Heat Exhaustion

Heat exhaustion is the most common heat-related condition observed in active populations ranging from athletes to recreational hikers. It is defined as the inability to continue exercise in the heat due to cardiovascular insufficiency (not enough blood pumped to the heart) and energy depletion that may or may not be associated with physical collapse.  Cardiovascular insufficiency refers to when the heart has difficulty providing enough oxygenated blood to all the working organs and muscles and is exacerbated by dehydration via extreme sweating without replacing fluids during exercise.  Heat exhaustion is one of the most common medical conditions reported to park rangers at the Grand Canyon and other recreational hiking areas.

It is important to note that heat illnesses are not on a continuum and that one condition does not lead to another condition, although having heat exhaustion one day can predispose an individual to heat illness the next day.  A common misconception is that heat exhaustion can lead to heat stroke.  This cannot happen because once the individual collapses from heat exhaustion they stop exercising in the heat, making exertional heat stroke impossible.  The body stops producing metabolic heat due to muscle contraction because exercise has ended, inhibiting exertional heat stroke.


How do you prevent heat exhaustion?

  • Individuals should adapt to exercise in the heat gradually — acclimatize — over 10-14 days by progressively increasing duration and intensity of work in hot conditions
  • Athletes should recognize heat exhaustion and exertional heat stroke signs and symptoms to indicate the need to slow, modify, or stop activity before a medical emergency arises
  • Being adequately hydrated before and during exercise can help prevent heat illnesses including heat exhaustion.  Maintaining blood volume is key in the prevention of heat exhaustion
  • Appropriate work to rest ratios based on environmental conditions is necessary in the prevention of heat illnesses.  Increasing rest break durations as ambient temperature increases is warranted.


What puts an individual at risk for heat exhaustion?

  • Exercising in hot and humid environment (air temp > 33°C)
  • Inadequate fluid intake before or during exercise resulting in dehydration
  • Inappropriate work to rest ratios with too much work compared to rest breaks
  • Body mass index > 27 kg/m

 Look for these symptoms in athletes when heat exhaustion are suspected:

Fatigue Nausea Fainting
Weakness Vomiting Dizziness / Light-headedness
Pale Chills Diarrhea
Heavy Sweating Decreased Urine Output / Dehydration Irritability
Headache Sodium Loss Decreased Blood Pressure
Decreased Muscle Coordination Hyperventilation Core body temperature between 36-40°C (96.8-104°F)

How do you treat an individual with heat exhaustion?

Individuals experiencing heat exhaustion should respond quickly to treatment.  If not, exertional heat stroke should be suspected.  In this case, obtain a rectal temperature and assess central nervous system function to rule out exertional heat stroke.  If the person is experiencing heat exhaustion, the rectal temperature should be < 40°C/104°F. To treat heat exhaustion:

  • Move the individual to a cool/shaded area and remove excess clothing
  • Elevate legs to promote venous return
  • Cool the individual with fans, rotating ice towels, or ice bags
  • Provide oral fluids for rehydration

When can the individual return to activity?

Returning to activity the same day of an episode is not prudent or advised. Individuals should wait at least 24-48 hrs before returning to activity and should gradually increase intensity and volume of exercise, and clothing and equipment.  Medical clearance is recommended to rule out other conditions.


  1. Armstrong LE. Exertional Heat Illnesses. Human Kinetics; 2003.
  2. Armstrong LE, Casa DJ, Millard-Stafford M, Moran DS, Pyne SW, Roberts WO. American College of Sports Medicine position stand: exertional heat illness during training and competition. Med Sci Sports Exerc. 2007;39:556–572.
  3. Binkley HM, Beckett J, Casa DJ, Kleiner DM, Plummer PE. National Athletic Trainers’ Association position statement: exertional heat illnesses. J Athl Train. 2002;37:329–343.