Updated June 20, 2025 at 1:27 PM MDT
It's not the heat that gets you — it's the humidity.
Millions will feel that firsthand over the coming days as a heat dome is expected to bring extreme heat and humidity for much of the U.S.
Those humid conditions will make temperatures feel even hotter and pose health risks to many.
Why is that?
Sheetal Rao, internal medicine physician at the University of Illinois Health with a focus on how the environment affects health, told Morning Edition that high levels of humidity simply make it feel hotter — and challenge our body's mechanisms to regulate temperature.
Here's what she had to say about humidity and how to stay cool:
Why humidity poses risks during extreme heat
One of the human body's main ways of cooling down is sweating, and that sweat evaporating off the body. When it's humid, sometimes that isn't enough, Rao said.
"When the air is so full of moisture, that evaporation doesn't happen as efficiently or effectively, and so we aren't able to cool down as well," Rao said.
Rao said high humidity poses the strongest risk to seniors and young children, as well as pregnant women.
But it can also come down to the exertion level of outdoor activity.
"So even a regular, healthy person who is out working very hard in the heat and humidity would be vulnerable," Rao said. "And then, of course, people who are outdoors more for their job or people who are unhoused and then certain medications as well can make people more vulnerable."
How can you stay safe during humid days?
While some people may initially feel that sitting by a fan alleviates the worst of the heat, Rao said that might not be enough on humid days. That's because the sweat on one's body doesn't have any place to evaporate off the skin because the air is already so full of water vapor.
"The fan kind of just cycles the same air around and it won't effectively cool people when it's very hot and humid," Rao added.
If you don't have air conditioning, Rao recommends getting some rest, taking regular breaks, closing blinds to cool down your home, hydrating and wearing loose-fitting clothes.
"But at some point when it gets very hot, air conditioning is likely the only intervention that's going to help you feel cool," Rao said. "And so there are places that people can go to get cool, like cooling centers, but also just public buildings like libraries and malls and things like that."
The digital version of this story was adapted for the web by Obed Manuel and edited by Suzanne Nuyen.
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