Heat Exhaustion

Heat Exhaustion: Overview

Heat exhaustion, and the more serious heat stroke (hyperpyrexia), is caused by a breakdown of the heat regulation system, for example from constant heat stress (day and night), insufficient physical fitness, being overweight, alcohol stress, too much physical stress, overly warm clothing, medications (e.g. "water pills"), infections and insufficient fluid intake.  Sweat production diminishes and the body temperature rises to temperatures of around 39-41°C.  The skin becomes dry and red; severe headaches occur.

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The Body's Heat Regulation Mechanisms. Because the body must maintain a relatively constant body temperature of around 37°C, it needs to be able to eliminate excess heat.  Most of the heat is conducted in the blood stream to the skin and then the heat is released in three ways:

  • Evaporation, in which excess heat is transferred to sweat which is then evaporated.  Evaporation causes heat loss, but evaporation does not take place so easily in high humidity.  So, on a hot, humid day, you can be dripping with sweat, but because the sweat is not evaporating it is not doing you any good.
  • Radiation, in which excess heat is carried by means of electromagnetic radiation to cooler objects around the body.  Blood vessels near to the skin dilate to provide a greater surface area.  However, where the temperature of the surrounding air approaches body temperature (37°C), this will not have much effect.
  • Convection, in which excess heat it transferred to air passing over the skin, particularly if it is windy.

Profuse sweating leads to a decrease in blood volume, i.e. dehydration.  You should not rely on feeling thirsty: by the time you feel thirsty you are already low on fluids.  The best indicator of proper fluid levels is urine output and color – your urine should be "copious and clear".  Dark urine means the body has concentrated the urine because it is low on fluids.  You also lose salts when sweating.

Heat illnesses include:

  • Heat Cramps – caused by insufficient salt.  Replace salt and fluid and stretch the affected muscle(s).
  • Heat Syncope (fainting) – In an effort to increase heat loss, skin blood vessels dilate to such an extent that blood flow to the brain is reduced resulting in faintness, dizziness, headache, increased pulse rate, nausea and vomiting.  Dehydration contributes significantly to this.
  • Heat Exhaustion – where fluid losses from sweating and respiration are greater than fluid reserves.  The lack of fluid causes the body to constrict blood vessels especially in the arms and legs (making the skin pale and clammy), have decreased urine output and make the affected person feel weak, dizzy and thirsty with possible nausea and vomiting.
  • Heat Stroke – A life-threatening condition where the core body temperature rises to above 41°C.  There are two types of heat stroke:

    • Fluid Depleted Heat Stroke – despite fluid loss due to sweating and inadequate fluid replacement, the person continues to function.
    • Fluid Intact (fast onset) Heat Stroke – heat challenge overwhelms body's active heat loss even though fluid level is sufficient.  The cause is generally overexertion despite already being overheated.  Signs include hot, pale skin (may be dry or wet); severe change in mental status and motor/sensory function; possible dilation and unresponsiveness of pupils to light.

Sweat not only contains substantial amounts of sodium (1gm per liter), but also modest amounts of potassium and small amounts of minerals such as iron and calcium.  Although it is possible to adapt to exercising in hot environments, one can not adapt to being dehydrated.  One key adaptation is that your sweat becomes more dilute (there are fewer electrolytes, including sodium, dissolved in it).  The 'weaker' sweat means that more sodium is being conserved inside the body; this preserved sodium 'pulls' more water into the blood, keeping blood volume high even in the face of fairly heavy sweating.  A key benefit is that the higher blood volume helps to keep heart rate fairly low during hot-weather exertion, making exercise feel less demanding and troublesome.

The extra blood volume also lessens the intense conflict which is usually set up between the muscles and skin during hot weather.  The muscles selfishly want more blood because of the oxygen it contains, while the skin demands blood for cooling.  The increased blood volume allows both demands to be met fairly successfully.

While your sweat glands are being stingy with sodium, your kidneys also help you adapt to the heat by holding on to water more tightly (lowering your urine output), which also boosts blood volume.  You become more resistant to both overheating and steep, hot-weather-related declines in performance.

Causes and Development

Heat exhaustion is mainly a problem in hot, humid environments.  Hiking in hot weather often causes fingers and hands to become swollen and puffy, so much so that it may become hard to make a fist.  If one is not taking in adequate electrolytes, the imbalance in salt levels between the blood stream, the cells, and the extracellular spaces essentially results in a trapping of the water in the tissues as the sodium is lost in sweat.  The same thing can happen if too much salt is being taken.

Treatment and Prevention

The only method of treatment is by cooling the afflicted person.  The entire body should be laid down in the shade and packed in moist compresses.  The patient should be taken as soon as possible to a hospital.

Other treatment consists of sitting or laying the affected person in the shade, elevating the feet and administering fluids slowly.  Putting half a teaspoon of salt and half a teaspoon of baking soda in each liter of water helps to replace lost electrolytes, as do 'oral rehydration salts'.  Cool (but not cold), wet cloths should be used to reduce body temperature; using a fan will increase cooling by evaporation.

General advice includes staying hydrated and cool.  Before exercising in hot weather (best avoided though, if possible), drink half a liter of water on waking up and add some salt to your breakfast to make sure you do not start the day dehydrated or low on sodium.

During exercise, wear light clothing to allow sweating; take on plenty of water and use cold, wet cloths to stay cool.  Drink and rest regularly.  Sweat consists sodium, chloride, potassium, proteins and fatty acids; in order to replace what you are losing, eat salty snacks and foods high in sodium and potassium.

After exercise, continue drinking regularly for the rest of the day to replace lost fluids.


Left untreated, this condition brings on a limitation of brain function, cramp attacks and finally coma.  The fatality rate for severe hyperpyrexia is 20%.

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Heat Exhaustion:

Signs, symptoms & indicators of Heat Exhaustion:

Symptoms - Metabolic

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Recommendations for Heat Exhaustion:


Increased Salt Consumption

Most commercial sports drinks have modest sodium content in order to make them more palatable.  A person exercising in heat needs as much as 2gm of sodium per liter for proper recovery – which would make sports drinks taste like seawater (there is about 0.5gm of sodium per liter of Gatorade).  Drinking large amounts of plain water is not ideal in itself because it shuts off the sense of thirst and produces more urine which results in further fluid loss even though you are dehydrated.

You can increase your sodium intake by eating sodium-containing foods.  Some people believe that table salt (sodium chloride, NaCl) is not a good way to get your sodium (note that there is 1gm of sodium in every 2.5gm of table salt), but others say that it is necessary.


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