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What Is Heat Exhaustion? Causes and Symptoms

· 8 min read
Pressure Pal Team
Pressure Pal Team
Health & Weather Insights Team

Heat exhaustion is the body still trying to cope with heat — and mostly succeeding, but only just. The cooling system is working at maximum, the person feels awful, and the line to heat stroke is closer than most people realize. Recognized early and treated well, it usually settles in an hour or two. Pushed past, it becomes a medical emergency.

This article covers what heat exhaustion actually is, what causes it, how it differs from milder heat illness on one side and heat stroke on the other, and the full set of symptoms to recognize.

A working definition​

Heat exhaustion is the most common moderate heat-related illness. Core temperature is elevated — usually somewhere between 38°C and 40°C — but the cooling system is still working. Sweating is heavy. The brain is essentially intact: the person knows who they are and where they are, even if they feel terrible.

It sits on a continuum:

  • Heat cramps — painful muscle spasms from heavy sweating and salt loss; otherwise feeling well
  • Heat exhaustion — the topic of this article; cooling system overloaded but functional
  • Heat stroke — cooling system has failed; core temperature above 40°C, mental status changed

The clinical importance of heat exhaustion is mostly about not letting it become heat stroke. Treated promptly, it is reversible. Ignored, it progresses.

How the body gets there​

Heat exhaustion develops when the heat the body is producing or absorbing exceeds the heat it can dump. Two routes lead there:

Salt and water depletion — the more common pattern. Heavy sweating over hours, replaced with plain water or not replaced at all, depletes both sodium and total body fluid. Blood volume falls, blood pressure dips on standing, and the body cannot sustain the high cardiac output required to keep cooling.

Water depletion alone — less common but more dangerous, more typical in older adults during heat waves. Thirst is blunted, fluid intake is low, sweating continues until it does not, and the person becomes profoundly dehydrated before they notice.

Most real-world cases mix both.

Common causes​

A few patterns produce most cases of heat exhaustion:

  • prolonged work or exercise in heat, especially in humid conditions where sweat does not evaporate well
  • being unacclimatized to a hot environment — the first three to seven days are the highest risk
  • inadequate fluid intake during sweating
  • sun exposure without shade or hats
  • wearing clothing that traps heat — turnout gear, uniforms, costume work, kitchen whites
  • medications that blunt sweating or thirst, or that increase fluid loss
  • alcohol use during heat
  • chronic illnesses that limit the cardiovascular response — heart disease, diabetes, kidney disease
  • pre-existing dehydration from gastrointestinal illness or fasting
  • being in a hot indoor environment without ventilation during a heat wave

Many of these stack. A roofer in a heat wave, in their first week back from medical leave, on a blood pressure medication, who skipped breakfast, will get there faster than any one factor would predict.

The full symptom list​

Heat exhaustion symptoms come from two sources: the cooling effort itself and the resulting dehydration and salt loss. The full list:

  • heavy, sometimes profuse sweating
  • skin that is cool, pale, and clammy to the touch
  • a fast, weak pulse
  • low blood pressure on standing — dizziness, light-headedness, sometimes fainting
  • muscle cramps, especially in calves, thighs, abdomen
  • headache, often throbbing
  • nausea and sometimes vomiting
  • fatigue and a profound sense of weakness
  • general feeling of malaise — "not right," "wiped out"
  • thirst that does not feel quenched by drinking
  • dark amber urine, or reduced urine output
  • goosebumps in heat
  • chills despite a hot environment
  • difficulty concentrating, but still oriented
  • shallow, rapid breathing
  • a slight elevation in core temperature, usually 38°C to 40°C

The clearest distinguishing feature compared to heat stroke is that the person remains mentally intact. They feel terrible but they are themselves.

