Heat
Heat exhaustion and heat stroke are separated by one category of symptom. Recognising which side of that line someone is on determines whether the situation is uncomfortable or immediately life-threatening.
Heat illness is a continuum rather than a set of discrete conditions, and progression can be rapid. What follows is field recognition, not clinical management — anything at the severe end requires emergency medical care.
Painful involuntary muscle spasms, typically in heavily worked muscles, occurring during or after exertion in heat. Core temperature is normal or near normal and mental status is unaffected.
Mechanism is debated. Electrolyte depletion, particularly sodium, is the traditional explanation and fits the observation that heavy salty sweaters are disproportionately affected. Neuromuscular fatigue theories account better for cramps that occur without measurable electrolyte disturbance. Both may operate.
Response: stop, move to shade, gently stretch the affected muscle, take fluid containing sodium. Resume only at reduced intensity, if at all.
The body is struggling to maintain cardiovascular function under combined heat and fluid loss. Core temperature is typically elevated but below 40°C.
Response: stop activity immediately and do not resume that day. Move to shade or air conditioning. Lie down with legs elevated. Remove excess clothing. Cool actively with wet cloths, fanning, ice packs to neck, armpits and groin. Give cool fluid if fully alert and not vomiting.
Improvement should be evident within about thirty minutes. If it is not, or if any change in mental status appears, treat it as heat stroke.
Thermoregulation has failed. Core temperature exceeds 40°C and multi-organ injury begins. Mortality is significant and rises with every minute above threshold.
Classic teaching describes heat stroke skin as hot and dry. In exertional heat stroke — the form that occurs in athletes, soldiers and hikers, as opposed to the classic form affecting elderly people in heat waves — the person is frequently still sweating profusely.
Waiting for dry skin before acting on suspected heat stroke costs lives. Mental status is the reliable sign; skin condition is not.
This inverts ordinary first aid instinct and it is the single most consequential point in this article. Outcome in exertional heat stroke correlates with time spent above the critical core temperature threshold. Cooling initiated on scene, before transport, produces markedly better outcomes than cooling delayed until hospital arrival.
Cold water immersion is the most effective method available and should be used wherever any body of water or container exists — a stream, a lake, a stock tank, a tarp filled with water and ice. Target cooling to roughly 39°C.
Where immersion is impossible: continuous dousing with cold water plus vigorous fanning, ice packs to neck, armpits and groin, removal of all clothing, and shade.
Call emergency services, then cool aggressively while waiting. Do not attempt to give fluids to anyone with altered consciousness.
Cool first, then transport. Every minute above the threshold increases organ damage, and the ambulance cannot cool faster than a stream can.
Unacclimatised individuals, particularly in the first days of a heat wave or after arrival in a hot climate. Those with recent illness, fever, vomiting or diarrhoea. People carrying a sleep deficit. Anyone taking diuretics, anticholinergics, antihistamines, beta blockers or stimulants. Those with prior heat illness, who carry elevated recurrence risk. Higher body mass and lower fitness both raise risk.
Move the activity out of peak heat, into early morning. Acclimatise deliberately before the season. Reduce intensity and duration when the heat index is elevated, rather than pushing through. Wear light loose clothing that permits evaporation. Take scheduled shade breaks rather than waiting for symptoms. Hydrate to a measured plan. Never train alone in extreme heat, because the person who cannot recognise their own altered mental status is the one experiencing it.
Check conditions before the session with the heat index and hydration calculator, which returns the National Weather Service risk band alongside fluid requirements for your intensity and body mass.
This article is general education, not medical advice. Suspected heat stroke is a medical emergency requiring immediate professional care.