong Kan, Ho Kim, Jan Kyselý, Eric Lavigne, Paola Michelozzi, Hans Orru, Nicolás Valdés Ortega, Mathilde Pascal, Martina S. Ragettli, Paulo Hilario Nascimento Saldiva, Joel Schwartz, Matteo Scortichini, Xerxes Seposo, Aurelio Tobias, Shilu Tong, Aleš Urban, César De la Cruz Valencia, Antonella Zanobetti, Ariana Zeka, Antonio Gasparrini

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Abstract

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.

Original languageEnglish
Article number097007
Number of pages8
JournalEnvironmental Health Perspectives
Volume127
Issue number9
DOIs
Publication statusPublished - Sep 2019

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