In many sports, heat is treated as a nuisance instead of a threat. Runners race on 110-plus days, and cycling races are often run no matter the temperature. That's a dangerous mistake. Even the most experienced, record-breaking athletes can succumb to heat stroke, as ultrarunner Michael Popov did when he died in Death Valley this past August.

Part of the problem lies in differentiating heat stroke, which can kill, from its milder cousin, heat exhaustion. The issue is that the symptoms aren’t all that different to the untrained eye, says Dr. Douglas Casa, chief operating officer of the Korey Stringer Institute at the University of Connecticut. In fact, the majority of symptoms are shared: decreased performance or weakness, collapse, muscle cramps, nausea, and headache being the most obvious.

The difference is that heat stroke victims experience central nervous system dysfunction—irrational behavior, irritability, emotional instability—and a rectal temperature greater than 104 degrees, something that is understandably difficult to measure in the field. Compounding the difficulty, heat stroke can strike when it isn’t entirely expected, like on milder days, if athletes don't get enough water or are out of shape.

WHAT HAPPENS: You’re creating more heat than you can dissipate. “Your core temperature gets so high that your cells are damaged,” says Casa. “Organs like the kidney, brain, and liver start to malfunction. If the body is above 105 degrees for more than 30 minutes, that’s when you run the risk of permanent complications or death.”

SIGNS AND SYMPTOMS: While symptoms like irrational behavior can be a tip-off, “rectal temperature is really the only way to be sure that the reason they’re having CMS dysfunction is due to heat stroke and not due to some other reason,” says Casa.

PREVENTION: Hydrate properly and don't exercise in serious heat if you're not properly acclimated.

TREATMENT: “Cool the person as quickly as possible, and as soon as possible, usually with whole-body ice immersion,” Casa says. “If that’s not possible, use rotating cold, wet towels to cover as much of the body as possible.”

THE MYTHS: You stop sweating. If you’re hydrated, you’re safe. Ice baths are bad.

THE REALITY: Nope, heat stroke victims keep sweating. It can occur before you have time to become dehydrated.  And ice is good, says Casa. “There used to be people thinking that cold water immersion as actually not good because you’d shiver or have peripheral vasoconstriction,” he says. “They’re wrong.”

THE BOTTOM LINE: You’ve got 30 minutes to get your temperature under control. Do whatever it takes to make that happen. Cool first and get to the hospital later.