Dangerously high temperatures across much of the United States have increased the risk of heat exhaustion and other heat-related illnesses. Knowing the signs of heat exhaustion and how to treat it can help prevent more serious problems.

What is heat exhaustion?

Heat exhaustion occurs when a person is no longer able to keep exerting themselves in the heat, usually because the heart can’t keep up with the demands on it. The body loses too much water and salt, usually through excessive sweating. It can happen during exercise, factory work or even a long wait at a bus stop in hot and humid weather.

Typically, when temperatures are high, someone experiencing heat exhaustion may start to feel dizzy, faint or nauseous, said Dr. Hany Atallah, an emergency medicine physician and the chief medical officer of Jackson Memorial Hospital in Miami.

“That means you’ve overheated, you’ve overexerted yourself,” Dr. Atallah said. “You really need to rest.”

Other signs of heat exhaustion include a headache, vomiting, muscle cramps and an increased heart rate

Who is most susceptible?

Heat exhaustion typically occurs in warmer months and during heat waves, said Dr. Jacquelyn Bowers, the director of emergency services at Ochsner L.S.U. Health System of North Louisiana. It most commonly affects people who are outdoors for long stretches without taking breaks to cool down and replace lost fluids. This can include outdoor workers, athletes and people who are homeless.