Extreme heat threatens long-term health as temperatures rise across US
Scientists warn prolonged exposure affects gut bacteria, long-term memory and accelerates aging process
(InvestigateTV) — A majority of Americans deal with extreme heat every year, and experts say the threat is only getting worse.
According to the National Weather Service, heat kills more people each year than tornadoes and hurricanes combined, and it is considered the deadliest natural hazard.
The summer of 2025 saw a number of extreme heat-related events and above-average temperatures across most of the lower 48 United States.
Heat closed the Washington Monument, damaged a bridge in Virginia and caused dozens to faint at a high school graduation in New Jersey.
As experts say unusually hot temperatures are expected to become more common in parts of the U.S., prolonged exposure can put both short-term and long-term health at risk.
Heat waves increasing in frequency and duration
In Phoenix, Arizona, Jordan Darby rides the bus to and from work and says she can feel a difference when temperatures start to rise.
“You always forget how hot it is until the summer comes and oh I got to deal with this again,” Darby said. “I know when I get into work I’m a little bit cranky just cause I’ve been out in the sun for an hour.”
The sunshine and warmer temperatures can quickly take a toll, even in a city where people are used to triple digits.
“I can handle 100, but like the moment it hits 110 I’m dying a little bit,” Darby said.
Across the country in Atlanta, firefighter Anare Holmes says during extreme heat, his team makes sure to stay hydrated and modify their diets to eat lighter meals, knowing how fast heat can make you sick.
“Not a one size fits all approach. These heat waves don’t play so you want to make the modifications you need for your body,” Holmes said.
The EPA explains extreme heat is when temperatures are substantially hotter or more humid than average for a location on a certain date. When unusually hot days are persistent, it’s known as an “extreme heat event” or a heatwave.
“The heat waves are undefeated, OK,” Holmes said.
Scientists warn they’re occurring more often in major U.S. cities. According to the federal government’s Fifth National Climate Assessment released in 2023, the average number of heatwaves has doubled since the 1980s, and the length of the heatwave season has increased from about 40 days to 70 days.
The threat isn’t just in the South or Southwest. The assessment also reports that since 2018, several major heatwaves have affected the U.S., including a record-shattering event in the Pacific Northwest.
In June 2021, temperatures reached 110 degrees Fahrenheit in Washington state and 116 degrees in Oregon. More than a hundred people died, and thousands more were hospitalized.
Immediate and long-term health risks
“In Oregon, the Pacific Northwest was thought of as a rainy place, temperate and pretty cool. The number of days in the summer above 90 degrees is already different in the past few decades,” said Dr. Anne Toledo, chief of urgent care at Kaiser Permanente Northwest.
With the risk of extreme heat rising across much of the U.S., Toledo says it’s important to know how quickly heat can affect your body.
“As someone gets more and more sick from being hot, you start to see things like headache, being a lot more irritable, feeling nauseous, having stomach symptoms,” Toledo said.
She explains extreme heat can dehydrate you and cause heat-related illnesses, meaning your body can’t properly cool itself — a condition that can be fatal.
“A person is confused, seems out of it, unusually lethargic, that is a very dangerous sign. Once someone has reached that point, that’s a sign the heat is basically toxic to the body and someone is at risk of organ failure,” Toledo said.
Experts also say there are potential long-term health impacts when it comes to extreme heat.
“Typically, we think of high temperatures of a 100, 105 but really above 85, your body starts to change,” said Dr. Michael Hartman.
A change, Hartman says, can take place in your stomach.
“Within our gut we have a microbiome which is full of bacteria fungi viruses and parasites and the heat actually changes the balance in good bacteria and bad bacteria,” Hartman said.
He points to a 2024 study from the University of California, Irvine, that shows how repeated exposure to high temperatures affects the internal organs of elderly mice. He says researchers saw an increase in bad gut bacteria, which can cause negative health effects, such as a weakened immune system.
“Keep your gut healthy, your whole body’s healthy,” Hartman said.
Hartman also explains that an imbalance in the GI tract can impact how your liver and brain communicate, which could lead to possible long-term memory problems over time.
“Ultimately, what we can take from just the one research is that we have to be mindful of how often we’re exposed to higher temperatures,” Hartman said.
Heat accelerates the aging process
Scientists also say extreme heat is making Americans age faster. According to a 2025 University of Southern California study, people in neighborhoods that experience more days of high heat show greater biological aging on average than residents of cooler areas.
Biological age is a measure of how well the body functions at the molecular level, as opposed to chronological age based on one’s birth date. Having a greater biological age is associated with a higher risk for disease.
Dense concentrations of pavement and buildings absorb and retain heat from the sun — a phenomenon known as the urban heat island effect. The EPA says it only accentuates the impacts of hot weather by causing even higher temperatures.
“Everything around us is designed for an average temperature,” said Heather Navarro, director of the Midwest Climate Collaborative at Washington University, which studies how “heat islands” can affect different communities.
“We’ve got folks looking at disparities in terms of how different people are experiencing heat,” Navarro said.
Cities implement cooling solutions
Navarro says planting trees, developing green spaces and using green building materials are possible solutions to make city living more comfortable, but she acknowledges those changes can take years to implement.
“Our region has been used to climate patterns for centuries. It’s going to require a shift in virtually everything,” Navarro said.
“Streets, sidewalks, trees that we grow, the building materials in our houses and so as, that changes that puts stressors on all of those things. It’s going to have economic impacts as well as health impacts all the way around,” she said.
To help protect more people from extreme heat, cities are trying to stop it from the top down, starting with the roof.
In Atlanta, Georgia, starting in June of 2026, new and replaced roofs must be made with a more reflective, eco-friendly material, known as “smart surfaces” that look like traditional roofing.
“We know how dangerous it can be. We estimate this will alleviate, or lower, temperatures by 2.5 degrees, and that, in many cases, is the difference between life and death for some folks,” said Liliana Bakhtiari.
Individual protection strategies
Hartman says this doesn’t mean we need to avoid living our everyday lives, but we need to take precautions when planning to spend hours in the heat, either for work or recreation.
You can also help keep your gut healthy by drinking plenty of fluids and eating fruits and vegetables to get your fiber.
“We can’t really change temperatures, but we can modify, we can adapt our routine. We can plan ahead,” Hartman said.
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