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Temperatures were in the 100s when Vanessa Dunn, a 29-year-old Los Angeles-based makeup artist, was driving back home to California from Virginia last summer. After hours on the road and drinking limited water, she was struck by a severe case of dehydration and heat stroke.

”I wasn’t drinking enough water because I didn’t want to stop to pee,” she says. When she finally pulled over for the night she felt light-headed, and she couldn’t keep food down when she tried to eat. She even threw up blood.

”I was in incredible pain, and dizzy,” she says. “[I went] to the ER, turned out there was blood because my throat was so dry.”

Her story is not unusual. In 2014, more than 13,000 people visited the emergency room because of a heat-related illness such as heat stroke, according to data from the Centers for Disease Control. And on average, about 675 people die in the U.S. every year from heat-related illnesses.

Heat stroke is the most severe form of heat-related illness. It’s less common than other issues such as heat exhaustion (characterized by heavy sweating, weakness, cold, pale or clammy skin, fainting, a fast or weak pulse, and nausea or vomiting) or heat syncope (fainting). But heat stroke can happen quickly, to anyone, and can result in irreversible damage or death.

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Heat stroke is an extreme elevation of your body temperature that occurs when your body stops being able to regulate itself, according to Dr. James Wantuck, chief medical officer at PlushCare, an online urgent care provider. “If a fever is like an infection turning up your body’s thermostat, heat stroke is like a broken air conditioner,” he says.

Your body does an expert job of keeping its temperature around 98.6 degrees Fahrenheit under normal circumstances….