Heatstroke occurs when the perspiration mechanism fails, and the body becomes unable to control its temperature. Body temperature can rise to 106°F or higher within 10 to 15 minutes.

"Heatstroke can kill and must be recognized as a medical emergency," says Michael Karp, M.D., assistant professor of clinical medicine at the Keck School of Medicine of USC.

He points out that it doesn't have to be over 100 degrees outside for someone to have heatstroke. “If there's particularly high humidity, there can be a danger at 75 degrees,” he says. “And if you're exerting yourself physically, the risk of heatstroke increases significantly."

Signs of heatstroke, which require immediate medical attention, include extremely high body temperature (above 103°F); red, hot, dry skin (no perspiration); rapid pulse; throbbing headache; dizziness; nausea and/or vomiting; confusion; and loss of consciousness. Luckily, heatstroke is entirely preventable.

“It's essentially just a matter of limiting your time in the sun and staying well hydrated," says Karp.

As a general rule of thumb, drink 16 fluid ounces of water an hour and a half-hour before going into the heat. Those working in the heat should drink one-half liter of water every half-hour.