Heat is the No. 1 cause of weather-related fatalities in the U.S., according to the National Weather Service.

When temperatures soar, older people and babies who can’t easily regulate their body temperatures are most vulnerable to the dangers of extreme heat.

How can you stay cool during a heat wave and fight heat exhaustion and heat stroke?

We consulted with Maj. Dr. Genevieve Hillis to answer your questions about why heat can be so dangerous and how you can stay healthy. She’s an Army major and an emergency medicine specialist who cares for patients at UCHealth University of Colorado Hospital on the Anschutz Medical Campus in Aurora and at UCHealth Broomfield Hospital.

During her military service, Hillis has treated patients with heat injuries all over the world.

“Heat is the No. 1 killer. We don’t think about it, but it’s true,” said Hillis.

Along with the young and old, heat waves can be extremely dangerous for people who must work outdoors when the mercury soars. Think of roofers or construction workers or, during the heat wave in July 2022, the Queen’s guards who have sweltered in hot red uniforms and fur hats.

Hillis once helped save the life of a woman whose core temperature had climbed to 106.2 degrees.

“That’s the worst I ever saw,” she said. “You have all these soldiers in full uniform working and carrying heavy loads. It’s really serious. Folks will get seizures, and some can suffer permanent brain injuries if their core temperatures get high enough.”

Hillis is also certified in wilderness medicine and studied tropical medicine in Africa. In 2019, she climbed Mount Kilimanjaro with fellow emergency medicine wilderness doctors. She is also a rock climber and teaches courses on how to do medical rescues in “high-angle” locations like canyons.

So, with Hillis as our guide, let’s start with the basics about the dangers of heat and how you can stay cool.

Why are heat waves and heat stroke so dangerous?

Heat can be especially dangerous because it can sneak up on people.

“Unlike cold, which hurts us pretty quickly, heat stroke can come on more gradually and folks don’t notice it,” Hillis said. “If you’re out in the cold, your fingers and toes will hurt long before you get frostbite.

“With heat, we gradually get sweaty. But then, as people don’t feel good, they may not feel as thirsty,” Hillis said. “As the world gets hotter, this is likely to be an increasing problem.”

Why is heat especially dangerous for older people?

In older adults, confusion can set in quickly.

“Their bodies don’t handle heat well. If you go see your mom or grandma and they’re not making a lot of sense or they’re asking the same question, and that’s not typical, make sure they’re drinking enough. They probably need to stay cool,” Hillis said.

Mobility also can be a problem.

“Some older people are not physically able to move. Maybe they can’t walk, or they use a wheelchair or a walker. They may not be able to drive. There are physical limitations to getting out of the heat.”

Older adults also are at risk of falls, especially if they are confused or are not feeling well.

“That’s another reason why heat is so dangerous. If you’re 85 and you don’t move around well, you can get hot and dehydrated, and you can fall and break a hip,” Hillis said.

Why is heat so dangerous for babies and younger kids?

Babies can’t cool themselves easily, and many parents make the mistake of wrapping their infants in too many blankets.

“If you’re hot and you’re wearing a tank top and shorts, then your baby is probably hot too,” Hillis said.

The physiology of babies’ bodies makes them more susceptible to heat.

“They’re more sensitive. Their bodies are little, but they have more surface area relative to their blood flow,” Hillis said. “Normally, when we get hot, we sweat and breathe more. Babies and little kids don’t do that as well and can get dehydrated really easily,” Hillis said.

Younger kids also can get sick from the heat. Toddlers and young kids might keep running around even if they’re hot.

“They might not be able to say, ‘I’m tired. I’m overheated,’” Hillis said. “They sometimes get sweaty and really red and flushed and keep going.”

What are heat cramps?

Children and adults can get heat cramps when they exert themselves and sweat excessively in hot temperatures. Heat cramps mark the least serious level of heat-related illnesses. Sometimes people who get overheated will experience cramps. Heat cramps are usually related to dehydration and lack of salt in the body. They can happen, for instance, when kids spend multiple hours running around in hot temperatures, like playing in an all-day soccer or lacrosse tournament.

“Along with cramps, they might also get headaches. They need to drink more water and get out of the heat,” Hillis said.

A person dealing with heat cramps typically doesn’t need to see a doctor or go to an ER.

Prevention of heat illnesses and dehydration is key, Hillis said.

“If you’re out at a soccer tournament all weekend, bring the kids over to the shade. Make sure they’re drinking plenty of water between games,” she said.

What is heat exhaustion?

Heat exhaustion is the next most severe form of heat illness.

Symptoms of heat exhaustion can include:

  • Rapid heartbeat.
  • Fast, shallow breathing.
  • Slightly elevated body temperature.

If a person with heat exhaustion has symptoms that don’t improve within about an hour, then they should seek medical care.

“We give them fluids and anti-nausea medicine,” Hillis said.

Emergency medicine doctors also will check kidney function.

“We check their urine to see how dehydrated they are. They usually need a lot of fluids,” Hillis said.

What is heat stroke?

“Heat stroke is really severe,” Hillis said. “These folks usually have passed out or have had a seizure. If they’re talking, they’re not making a lot of sense. They can’t always say where they are. Their core temperature will be high.”

Other signs of heat stroke can include:

  • High body temperature.
  • Lack of coordination.
  • Hot and dry skin or profuse sweating.
  • Rapid heart rate and breathing.
  • Throbbing headache.

