Heat exhaustion is a heat-related illness in which overheating and cardiovascular strain cause marked weakness and a loss of physical capacity. The German term is Hitzeerschöpfung. Heavy sweating can cause water and salt losses that add to the strain. During evasion, the person may consequently lose the ability to keep moving, negotiate terrain safely and reach a suitable resting place in time.
Heat and circulation
Working muscles produce heat. Solar radiation and heat from hot air and the surroundings add to that load. The body releases heat through the skin: skin blood vessels dilate, more blood reaches the surface, and evaporating sweat removes heat from it. High humidity limits evaporation. Impermeable clothing and protective equipment can restrict heat and water vapour transfer; their weight also increases exertion. High internal heat production can therefore become dangerous even in moderate ambient temperatures.
The circulation must supply working muscles, the increased skin blood flow needed for cooling and the vital organs. If it cannot sustain those demands, physical capacity falls. Substantial fluid losses can reduce circulating blood volume and worsen the problem. Heat exhaustion and dehydration can occur together, but drinking alone cannot remove the heat load. Additional heat production and heat gained from the environment must also be reduced.
Warning signs during evasion
Unusual weakness, a marked drop in performance, headache, dizziness, nausea or vomiting during or after heat exposure call for stopping and cooling. Heavy sweating and thirst may accompany them. Someone finding it increasingly difficult to maintain their previous pace while developing these symptoms should avoid forcing another long stretch of movement. Dizziness increases the risk of falling on a slope, an embankment or while climbing over an obstacle.
These symptoms suggest heat illness but do not establish a specific cause. Low blood sugar, a severe infection or an electrolyte disturbance can look similar. Particularly after drinking large amounts, headache with nausea must not simply be interpreted as a need for more water: excessive fluid intake can dangerously dilute sodium in the blood. These symptoms alone do not allow a reliable distinction.
Stop exertion and cool
First stop exertion and put down the carried load. If dizzy, sit or lie down so that a circulatory collapse does not cause a fall. Use an immediately reachable shaded, cooler resting place. If the present location exposes the person to an immediate risk of gunfire or capture, a short move into reachable protection may be necessary; a long search for an ideal hide produces more muscular heat. The resting place should permit cooling while limiting the view of the person.
Open or remove unnecessary heavy clothing and layers that trap heat. Use cool water actually available to wet the skin or light clothing, and fan air over it. Evaporation removes heat; additional air movement assists it. A completely enclosed covering with little air permeability can impede that heat loss. Shade reduces solar heat input but does not replace the release of heat already stored in the body. In high humidity, little water may evaporate even from wet skin.
If water or cooling supplies are absent, stop exertion immediately and release unnecessary trapped heat. If no shade is immediately reachable, first rest where stopping is possible instead of producing additional heat through a strenuous search. Available loose clothing can shield the skin from direct solar radiation; open and loosen it so that air reaches the skin. An additional impermeable wrapping traps heat and water vapour. If an immediate threat makes relocation unavoidable, keep its distance and exertion as small as possible. If heat loss remains insufficient, the body continues to gain heat despite resting.
Drinking and replacing losses
Someone who is awake, mentally clear and able to swallow safely can drink available safe drinking water in small, repeated sips. Large amounts at once are often poorly tolerated when nauseated. If drowsy, confused or unable to swallow safely, take no fluid by mouth: it may enter the airway. Repeated vomiting limits fluid replacement and is a reason to regard the situation as more serious.
Sweat contains water and salt. After prolonged heavy sweating, available food or an appropriate electrolyte drink can help replace losses. Salt tablets or large quantities of salt taken on speculation are unsuitable for self-aid. Large fluid volumes should not be forced: they can further disturb the electrolyte balance.
Serious signs and the course of illness
Confusion, disorientation, abnormal speech or coordination, a seizure or unconsciousness during heat exposure immediately raise suspicion of severe heat illness, including heat stroke. The brain and other organs may be damaged. Profuse sweating does not rule out heat stroke. Start cooling with available means immediately; do not wait for sweating to stop, a temperature reading or a fixed waiting period. Declining mental orientation also makes choosing a location, using equipment and drinking safely unreliable.
If symptoms worsen despite rest and cooling, persist, or include repeated vomiting or collapse, medical care is urgently needed. Use a genuinely reachable connection and available assistance early, while continuing cooling. Brief improvement does not justify another long march; renewed weakness or dizziness indicates that the load is still too high.
Military terms
US military usage distinguishes heat exhaustion and heat stroke as separate heat illnesses. British operational medicine describes illness caused by exertion as Exertional Heat Illness, or EHI, and separates mild from severe forms. In mild illness, the ability to continue physical activity breaks down; brain dysfunction determines the severe form. S3F names cooling with basic means: Shade, Strip, Spray, Fan.