With extreme heat plaguing parts of the U.S., Fox News medical contributor Dr. Janette Nesheiwat issued a warning over the "no. 1 weather-related killer in this country."

"It's summertime. We want to go out. We want to have fun, but we need to do it safely, especially because heat is the no. 1 weather-related killer in this country," Dr. Nesheiwat said on "Mornings with Maria" Thursday.

 "It's very important to be aware of what the temperature is outside."

EXCESSIVE HEAT WARNINGS REMAIN IN PLACE ACROSS US, EXPECTED TO LAST AT LEAST THROUGH MONDAY

Parts of the U.S. Southwest and West have been under Excessive Heat Warnings and advisories for the past two weeks. States including California, Arizona and Texas have seen temperatures rise well into triple digits. 

In California, a 65-year-old man died in his car at Death Valley National Park on Monday when temperatures reached 126 degrees, and National Parks officials believe his death was due to the extreme heat.

"Once it hits about 90 degrees, that's when we start to take precaution," Dr. Nesheiwat said. "But even lower than that, even a lower temperature, if it's humid outside, then you have to even be more careful."

Southern California was under an Excessive Heat Warning on July 3 when the man was found in his car off North Highway.

This is the fourth heat-related hiking death in a national park in two weeks.

EXPERTS SAY AMERICA'S HEAT WAVES WILL BECOME MORE FREQUENT AND ENDURING AFTER A DOZEN DIED IN TEXAS

"I've been having patients come in with body aches, cramping, headache, very, very thirsty, sometimes their temperatures are elevated, they feel nauseous, they feel a little bit confused and disoriented," Dr. Nesheiwat explained.

Dr. Nesheiwat added while there are some groups that are more at risk, heat stroke and heat-related illnesses can "affect anyone."

"We don't want heat exhaustion or heat cramps to progress into a heat stroke, which can affect anyone," she said. But those who are at high risk are the elderly, those who have high blood pressure or are taking certain medications, young children, and even athletes. I've had athletes come into the E.R. dehydrated. They're feeling unwell. They're feeling faint. Their temperatures are 103,104. Sadly, we lost an NFL player, Marion Barber, last year. He was only in his thirties and he died of a heat stroke. So it can affect anyone."

On July 3, a 57-year-old woman died from a heat-related illness in a remote area of the Grand Canyon National Park. Officials said she was attempting an 8-mile hike during 100-degree temperatures.

Last week, a father and son in Texas died hiking in Big Bend National Park during extreme heat. 

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Dr. Nesheiwat advised Americans to stay indoors when there are such extremely high temperatures but also shared ways to respond if an individual experiences heat-related symptoms.

"If that happens to you, what you want to do is get out of the heat right away. You want to go indoors where it's air-conditioned. Drink some cool water, remove the clothing, and if you're outdoors, you want to wear loose clothing."

"But after taking these steps to try to feel better, if you don't feel better, that's when you want to seek medical attention and medical care right away," she noted.

FOX Weather's Emilee Speck contributed to this report.