Heat stroke or heat exhaustion (referred to medically as exertional heat illness, or EHI) can not only interfere with your training, but have a severe impact on your health, so it's important to take precautions. This means being aware of the symptoms, and not simply dismissing them as the side effect of an intense workout.

If you find yourself experiencing light-headedness, excessive sweating and thirst, a higher than normal heart rate, rapid breathing, headache or nausea, then you should stop what you're doing immediately, and move somewhere shaded or air-conditioned, says Michael Bergeron, Ph.D. , the heat, hydration, and research advisor to Major League Soccer. More extreme symptoms, such as slurred speech or loss of consciousness are considered medical emergencies, and you should call 911.

Medical professionals advise exercising first thing in the morning when it's still cool, or waiting until the temperatures have eased in the evening. "When the temperature is above 80 degrees and humidity is greater than 75 percent, the risk for heat injury is high," says Micah Zuhl, Ph.D. , an assistant professor in the department of health, exercise and sport sciences at the University of New Mexico. If the temperature is above 89F (32F), it might be worth skipping your workout.

In addition to wearing sunscreen, it's important to avoid diuretics like caffeine or alcohol when training in hot weather, and to hydrate before, during and after any exercise session.

Hydration is about so much more than H20," says Chris Mohr, Ph.D. , the co-owner of well-being consultancy Mohr Results, Inc. "Hydration is about sodium, and potassium, and sweat, and pee (yes, pee!). It's also not just about what you drink, but what you eat."