Originally Published: 15 JUN 22 06:27 ET

Updated: 15 JUN 22 13:40 ET

By Jen Christensen, CNN

(CNN) -- Carilion Franklin Memorial Hospital in Rocky Mount, Virginia, is full. It's managing its share of Covid-19 cases, as well as more typical problems for this time of year, like boating accidents. But staffers have also had to care for people who are sick because of something that's deceptively dangerous: extreme heat.

High temperatures in the area this week are in the 90s, but when you factor in humidity, the heat index climbs as high as 104.

"We've had people just coming in today after mowing their lawn," Dr. Stephanie Lareau, an emergency room physician, said Tuesday. "Luckily, sports hasn't started back yet, so we haven't seen a lot of the youth population. We see a lot of heat-related cases when football practice starts."

Of all the natural disasters, heat is the No. 1 killer, studies show. And as temperatures continue to rise because of the climate crisis, scientists expect it to make even more people ill.

Heatwaves are already happening more often. In the 1960s, Americans saw about two heatwaves a year; by the 2010s, there were six per year, according to the US Environmental Protection Agency.

Heat-related illness is the leading cause of death and disability among US high school athletes, according to the US Centers for Disease Control and Prevention. But it can be a problem for anyone taking part in everyday activities like mowing the lawn or going for a walk.

The National Weather Service has issued excessive heat advisories this week for a swath of the country stretching from the Upper Midwest to the Southeast. From Dayton to Durham, doctors have been encouraging people to stay indoors as much as possible, warnings that are complicated by the fact that half a million didn't have power as of Tuesday because of severe storms, according to PowerOutage.US.

The heat has forced school closures in Minnesota and Milwaukee, and it shut down horse races in Kentucky and Indiana. Even UC Davis cut its graduation short after attendees made dozens of calls for medical attention because of heat exposure.

What heat does to the body

Two of the most common heat-related conditions are heatstroke and heat exhaustion.

With heatstroke, the body can't cool itself. Its temperature rises quickly, and its natural cooling mechanism -- sweat -- fails. A person's temperature can rise to a dangerous 106 degrees or higher within just 10 or 15 minutes. This can lead to disability or even death.

A person who has heatstroke may sweat profusely or not at all. They can become confused or pass out, and they could have a seizure.

Heat exhaustion happens when the body losses too much water or salt through excessive sweating. That can come with symptoms like nausea, dizziness, irritability, thirst, headache and elevated body temperature.

With both conditions, emergency help is needed quickly. While waiting for assistance, bystanders can try to cool the person by moving them to the shade and giving them with water.

Extreme high temperatures can also put significant strain on the heart or make breathing more difficult.

These temperatures can be linked to at least 17 causes of death, most of them related to heart and breathing issues but also including suicide, drowning and homicide.

Studies have shown that exposure to extreme heat can also contribute to mental health issues, problems for pregnant women and poor birth outcomes.

Who is vulnerable

The elderly, children and people with chronic diseases and mental health problems are at the highest risk of heat-related illness, along with people who take certain medicines, according to the CDC.

But people who are young and otherwise healthy aren't immune, according to Dr. Aaron Bernstein, interim director of the Center for Climate, Health, and the Global Environment at Harvard T.H. Chan School of Public Health.

He conducted a study, published in January, that found that nationwide exposure to heat has increased the number of children who go to the ER for any reason in the summer.

These visits weren't just on the hottest days; they were across a range of days with warmer temperatures.

A child born in the US today will experience 35 times more life-threatening heat events then someone born in 1961, his research showed. And that's given the best-case scenario, with the world seeing a rise in temperatures of only 1.5 degrees Celsius over the next couple of decades -- "which we ain't doing," Bernstein added. "This is the most conservative scenario projected.

"It's a big change in only 60 years," he added. "That, to me, is a major problem."

Extreme heat doesn't kill as many children as elderly people, but these "climate shocks," as Bernstein calls them, can put added stress on a child's life. That has a cumulative effect, and it can be just as damaging as poverty or any other stressor, contributing to significantly higher rates of substance use problems and health problems like cancer and heart disease, Bernstein said.

"We need to focus on these climate shocks and buffering children, because they can pose such lifelong health threats," he said. "It is devastating to your lifetime health potential."

It's not just exposure to extreme temperatures that's a problem. Higher temperatures increase particle and ozone pollution and contribute to hundreds of thousands of additional deaths of all ages around the world, according to a study published last year.

"There's a direct linear relationship between the concentration of ozone outdoors and the temperature, and so that's projected to be more of a problem as as our climate gets warmer," said Dr. John Balmes, a medical spokesperson for the American Lung Association. "And then, of course, those really hot, dry days in the summer are often when we have wildfires, too."

Exposure to wildfire smoke, which is largely made of particle pollution, can also increase the risk of heart and breathing problems.

What officials can do about heat-related issues

Some US cities feel the brunt of heat-related problems more than others. Phoenix has more than 100 100-degree days each year, on average; it had 145 such days in 2020. The city has created the country's first publicly funded Office of Heat Response and Mitigation to focus on problems related to high temperatures. It's working across city government to integrate plans to manage heat-related problems from all angles.

Efforts are underway to create electric vehicle infrastructure to reduce the reliance on the fossil fuel emissions that make climate change worse. The city is working with homeless initiatives to get unhoused people access to water and shelter, and the city's created a tracking system so riders know when a certain bus is coming and don't always have to stand outside.

"There are many pieces of the pie that can help with the heat challenge," said the program's director, David Hondula. Having someone who is focused on the issue can get other city departments to think about how they can ease the problem. It's a big undertaking and one that, if done right, could save lives.

"We're moving, regionally, very much in the wrong direction with heat-associated deaths, seeing a more than a 400% increase since 2014. That far outpaces anything we'd expect in terms of population growth, demographic change," Hondula said.

Miami has also added a chief heat officer to its ranks, although that office is not funded through the city. Just this month, Los Angeles' city council voted to create a position of chief heat officer.

How to protect yourself

To avoid heat-related illness, there is something you can do. Lareau emphasizes the need to stay hydrated; make sure you drink water before you notice that you're thirsty.

Take periodic breaks from the heat when you have to be outside.

Let yourself acclimate to high temperatures before you start running marathons or doing in any other extreme outdoor exercise.

And wear sunscreen: People who are sunburned have less of an ability to regulate their body temperature.

She said it's important to keep an eye not just on the temperature but on the heat index, because it takes into account humidity, and that can matter more for heat-related illness.

She also advises people to help monitor those who are very young or very old, because they're not able to regulate their body temperature as well. When planning activities, try to keep them out of the heat, and check in on neighbors.

"People often think of doing that during snowstorms, but the heat can be as dangerous for the elderly, especially if they don't have air conditioning," Lareau said. "So if you can offer to cut their grass or do their chores for them if you're younger and healthier and can withstand the heat a little bit better, everyone benefits."