Heat exhaustion or heat stroke: how to tell them apart
The two conditions arrive on the same Phoenix afternoon and are not the same emergency. What the CDC’s symptom list actually says, and why sweating is a poor test.
Short answer
Heat stroke comes with confusion, slurred speech, fainting, a seizure, or skin that has gone very hot: the CDC’s guidance is to call 911 and cool the person while help is on the way. Heat exhaustion shares the milder symptoms (headache, nausea, dizziness, weakness, heavy sweating) and the CDC sends that one to a clinic or emergency room rather than home to the sofa. The two look alike for the first twenty minutes, so the deciding question is never how hot the person seems.
Two conditions arrive on the same afternoon, in the same heat, and look alike for the first twenty minutes. Heat exhaustion and heat stroke are not a mild version and a severe version of one illness. They are two entries on the CDC’s heat-illness list with different symptom lists, different urgencies, and one symptom that decides between them.
This page describes what the CDC and the National Weather Service publish. It cannot tell you which one you are looking at, and it does not try to.
The difference in one table
The columns below are the CDC’s own symptom sets, taken from its NIOSH heat-illness page. The wording in the middle and right columns is a shortened version of the CDC’s; the tool linked at the foot of this page prints the full text.
| Heat exhaustion | Heat stroke | |
|---|---|---|
| What the CDC says is happening | The body is losing more fluid and salt than it is taking in | The body can no longer control its temperature |
| Symptoms | Headache, nausea, dizziness, weakness, irritability, thirst, heavy sweating, an elevated body temperature, passing much less urine than usual | Confusion, slurred speech, fainting or being unable to be woken, a seizure, skin that feels very hot or sweating that has stopped |
| The CDC’s response | Take the person to a clinic or emergency room for medical evaluation and treatment | Call 911 for emergency medical care |
| The trap | People treat this one at home | People wait for the skin to go dry |
| Where it can lead | Heat stroke, if it is ignored | A body temperature of 106 °F or higher within ten to fifteen minutes |
That last row is the reason the two are worth telling apart. The CDC describes heat stroke as a state the body reaches within minutes, not hours, which means the useful decision point is early, and it is made on confusion, speech and consciousness rather than on a thermometer.
The symptoms that decide it
Five symptoms on the site’s Heat-Illness Triage Guide are treated as red flags, meaning they return the emergency reading on their own whatever else is ticked. They are the CDC’s heat-stroke symptoms, and they are worth knowing in the plain form a bystander would use:
- Confusion, or not making sense when speaking
- Slurred speech
- Fainted, or cannot be woken
- A seizure
- Skin that feels very hot, or sweating that has stopped
Everything else on the list (headache, nausea, dizziness, weakness, thirst, cramps, a rash) belongs to one of the milder conditions, and the triage guide sorts those too. But a red flag short-circuits the sort. That is a deliberate choice on the tool: being wrong in that direction costs an ambulance ride, and being wrong in the other direction costs a life.
Why sweating is a poor test
Most people carry a picture of heat stroke that includes dry skin, and it is not wrong so much as incomplete. The CDC’s symptom line is:
Hot, dry skin or profuse sweating
A person in heat stroke can be sweating heavily and still be in heat stroke. The dryness people look for is what happens when the sweating mechanism has already failed, which is later in the sequence. If you are watching for a dry forehead before you act, you are watching for the wrong moment.
The same caution applies to how hot the person feels. An elevated body temperature appears on the CDC’s heat-exhaustion list as well, so a warm-feeling person is not automatically the emergency case, and a person who feels less hot than you expected is not automatically fine. The symptom that separates the two is neurological (confusion, speech, consciousness), not thermal.
Hours of a normal day at or above a 90 °F heat index
Counted hour by hour from the Phoenix Sky Harbor normals, 1991–2020. Eight months of the year never reach the line. The four that do are the four months the body never gets an easy hour.
NOAA NCEI U.S. Climate Normals 1991–2020, hourly summaries, Phoenix Sky Harbor (USW00023183). The heat index is this site’s arithmetic: the NWS Rothfusz regression applied to the normal temperature and the normal dew point for the same hour. A normal temperature and a normal dew point do not always occur in the same hour, so these are typical conditions rather than an observed heat index.
What a normal July does to the count
Sixteen hours out of twenty-four is the figure for July, and it is the same in August. June manages eleven and September eight, and no other month reaches the line at all.
Read that against the symptom lists above and the shape of the problem changes. Phoenix does not present a hot afternoon that a careful person can work around. It presents a day with two or three cool hours in it, which is a scheduling problem before it is a medical one: the reason the site builds a Heat-Safe Hours Planner rather than a single warning.
The two months that reach a heat index of 100 °F are July and August, five hours each, which is worth holding next to the fact that July’s normal low is 84.5 °F. The hours at or above 90 are mostly the hours either side of the hottest part of the day, and they include the late evening.
Rhabdomyolysis, the one most people have not heard of
The CDC lists a fourth condition that is not heat exhaustion and not heat stroke: rhabdomyolysis, where damaged muscle tissue releases proteins and electrolytes into the blood. It can cause dangerous heart rhythms, seizures, kidney damage or death.
Its signs are dark urine the colour of tea or cola, muscle aches or cramps worse than you would expect, an unusual weakness, and being unable to finish a workout you could do before. It shares its ranking with heat stroke on the triage tool, and the reason is in the CDC’s own wording:
You can’t tell by symptoms alone if you have rhabdo. The only way to know for sure is to see a healthcare provider.
That sentence is the whole section. Dark urine after hard work in the heat is not a wait-and-see symptom.
Where this page stops
This is a description of published guidance, not a diagnosis, and the two are further apart than they look. Several of these conditions share symptoms; a person can be sweating and in heat stroke; and this page has no way to examine anyone.
What it can do is put the CDC’s own words in front of you and the Heat-Illness Triage Guide one click away, so the decision path matches the published one rather than someone’s recollection of it. If there is any doubt about a person who is confused, cannot be woken, or has had a seizure in the heat, the guidance on the page is to call 911 and stay with them. Everything else on this page is secondary to that.
The CDC names the people for whom all of this arrives sooner: adults aged 65 and over, infants and young children, anyone with a chronic medical condition, and (the entry that reads as though it were written for this city) people without air conditioning. If that is someone in your household, the highest-risk article in this section goes through it at length, and the outdoor-work article covers the version of the problem that has a shift pattern attached.
- CDC National Institute for Occupational Safety and Health: “Heat-related Illnesses”
Every symptom in the comparison table, the first-aid steps for heat stroke, heat exhaustion, heat syncope, heat cramps and heat rash, and the warning that heat stroke can present with profuse sweating rather than dry skin.
- NOAA National Weather Service, Phoenix: heat safety and heat-illness first aid
The local office’s wording for the heat-illness symptoms it prints for the Phoenix area, used where it says something the national page does not.
- CDC National Institute for Occupational Safety and Health: Rhabdomyolysis (rhabdo)
The signs of rhabdomyolysis, and the CDC’s own statement that symptoms alone cannot tell you whether you have it.
- CDC: Heat and Health Index and heat risk factor guidance
The list of people the CDC names as at higher risk from heat.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, hourly summaries, Phoenix Sky Harbor (USW00023183)
The hours of a normal day at or above a 90 °F heat index in the chart, and the normal July afternoon temperature.