Symptoms of heat exhaustion include fatigue; goose bumps; weakness; headache
; dizziness; nausea; vomiting; decreased coordination; possible fainting; and skin that is cool, moist, pale, or flushed.
Heat exhaustion occurs when you cannot sweat enough to cool your body. It most commonly happens when exercising intensely in a hot, humid condition.
The easiest way to avoid heat disorders is to drink fluids before, during and after exercise. The body's fluid needs vary with exertion, climate, humidity, terrain, and other factors. The new fluid recommendations say that runners should "obey your thirst" and drink when their mouth is dry and they feel the need to drink. Drink before workouts and make sure you have access to fluids if exercising longer than 30 minutes. During longer workouts, some of your fluid intake should include a sports drink to replace lost salt and other minerals (electrolytes).
It's also important that runners take weather conditions into account when determining how fast or far they're running. Brutal heat and humidity mean you should scale back your performance goals. Don't try to beat the heat. Hot and humid conditions are not the time to try to push your pace. Slow down, take walking breaks, and save your hard efforts for cooler weather. Stick to the treadmill if you want to run your usual pace and distance.
More: Tips for Preventing Heat-Related Illnesses
Tips for Racing in Hot Weather
If you experience symptoms of heat exhaustion, stop running immediately. Drink fluids and get in the shade or an air-conditioned room as soon as possible. Loosen or remove excess clothing.
"Exercise and Fluid Replacement", ACSM Position Stand, American College Of Sports Medicine, Medicine and Science In Sports & Exercise, 2007.
MayoClinic.com - Heat Exhaustion
Tamara Hew-Butler, DPM, Joseph G. Verbalis, MD, and Timothy D. Noakes, MBChB, MD, DSc, "Updated Fluid Recommendation: Position Statement From the International Marathon Medical Directors Association (IMMDA)," Clinical Journal of Sports Medicine, 2006;16:283–292)