Why the heat index matters more than just the temperature

The intense heat of summer is more than an inconvenient seasonal phenomenon—it’s deadly. Heat is the leading cause of weather-related deaths in the United States, and as global temperatures continue to rise, the threat extreme heat poses isn’t going away. But the number on the thermometer only tells half of the story. The heat index—or “feels like” temperature—combines the air’s temperature and humidity to measure how hot it actually feels to your body. 

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It’s important to pay attention to the heat index, says Janessa Webb, WCCB Charlotte chief meteorologist. “It will feel different as you step outside, and if you are not prepared, it can really sneak up on you and catch you off guard.” 

High humidity makes it harder for the body to sweat and cool itself down effectively, explains Kregg Laundon, emergency medicine physician and chief of emergency medicine at Southeast Georgia Health System. 

“As heat stress worsens, people can develop dehydration, electrolyte abnormalities, heat cramps, heat exhaustion, and sometimes even eventually heat stroke,” Laundon says. 

But experts say heat-related illnesses are largely preventable. Here’s what you need to know about the heat index this summer and what you can do to stay safe when the temperature and humidity soar. 

What is the heat index?

The heat index measures the humidity, or the amount of water vapor in the air, along with the temperature. Days with a high heat index feel like stepping into a steam room, compared to the dry heat of a sauna, Webb explains. “It’s that swampy heat.” When it’s 90 degrees outside with 70 percent humidity, it can feel like a much hotter 106 degrees.     

The South and Southeastern U.S. deal with high levels of humidity more often than the rest of the county because of their proximity to warm water from the Gulf of Mexico and the Atlantic Ocean.

Physicist Robert Steadman created the heat index in 1979. That same year, the National Weather Service (NWS) developed its own heat index using Steadman’s work as inspiration. The NWS still uses a version of this heat index today, and it’s mostly accurate, says David Romps, professor of climate physics in the Department of Earth and Planetary Science at the University of California, Berkeley.

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But there’s a problem, Romps says. Steadman ran into an issue with his calculations and couldn’t compute the heat index above 88 degrees. His equations assumed that sweat would continue to build on the skin until the air around it became oversaturated with moisture. However, excess sweat would normally drip off the body, which is something Steadman didn't consider. There also weren’t as many heat waves in the 1970s as there are today, so not having numbers for extremely hot and humid days wasn’t as much of a concern back then as it is now, Romps says. Today’s heat waves also move slower and stick around for longer than they used to.   

“Once the temperature falls outside of the range defined by Robert Steadman, no one actually knows what happens beyond that,” says Yi-Chuan Lu, postdoctoral researcher at UC Berkeley, who co-authored a study on the heat index with Romps.

Without Steadman’s work as a model for hotter temperatures, alternative calculations served as stand-ins and they underestimate how hot it feels by 10 or 20 degrees in some cases, says Romps. “It's only in [severe heat waves] where the values they're giving are wrong, but they are substantially wrong,” Romps says.

Steadman’s heat index is based on ideal circumstances—how the temperature feels to a healthy person who is in the shade, wearing the right clothes for the weather, and with as much drinking water as they need.

Do we need to rethink how we measure the heat index?

Lu and Romps revisited Steadman’s research and found a way to extend his calculations for the higher temperature and humidity ranges. They wanted to expand on a 2010 study that looked at whether there is a certain point where things get too hot for the planet to remain habitable. They say their numbers are more accurate than the NWS. 

When it’s 100 degrees outside with 65 percent humidity, the current NWS information says it feels like 136 degrees. Building off of Steadman’s heat index, Lu and Romps say it actually feels like 153 degree. The math the NWS uses to determine a “feels like” temperature at higher temperatures and levels of humidity doesn’t have the same focus on how the human body responds to heat as Steadman’s original work. This is why the NWS heat index and the one proposed by Lu and Romps are so different, Romps says. “It turns out that the human body does fine with heat until it doesn't, and when it doesn't, things go badly very quickly.”

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Lu says the NWS is aware of the updated heat index he and Romps made. “Per long-standing practice, NOAA does not endorse or critique external studies,” an NWS spokesperson said over email. 

“NOAA’s National Weather Service (NWS) is evaluating its heat forecast tools to identify areas for improvement and potential modifications," the NWS spokesperson said. 

“Because the heat index has inherent strengths and limitations, the NWS utilizes a suite of three distinct tools to evaluate and communicate extreme heat: the Heat Index, Wet Bulb Globe Temperature (WBGT), and HeatRisk. Together, these tools provide a more complete picture than just air temperature alone and provide a better metric for communicating the risks from heat to the public." 

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Updating the heat index is valuable, but it isn’t enough to save lives, says Tarik Benmarhnia, professor of epidemiology at the University of California, San Diego. 

Communities most impacted by the heat index may not have access to the information, Benmarhnia explains. 

And even if they do hear the local weather report, “knowing that [it] is 150 Fahrenheit with an index, 130 with another index, may not change that much the reality,” he adds. 

“Unfortunately, because there's so many things that need to be undertaken to actually make a difference and change behaviors and everything, this is just one part of the big, big problem,” Benmarhnia says. He says we need experts from various fields to contribute and suggests incorporating health data into these indexes from the real world as a next step to improve them more.

What you can do to stay safe when the heat index is high

Laundon recommends drinking two waters for every electrolyte drink to stay hydrated. Electrolytes restore the salt lost when we sweat, without taking in too much sugar. He also suggests resting regularly in the shade or inside with air conditioning, wearing appropriate clothing for the weather, and avoiding the peak heat from midday into the late afternoon. 

“People should check on older neighbors, young children, and anyone without reliable air conditioning,” he adds. 

Laundon says to be aware that a wide variety of medications, ranging from ACE inhibitors that lower blood pressure to common painkillers like ibuprofen, can lower kidney function and cause the medicines to build to toxic levels or cause dangerous changes in the balance of electrolytes in your body.

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Cramping is an early symptom of heat exhaustion, says Laundon. Other signs to look out for include headache, dizziness, excessive vomiting, and shortness of breath, he says. Heat stroke is more serious than heat exhaustion and causes confusion, seizures, or even loss of consciousness.

If you see someone in trouble, call an ambulance, pack ice on their head and armpits, and put their feet in water while you wait for help, Laundon says. You can also crank up the air conditioning and offer them water. 

“The way to prevent [ending up in the ER] is to have a game plan, because people have different resources, and there's not going to be one plan that fits all for everybody,” says Laundon.