In a recent study published in Environmental Research Letters, a pair of researchers at the University of California at Berkeley (UC Berkeley) discuss how the heat index, also known as the apparent temperature, can often underestimate the perceived temperature during excessive heat waves, and the toll this heat takes on the human body. The study discovered that the highest heat index over the Midwest of the United States taxes the cardiovascular system, to include skin blood flowed elevated several times over. This study has the potential to give us a better understanding of current heat waves and the associated health impacts, as well.
The heat index was created in 1979 by textile physicist, Dr. Robert Steadman, who utilized simple equations summer conditions, or essentially to determine how hot it feels. Dr. Steadman viewed this new index as a complement to the widely used wind chill factor to estimate how cold it feels.
"Most of the time, the heat index that the National Weather Service is giving you is just the right value. It's only in these extreme cases where they're getting the wrong number," said Dr. David Romps, a Professor in the Department of Earth and Planetary Science at UC Berkeley, and lead author of the study. "Where it matters is when you start to map the heat index back onto physiological states and you realize, oh, these people are being stressed to a condition of very elevated skin blood flow where the body is coming close to running out of tricks for compensating for this kind of heat and humidity. So, we're closer to that edge than we thought we were before."
While the heat index has been widely embraced in the United States, to include the National Weather Service (NWS), Dr. Romps and his graduate student, Yi-Chuan Lu, had the goal of expanding on Dr. Steadman’s work to incorporate all temperatures and humidities between 0% and 100%.
"The original table had a very short range of temperature and humidity and then a blank region where Steadman said the human model failed," Lu said. "Steadman had the right physics. Our aim was to extend it to all temperatures so that we have a more accurate formula."
After publishing a revised heat index equation earlier this year, the researchers then published this most recent study by applying the top 100 heat waves that happened between 1984 and 2020. While they discovered small disagreements with what the NWS reported at the time, they also discovered extreme scenarios where the NWS heat index wasn’t even close. The study highlights a number of past heat waves in the United States as evidence to support their hypothesis.
"I'm no physiologist, but a lot of things happen to the body when it gets really hot," Romps said. "Diverting blood to the skin stresses the system because you're pulling blood that would otherwise be sent to internal organs and sending it to the skin to try to bring up the skin's temperature. The approximate calculation used by the NWS, and widely adopted, inadvertently downplays the health risks of severe heat waves."
Going forward, Earth is unlikely to reach heat indexes that will be of concern for human health for the next few decades, but less severe heat waves will still unfortunately continue worldwide.
Sources: Environmental Research Letters, New York Times, Journal of Applied Meteorology and Climatology
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