When a person suffers severe heat stroke, they need medical help immediately. Call 911 or take the person to the nearest hospital emergency department.

If a person doesn’t get treatment for heat stroke, their body will begin to break down.

They can suffer from a medical syndrome known as rhabdomyolysis.

“That means that muscle tissue is breaking down. You get proteins in the blood and that causes renal failure. These patients end up on ice and often have to go on dialysis. It’s really serious. They can have seizures and permanent brain injuries if their core temperature gets high enough.”

What are the symptoms to watch for with illnesses related to extreme heat?

Early symptoms of heat distress can be a red, flushed face, cramping and dehydration.

Symptoms can worsen quickly. For more serious heat exhaustion or heat stroke, symptoms can include:

  • Passing out.
  • Confusion and repetition.
  • Kidney problems.

Once a person has suffered from a heat-related illness, are they more likely to get heat exhaustion or heat stroke again?

Yes, Hillis said.

“Once you have had a heat injury, you are at high risk for having one again. You’ve got to be really careful,” she said.

How can you tell if an infant or toddler is sick from the heat?

“If babies are too hot, you’ll see what we call ‘a change in mental status,’” Hillis said. “They’re not talking. They’re less responsive. They’re not interacting. They’re not getting excited or cooing. They may look sleepy or are not waking well. Their mouths will be dry. They won’t be drooling or have tears. They may be really red and flushed, especially if parents are piling blankets on them.”

What are the symptoms of heat exhaustion or heat stroke in a child?

“Younger kids might not say, ‘I’m tired. I’m overheated,’” Hillis said.

But if they’re suffering from a heat-related illness, they will likely be flushed and may have a headache.

It’s also really common for kids to start vomiting when they’re dealing with heat stress.

What if I’m far from medical help? For instance, we’re out in the backcountry and are hiking, mountain climbing or rafting when a child or adult gets sick from the heat?

Hillis advises people to use whatever tools they have to cool a person down. If you have access to ice, make ice packs to cool areas of the body, like the armpits and groins.

If you don’t have ice, but you do have water, dunk a sheet or a tee-shirt in water, then wrap the wet fabric around the person’s body.

“If you’re out river rafting, use the cool water. Put the person’s hands and feet in the cool water,” she said.

If a child or adult is vomiting uncontrollably, try to get medical help as quickly as you can.

“Don’t chug water. Just do little sips,” Hillis said.

People who are severely dehydrated usually can’t hold down fluids. If they try to drink too much water, they may just vomit again, and the dehydration will worsen. That’s why getting medical help is so essential.

Is heat stroke more likely at high elevations?

Dehydration is more common at high elevations, and that makes it easier also to suffer heat stroke, Hillis said.

For instance, if you’re hiking at high elevations, the impact of the sun can be much stronger.

“You lose much more water at high elevations. People don’t realize how quickly they can get overheated in the mountains. When you’re hiking with no shade, it’s like walking on a mirror. It’s higher and dryer, and you hyperventilate and breathe more. When you do that, you lose more fluids,” Hillis said.

“Once you’re dehydrated and down on fluids, your risk of heat injury goes up,” she said.

She advises people always to stay hydrated. Drink plenty of water. Or if you need to replenish salts in your body, stay away from sugary drinks like Gatorade and instead drink sodium-infused fluids. Some brands include Liquid I.V., ORS (Oral Rehydration Solution) and UCAN hydrate.

When do you need to take a child or older person to a doctor or emergency provider for help?

If a child is vomiting repeatedly and can’t hold down fluids, seek medical care quickly.

“Once they start throwing up, you’ve got to get help,” Hillis said.

How can I stay cool during heat waves?

The basic guidelines for staying cool include:

  • Getting out of the heat whenever possible.
  • Drinking plenty of water.
  • Wearing a hat.
  • Keep skin covered with light layers.
  • When you’re in extreme heat, force yourself to take breaks every hour for about 10 minutes so you can rest and rehydrate.

Learn more from experts at the U.S. Centers for Disease Control and Prevention about how to “beat the heat.”

Most adults feel heat coming on and act right away. They drink more water and take breaks in the shade.

If you don’t have air conditioning, seek out cool public places like libraries, senior centers or recreation centers. As temperatures in cities increase, more leaders are creating designated cooling centers so at-risk people can escape the heat.

Always bring water with you and drink it regularly. And consider carrying a little spray bottle with you. There are even tiny, portable fans. You can fan yourself and enjoy a spritz of water on your body.

If you own your home, look for rebates from utility providers to install evaporative coolers, which are energy-efficient ways of using moisture and fans to cool people in dry climates.

If you rent your home or want an inexpensive, portable cooling system, Hillis recommends a clever do-it-yourself option. You can fit a small basic fan in the top of an orange 5-gallon Home Depot bucket. Check out this video to see to create this inexpensive cooling system.

Why is extreme heat even more likely in humid environments?

When there’s a great deal of humidity, it’s harder to remove heat from the body.

“In the south, if you’re outside and it’s over 85 degrees with 85% humidity, you can get heat exhaustion very quickly,” Hillis said.

In dry climates, it’s much easier to use moisture — like a wet sheet or T-shirt — to draw heat out of the body.

“The water is cooler than the body because there’s more humidity. The water will evaporate and will suck the energy out of the body,” Hillis said. “But in a humid environment, you can’t use evaporation. There’s too much humidity in the air.”

This article originally appeared in UCHealth Today, a publication of UCHealth, the hospital associated with the University of Colorado School of Medicine.