Temperatures continue to set records across the U.S. — and the extreme heat can have a life-threatening impact on your body.

More than a third of Americans were under extreme heat advisories, watches and warnings over the past week as a blistering heat wave baked much of nation, with day after day of triple-digits in the Southwest, even topping 110 degrees for an unprecedented stretch in Phoenix. 

Forecasters said the long-duration heat wave is extremely dangerous, especially for children, older people, homeless residents and other vulnerable populations.

"Excessive heat is the leading weather related killer in the United States," the National Weather Service warns. Some hospitals are already seeing a rise in heat-related illnesses.

Here's what do know about how heat affects your body and health:

Why is extreme heat so risky?

Exposure to excessive heat can have wide-ranging impacts on anyone, from the direct effects of heat on the body to the worsening of existing conditions.

"Rapid rises in heat gain due to exposure to hotter than average conditions compromises the body's ability to regulate temperature and can result in a cascade of illnesses, including heat cramps, heat exhaustion, heatstroke, and hyperthermia," the World Heath Organization's website states. "Temperature extremes can also worsen chronic conditions, including cardiovascular, respiratory, and cerebrovascular disease and diabetes-related conditions."

It doesn't help that people often underestimate heat, Jeff Goodell, author of the book "The Heat Will Kill You First," told CBS News.

Instead of viewing it as a health risk, some brush off warmer temperatures as "better beach weather," he says — but what many don't understand is there are thresholds of heat our body can't cope with.

"Our body has a pretty narrow range of temperatures which it can handle," Goodell says, "and when it starts to get too hot, our heart starts pounding and it's pushing blood out towards the surface of our skin in a desperate attempt to cool that blood down, which it does by, you know, our body starts sweating ... and that sweating cools the blood and that, in theory, cools the body. But that mechanism only works so far."

It's also a threat that's mostly "invisible," compared to other extreme weather conditions like hurricanes or tornadoes, which can add to people's lack of concern.

"It's very easy to visualize the threat of a storm or floods," Goodell notes. "The problem with heat is it's invisible, so it's very hard to communicate about that."

The grim toll of extreme heat can add up. A recent study analyzed data from across Europe during last year's record-hot summer and concluded that more than 61,600 people died from the effects of heat across the continent between May 30 and Sept. 4, 2022.

How does your body break down in the heat?

What happens in extreme heat is your body "tries to compensate," Dr. Tochi Iroku-Malize, family physician and president of the American Academy of Family Physicians, told CBS News.

Your body starts to get warmer, it overheats and then as it tries to regulate itself, it can lead to different things — the first of which we're all familiar with: sweat. But as the heat progresses, other symptoms may appear, such as fever, muscle cramps or weakness, pale or cold skin, heightened heart rate and nausea. 

These are signs you're experiencing heat exhaustion, which can quickly become the even more dangerous heat stroke as your fever rises. 

"At this point your system is starting to shut down so you're not sweating, you're going to have trouble breathing, you may have fainting, you may have seizures," Iroku-Malize says of heat stroke.

• How to prevent heat stroke and spot symptoms as U.S. bakes in extreme heat

With these heat-related illnesses, almost every system of the body is impacted, she explains, urging people to take high temperatures seriously.

"If you think about muscle cramps, we're talking about your musculoskeletal system. Fatigue and headaches, that's your nervous system. Nausea and vomiting, it's the nervous system but it's now affecting your digestive system," she says, adding your central nervous system may also trigger other parts of your body like your cardiac and respiratory system as well.

It can even impact your brain and behavior.

Keeping our body temperature regulated is what keeps our organs functioning properly, so when extreme heat disrupts that, "your brain is the first thing to be affected," Iroku-Malize says.

"You feel dizzy, you feel lightheaded, and as it progresses, you now start to feel irritable or confused or you may have delirium, you may faint, we may lose consciousness or in extreme situations you may start to seize," she explains. "When dehydration causes seizures, it can lead to permanent brain damage and sometimes even death, so this is not something to be taken lightly."

Who's most at risk?

While anyone is at risk from heat-related illnesses, children, pregnant women and older adults are more susceptible to adverse effects because they're less able to regulate body temperature, according to the National Institutes of Health. 

"Other at-risk groups include individuals working outdoors, outdoor athletes, the socially isolated and those with incomes below the federal poverty level, as well as communities of color," the organization's website notes.

Goodell adds anyone who has heart conditions or circulatory problems are also at higher risk.

"That mechanism begins to break down, and that's when you start moving into the land of heat exhaustion and heat stroke and ultimately, if it's too hot for too long, death," he says.

Where you live is also a factor of vulnerability.

Densely populated cities can experience an urban "heat island" effect, where manmade surfaces absorb sunlight, leading to higher temperatures than nearby areas with more grass and tress. This is especially hazardous when temperatures fail to cool off sufficiently at night.

How does humid heat affects the human body differently from dry heat?

The combination of high temperatures and humidity can be especially dangerous.

Hot, dry environments allow sweat to evaporate, which allows the body to cool itself. However, there are limits to how much people can sweat, and those higher temperatures mean the body gains more heat.

Researchers who have studied the limits to how much heat humans can tolerate note that even less-extreme temperatures with higher humidity can put stress on the heart and other body systems.

The bottom line? Not matter what, stay hydrated and take steps to reduce your time in the heat to stay safe.

-The Conversation and The Associated Press contributed to this report.

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