AILSA CHANG, HOST:
This summer, NPR's Science Desk has been looking into the science of sweat. As the planet gets hotter, it turns out perspiration isn't what it used to be. And even though we've gotten used to checking the outdoor temperature or even humidity, there could be another measurement to pay close attention to. In the final installment of this sweat series, climate reporter Lauren Sommer has this look at what the future may hold for the human body's ability to cool down effectively.
LAUREN SOMMER, BYLINE: You know the feeling. It's humid, muggy.
KRISHNA ACHUTARAO: It's extremely uncomfortable.
SOMMER: Krishna AchutaRao is a climate scientist at the Indian Institute of Technology, Delhi. In March, temperatures started spiking in northern India, and it stayed relentlessly hot for two months.
ACHUTARAO: If you're outdoors, if you have to work, these things are quite dangerous.
SOMMER: And the danger wasn't just the heat; it was the humidity - because that makes it tougher for our sweat to do its job. Larry Kenney is a professor of physiology.
LARRY KENNEY: Only sweat that evaporates has any ability to cool the body.
SOMMER: At his lab at Penn State University, he studies how our bodies deal with humidity by putting test subjects into a climate-controlled room. They walk on a treadmill, and he turns up the heat. The more humid he makes it, the harder it is for people to get their core temperatures down.
KENNEY: When it gets close to the humidity of the sweat on the skin, it can no longer evaporate.
SOMMER: Evaporation is the magic of sweating. When sweat leaves your skin, it takes the heat with it. But in really humid air, the sweat just sits there, so you're not getting any cooler. That's when humans can die within a matter of hours even just sitting in the shade. Scientists have a way of measuring this heat-humidity combo. They call it the wet-bulb temperature. The higher the wet-bulb temp, the harder it is for your body to cool off by sweating. Some heat waves around the world, including the one this year in India, are getting close to the wet-bulb danger zone.
KENNEY: Heat is the most deadly of all weather-related fatalities, much more so than tornadoes, hurricanes, all other things combined.
SOMMER: And heat waves are getting worse as the climate gets hotter. Kenney's research has found that humans can endure less humidity and heat than previously thought.
KENNEY: So it's important to know that things are a little bit worse physiologically than we thought in terms of impending climate change.
SOMMER: So Kenney says outdoor workers, sports teams and the very young and old need to pay attention to the wet-bulb temperature, too, because it's not just how hot it is outside; it's also whether you can sweat it out. Lauren Sommer, NPR News.
(SOUNDBITE OF BUN B AND STATIK SELEKTAH SONG, "SUPERSTARR")
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