How it differs from heat stroke​

The line between heat exhaustion and heat stroke is where many of the worst clinical errors happen. The key differences:

  • Mental status — intact in heat exhaustion, altered in heat stroke. Confusion, irritability, slurred speech, seizures, or loss of consciousness move the diagnosis to heat stroke.
  • Core temperature — usually under 40°C in heat exhaustion, above 40°C in heat stroke.
  • Sweating — heavy in heat exhaustion; can be heavy in exertional heat stroke but typically stops in classic heat stroke.
  • Skin — cool and clammy in heat exhaustion; hot in heat stroke, dry or wet depending on type.
  • Response to cooling and fluids — heat exhaustion improves over 30 to 60 minutes; heat stroke does not improve without aggressive cooling and medical care.

The simplest practical line: heat exhaustion still feels like the person is themselves. Heat stroke does not.

Who is at the highest risk​

The risk groups for heat exhaustion overlap heavily with those for heat stroke:

  • outdoor workers in construction, agriculture, roofing, landscaping
  • athletes in early-season training in heat
  • military personnel in training
  • older adults during heat waves
  • people with chronic heart, kidney, or metabolic disease
  • adults on diuretics, antihypertensives, anticholinergics, antipsychotics, or stimulants
  • people who use alcohol heavily, especially in heat
  • pregnant women in the third trimester
  • people without access to air conditioning
  • people recently arrived from a cooler climate

Children are also vulnerable, especially during sports, and their early signs are usually behavioral — fussiness, refusing fluids, low energy — rather than complaints.

What to do​

The treatment is straightforward and almost always effective when started early:

  1. Stop the activity. No "just one more set."
  2. Move to a cooler environment — air conditioning if possible, deep shade if not.
  3. Loosen or remove unnecessary clothing.
  4. Apply cool water to skin and use the strongest air movement available. Cold packs to neck, armpits, and groin help.
  5. Sip cool fluids steadily. Sports drinks or oral rehydration solution work better than plain water in someone who has been sweating heavily.
  6. Lie down with feet slightly elevated if light-headed.
  7. Stay with the person and monitor for the next 30 to 60 minutes.

If they are not noticeably better in 30 minutes, or if mental status changes, or if vomiting prevents fluid intake — escalate to emergency services and treat as heat stroke until proven otherwise.

What not to do​

Several reflexes that make things worse:

  • pushing the person to "walk it off"
  • giving large volumes of plain water all at once — invites cramps and nausea
  • giving alcohol or caffeine
  • giving acetaminophen or ibuprofen — heat illness is not a fever
  • waiting too long to escalate when symptoms persist
  • letting them drive themselves home

Prevention​

Most heat exhaustion is preventable, and the prevention measures are the same ones that prevent heat stroke:

  • watch the forecast for heat advisories and humidity readings
  • acclimatize over one to two weeks before heavy work or training in heat
  • shift outdoor activity to early morning or evening
  • hydrate consistently across the day rather than catching up later
  • include some salt with fluids during long heat exposure
  • wear loose, light, breathable clothing
  • use sunscreen and a hat in direct sun
  • take real breaks in shade or air conditioning
  • limit alcohol on hot days
  • check on elderly neighbors and relatives during heat waves
  • review medications with a clinician before a heat-heavy season

The work-rest cycle matters more than the total hours. Pushing through early warning signs is the single most common path into heat exhaustion.

Where Pressure Pal fits in​

Pressure Pal centers on the barometric pressure forecast and is most directly useful for migraine and weather-sensitivity tracking. On hot days, pair it with a standard weather app's heat index, humidity, and dew point views, and you have a useful daily picture for planning around the worst hours — especially relevant if you are in a high-risk group or care for someone who is.

A daily forecasting habit makes the early warning signs of heat exhaustion easier to spot because you already know which days are likely to push the system.

Bottom line​

Heat exhaustion is the body's cooling system at maximum, with the person still intact mentally but feeling profoundly unwell. It is caused by heat exposure, salt and water loss, and the factors that make cooling harder. Symptoms cluster around heavy sweating, pale clammy skin, weakness, dizziness, nausea, headache, and a fast weak pulse.

Caught early, it responds quickly to shade, cooling, and slow rehydration. Pushed past, it becomes heat stroke. The treatment is simple, the prevention is simpler, and the recognition is the part that takes practice.