Heat exhaustion

Heat exhaustion is a severe form of heat illness. It is a medical emergency. Heat exhaustion is caused by the loss of water and electrolytes through sweating. 
The United States Department of Labor makes the following recommendation, "Heat illness can be prevented. Remember these three things: water, rest, and shade."
Causes.
Common causes of heat exhaustion include:
Especially during physical exertion, risk factors for heat exhaustion include:
Signs and symptoms.
Symptoms of heat exhaustion include nausea, dizziness, irritability, headache, thirst, weakness, vomiting, high body temperature, excessive sweating, and decreased urine output.
Treatment.
First aid.
First aid for heat exhaustion includes:
Emergency medical treatment.
If an individual with heat exhaustion receives medical treatment, Emergency Medical Technicians (EMTs), doctors, and/or nurses may also:
Prognosis.
If left untreated, heat exhaustion may progress to heat stroke.