Sweating is the way in which the body maintains it’s core temperature at 37 degrees centigrade. This results in the loss of body fluid and electrolytes (minerals such as chloride, calcium, phosphate, magnesium, sodium and potassium) and if unchecked will lead to dehydration and eventually circulatory collapse and heat stroke. The effect of fluid loss on the body is as follows:

% body weight lost as sweat
Physiological Effect
2%
Impaired performance
4%
Capacity for muscular work declines
5%
Heat exhaustion
7%
Hallucinations
10%
Circulatory collapse and heat stroke

Dehydration can affect an athlete’s performance in less than an hour of exercise — sooner if the athlete begins the session dehydrated.
Dehydration of just one to two percent of body weight (only 1.5-3 lb.. for a 150-pound athlete) can negatively influence performance.
Dehydration of greater than three percent of body weight increases an athlete’s risk of heat illness (heat cramps, heat exhaustion, heat stroke).
High-body-fat athletes can have a harder time with exercise and can become dehydrated faster than lower-body-fat athletes working out under the same environmental conditions.
Poor acclimatization/fitness levels can greatly contribute to an athlete’s dehydration problems.
Medications/fevers greatly affect an athlete’s dehydration problems.
Environmental temperature and humidity both contribute to dehydration and heat illnesses.
Clothing, such as dark, bulky, or rubber protective equipment can drastically increase the chance of heat illness and dehydration.
Wet bulb temperature measurements should be taken 10-15 minutes before practice, and the results should be used with a heat index to determine if practices or contests should be started, modified or stopped.
Even dry climates can have high humidity if sprinkler systems are scheduled to run before early morning practices start. This collection of water does not evaporate until environmental temperatures increase and dew points lower. Dry climate areas should take wet bulb and temperature readings 10
to 15 minutes before practice or contests.
A Heat Index chart should be followed to determine if practice/contests should be held.
A Heat Index chart should come from a reputable source like the National Oceanic and Atmospheric Association.
A relative humidity of 35 percent and a temperature of 95 degrees Fahrenheit are likely to cause heat illness, with heat stroke likely.
A relative humidity of 70 percent and a temperature of 95 degrees Fahrenheit are very likely to cause heat illness, with heat stroke very likely.