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Environ Health Perspect. 2019 Sep;127(9):97007. doi: 10.1289/EHP5430. Epub 2019 Sep 25.

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

Author information

1
Department of Public Health, Environments and Society, London School of Hygiene and Tropical Medicine, London, UK.
2
Center for Statistical Methodology, London School of Hygiene and Tropical Medicine, London, UK.
3
Universidad de Buenos Aires, Facultad de Ciencias Sociales, Instituto de Investigaciones Gino Germani, Buenos Aires, Argentina.
4
Section of Sustainable Health, Department of Occupational and Environmental Medicine, Umeå University, Umeå, Sweden.
5
School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut, USA.
6
National Institute of Environmental Health Science, National Health Research Institutes, Zhunan, Taiwan.
7
Institute of Advanced Studies, University of São Paulo, São Paulo, Brazil.
8
Department of Public Health, Universidad de los Andes, Santiago, Chile.
9
Institute of Research and Development, Duy Tan University, Da Nang, Vietnam.
10
Department of Environmental Health, Faculty of Public Health, University of Medicine and Pharmacy at Ho Chi Minh City, Ho Chi Minh City, Vietnam.
11
Department of Environmental Health, National Institute of Public Health, Cuernavaca, Morelos, Mexico.
12
Department of Public Health and Clinical Medicine, Umeå University, Sweden.
13
Technological University Dublin (TU Dublin), Dublin, Ireland.
14
Department of Environmental and Occupational Medicine, National Taiwan University (NTU) Hospital, Taipei, Taiwan.
15
Institute of Occupational Medicine and Industrial Hygiene, NTU Hospital, Taipei, Taiwan.
16
Department of Epidemiology and Preventive Medicine, School of Public Health and Preventive Medicine, Monash University, Melbourne, Australia.
17
Climate, Air Quality Research Unit, School of Public Health and Preventive Medicine, Monash University, Melbourne, Australia.
18
Department of Pediatric Infectious Diseases, Institute of Tropical Medicine, Nagasaki University, Nagasaki, Japan.
19
Faculty of Health and Sport Sciences, University of Tsukuba, Tsukuba, Japan.
20
Department of Family Medicine and Public Health, University of Tartu, Tartu, Estonia.
21
Department of Statistics and Computational Research, University of València, València, Spain.
22
Biomedical Research Center Network of Epidemiology and Public Health (CIBERESP), Madrid, Spain.
23
Department of Environmental Health, School of Public Health, Fudan University, Shanghai, China.
24
Graduate School of Public Health, Seoul National University, Seoul, Republic of Korea.
25
Institute of Atmospheric Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic.
26
Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Czech Republic.
27
School of Epidemiology and Public Health, Faculty of Medicine, University of Ottawa, Ottawa, Canada.
28
Air Health Science Division, Health Canada, Ottawa, Canada.
29
Department of Epidemiology, Lazio Regional Health Service, Rome, Italy.
30
Santé Publique France, Department of Environmental Health, French National Public Health Agency, Saint Maurice, France.
31
Swiss Tropical and Public Health Institute, Basel, Switzerland.
32
University of Basel, Basel, Switzerland.
33
Department of Environmental Health, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, USA.
34
Department of Environmental Engineering, Graduate School of Engineering, Kyoto University, Kyoto, Japan.
35
Department of Global Ecology, Graduate School of Global Environmental Studies, Kyoto University, Kyoto, Japan.
36
Institute of Environmental Assessment and Water Research (IDAEA), Spanish Council for Scientific Research (CSIC), Barcelona, Spain.
37
Shanghai Children's Medical Center, Shanghai Jiao Tong University School of Medicine, Shanghai, China.
38
School of Public Health, Institute of Environment and Population Health, Anhui Medical University Hefei, China.
39
School of Public Health and Social Work, Queensland University of Technology, Brisbane, Australia.
40
Institute for the Environment, Brunel University London, London, UK.

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.

PMID:
31553655
DOI:
10.1289/EHP5430
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