Most people on the planet probably don’t need to read the news to know that there’s a heat wave on. The temperature in Britain last week was high enough to melt roads and buckle train rails, per the Guardian. The Hindustan Times notes that high temperatures and humidity combined for a heat index that made Delhi feel like 132 degrees. Also in Delhi, something called the “wet bulb temperature” soared above 92 degrees. That might not sound so bad, but it is. According to the National Weather Service, the process for finding wet bulb temperature essentially simulates the body’s built-in cooling system, i.e., perspiration.
The measurement is determined by wrapping the bulb of a thermometer in wet fabric and comparing the drop in temperature to a dry bulb thermometer showing the actual air temperature. The wet bulb reading is always lower than the air temperature, unless there is enough relative humidity to reach adiabatic saturation, i.e., the point at which heat no longer enters or exits a system. According to an essay in Scientific American, a wet bulb reading of 95 degrees is generally considered the upper limit of safety, beyond which “the human body can no longer … maintain a stable body core temperature.”
However, the essay’s authors—researchers from Penn State—recently tested that limit in a laboratory setting for the first time. They rounded up a cohort of healthy, young adults and had them perform light-duty daily tasks under various combinations of heat and humidity. Findings show that the human body reaches a “critical environmental limit” and begins showing signs of heat stress at a considerably lower wet bulb reading of 88 degrees, which can happen when it’s 100 degrees with relative humidity of 60. With intense heat waves becoming more common, the researchers say societies in hot-humid regions need to adapt, and fast, because the human body didn't evolve to survive if its sweat can't evaporate. (More heat wave stories.)
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