Houston’s hot and sticky summer climate is the perfect environment for The University of Texas School of Public Health researcher Dr. Kai Zhang, assistant professor in the division of epidemiology, human genetics and environmental sciences,who developed an interest in the health impact of climate change and extreme heat as a student at the much cooler (temperature-wise) University of Michigan. Since joining The University of Texas School of Public Health in 2012, Zhang has focused his research on learning more about the factors that contribute to heat-related health effects—from humidity to the heat index.
So is it the heat, or the humidity?
“Actually, both,” says Dr. Zhang. “In our latest study in the journal Environmental Research,my colleagues and I ranked weather variables linked to heat-related deaths in Chicago, Detroit, Philadelphia, and Phoenix, using a sophisticated statistical learning method called random forests. Apparent temperature—how warm or cool our bodies perceive the air to be—was the most important predictor of heat-related mortality. But we were surprised to find that absolute humidity—the total amount of water vapor in the air—was a close second. This study shows that it is an important weather variable in terms of its relationship to heat-related mortality.”
So absolute humidity takes the heat up a notch?
“When humidity increases, the way we feel heat is different. Humidity makes it more difficult for sweat to evaporate, which prevents the body from cooling off. Everything is more uncomfortable. That’s why 90 degree weather in a city like Houston or Chicago, can feel as bad or worse as 108 degree weather in a city with a drier climate, like Phoenix.”
“Much of the epidemiological literature on heat-related illnesses uses relative humidity—the ratio of the actual amount of moisture in the air compared to the maximum amount of moisture air could hold at a specific temperature—as a metric for air moisture. But this study is the first suggesting that absolute humidity may be a more important metric, because it better reflects physiological heat exposure. And there have been previous studies linking absolute humidity to health. For example, studies show absolute humidity to be a major driver of the influenza season in temperate regions.”
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