Department of Public Health, Environments and Society, London School of Hygiene and Tropical Medicine, London, UK.
Center for Statistical Methodology, London School of Hygiene and Tropical Medicine, London, UK.
Environ Health Perspect. 2019 Sep;127(9):97007. doi: 10.1289/EHP5430. Epub 2019 Sep 25.
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.
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.
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.
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.
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.
有强有力的实验证据表明，如果湿度较高，高温带来的生理压力会更大。然而，开发出来的热指数并没有始终比干球温度更好地预测死亡率。
我们旨在通过使用大型多国家数据集，阐明湿度除温度以外对夏季每日死亡率预测的潜在贡献。
在 24 个国家的 445 个城市中，我们使用时间序列回归模型拟合夏季死亡率，该模型采用温度的分布式滞后非线性模型（DLNM）（最大滞后 3 天），并补充了一系列相对湿度（RH）及其与温度相互作用的术语。使用荟萃分析技术总结城市特异性关联。
在温度项中添加 RH 的线性项略微改善了拟合度，RH 增加 23%（第 99 百分位数异常）与死亡率降低 1.1%[95%置信区间（CI）：0.8，1.3]相关。允许 RH 项呈曲线状或添加 RH 与温度相互作用的项并没有改善模型拟合度。与湿度相关的风险降低由滞后 0 时的正系数组成，而滞后 1-3 天时的负系数则相形见绌。主要结果在模型微小变化和用绝对湿度代替 RH 时具有广泛的稳健性。用表观温度（一种结合湿度和温度的指标）代替温度略微降低了拟合优度。
在这项大型跨国研究中，夏季湿度与死亡率之间没有正相关关系，这与生理研究的预期相悖，尽管与之前发现热指数对死亡率预测改善作用证据很少的流行病学研究一致。湿度与死亡率之间存在平均负相关的结果应谨慎解释；滞后结构的解释尚不清楚，表明需要进一步研究以阐明。https://doi.org/10.1289/EHP5430.
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