Heat Stroke vs Heat Exhaustion: When to Call 911

Heat stroke vs. heat exhaustion: the one sign that tells you to call 911

We live in a place where the heat can kill people, and every summer I watch families hesitate at exactly the wrong moment because they are not sure which one they are looking at. So I am going to give you the dividing line first, before anything else, because in two decades of practicing medicine I have learned that the minutes spent deciding are the minutes that matter most.

The line is mental status. Heat exhaustion is miserable but the person is still themselves — thirsty, weak, nauseated, drenched, wrung out, but oriented and making sense. Heat stroke means the brain is affected: confusion, agitation, slurred speech, stumbling, a seizure, or unconsciousness, together with a core temperature above 104°F (40°C). Formally, heat stroke is defined as that high core temperature plus central nervous system dysfunction. Practically, if the person is not acting right in the heat, you do not need a thermometer to make the call.

If you see altered mental status: call 911 and start cooling now

Heat stroke is a true emergency, and cooling speed drives survival. Call 911 first, then begin cooling while you wait — do not choose one or the other. Get them out of the sun, strip off clothing and any gear, and cool aggressively: cold water immersion if there is any way to do it, otherwise soak them with water and fan hard, and pack ice at the neck, armpits, and groin. Do not wait for the ambulance to start.

One thing people get wrong with the best intentions: do not pour fluids into someone who is confused or barely conscious. They cannot protect their airway and you can cause them to aspirate. Oral hydration is for people who are alert and making sense. Everyone else gets cooling and paramedics.

The spectrum below heat stroke

Heat illness is a continuum, and everything on this list can be managed at home as long as thinking stays clear and temperature stays under 104°F:

Heat cramps — painful muscle cramping during or after exertion. Rest, fluids with electrolytes, gentle stretching and massage of the muscle.
Heat edema — swollen feet and ankles in the first days of hot weather. Elevate the legs. Diuretics are not the answer here and can make things worse.
Heat syncope — fainting or near-fainting, usually after standing a while in the heat. Lie flat, elevate the legs, cool and rehydrate.
Heat rash — remove the clothing trapping the sweat and let the skin cool and dry. Skip the heavy ointments; they make it worse.
Heat exhaustion — heavy sweating, weakness, headache, nausea, dizziness, fast pulse, clammy skin, still mentally clear. This is the one that sits right at the edge, and it can progress. Treat it seriously and watch the person.

Home treatment when there is no altered mental status

Stop the activity immediately. Get into air conditioning or deep shade and lie down, legs elevated if there is any faintness. Take off the extra clothing. Cool by evaporation, which is the most effective thing most people can do at home: spray or sponge with tepid water and fan the skin, or lay cool wet towels or ice packs on the neck, armpits, and groin. A cool shower works too.

Then rehydrate with water plus salt, not water alone. The CDC’s simple homemade oral rehydration solution is a quarter to a half teaspoon of table salt stirred into one liter of water, with a splash of citrus juice or a little sugar to make it drinkable. The sugar is not decoration — sodium and glucose together are absorbed better than either alone, which is the whole principle behind oral rehydration. A commercial sports drink does the same job. So does plain water alongside salty food, which the CDC notes is equally effective and often more palatable than choking down salt water.

Eat your water: the foods that rehydrate

Sweat is mostly water and sodium, so the goal is to replace both. Food does a lot of this work, and in an Arizona summer it goes down easier than another warm water bottle.

High-water produce: watermelon, cantaloupe, cucumber, oranges and citrus, strawberries, tomatoes, lettuce, celery. Free water plus potassium.
Potassium-rich: bananas, oranges, potatoes, yogurt.
Salt to pair with the water: salted crackers, pretzels, salted nuts, broth and soups. This is the half people skip, and it is the half that matters most, because what you lose in sweat is predominantly sodium.
Water and electrolytes together: yogurt, milk, broth-based soups, fruit smoothies.

My favorite version of this, and the one I actually tell patients: salt your watermelon. A light sprinkle of salt on cold watermelon gives you water, potassium, sugar, and sodium in one bite, it tastes better than plain melon, and children will eat it without an argument. Same logic behind salting the cantaloupe or having a pickle with lunch. It is the oral rehydration principle disguised as a snack.

The trap: too much plain water

This is the one almost nobody sees coming. Drinking large volumes of salt-free water during heavy sweating can drop your blood sodium far enough to cause exercise-associated hyponatremia, and it looks like heat stroke — confusion, and in severe cases seizures. If someone becomes confused in the heat but is not especially hot, over-hydration deserves as much suspicion as heat. This is exactly why the guidance is water plus sodium, and why the hydration advice from the Wilderness Medical Society is to drink to thirst rather than force a rigid schedule. Rigid schedules produce both underhydration and overhydration; thirst is a better instrument than a stopwatch.

Go to the ER, even without altered mental status, if:

Symptoms are not improving quickly with rest and cooling.
The person cannot keep fluids down, or keeps vomiting.
Urine is dark or cola-colored. That can mean muscle breakdown, and it threatens the kidneys.
Mental status changes at any point. That is no longer heat exhaustion.

Prevention actually works better than treatment

The Wilderness Medical Society is blunt that prevention outperforms treatment, and the interventions are unglamorous and effective:

Acclimatize before you need to. Roughly 80 percent of heat acclimatization is built in about a week of moderate exertion in the heat, an hour or two a day; even three 90-minute outdoor sessions in one week can cut heat strain meaningfully. The person who goes from air conditioning straight into a Saturday hike is the person who gets hurt.
Know your own risk. Prior heat injury makes recurrence more likely. Higher body mass raises risk. So do a number of common medications — diuretics, beta-blockers, anticholinergics, antihistamines, antipsychotics — along with alcohol and stimulants. And a recent sunburn impairs your ability to sweat for about a week, which is worth respecting before you head back out.
Move the clock, not the effort. Early morning or evening, frequent shaded rest breaks, and take off the heat-trapping gear when you can.
Air conditioning is the intervention. Home, a mall, a library, a cooling center — anywhere. Fans are useful up to a point, but above roughly 99°F they stop helping and in very dry air they can make things worse, which describes Phoenix in July precisely.
Check on people. Older adults, anyone isolated, anyone with limited mobility. A daily phone call in July is a medical intervention here.

Measure, don’t guess

If you keep wilting in heat that other people tolerate, that is information, not weakness. Thyroid function, anemia, blood sugar, kidney function, hormone status, and your medication list all change how you handle a Mesa summer, and every one of them is measurable. That is the conversation worth having in the fall, before next June. Related reading: the six daily habits that carry the most weight, signs of magnesium deficiency and what masquerades as it, and what it costs to stay healthy and active in retirement.

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To Health and Wellness,
Dr. Tallman

This page is general education, not a substitute for emergency care. If you suspect heat stroke, call 911.

References

American Family Physician, 2025 — Wilderness Medical Society guidelines on prevention and treatment of heat illness.
American Family Physician, 2026 — heat-related illnesses: recognition and management.
The New England Journal of Medicine, 2022 — treatment and prevention of heat-related illness.
The New England Journal of Medicine, 2019 — heatstroke.
The New England Journal of Medicine, 2024 — climate change, extreme heat, and health.
The New England Journal of Medicine, 2023 — preventing heat-related illness among outdoor workers.
Journal of Occupational and Environmental Medicine, 2021 — prevention of occupational heat-related illnesses.
CDC Yellow Book — heat and cold illness in travelers.