Heat Stress Symptoms: Exhaustion vs Heat Stroke

The short answer

Heat stress symptoms usually start with heavy sweating, cramps, tiredness, dizziness, headache or nausea. Those point to heat exhaustion, and they often ease once you cool down and sip water. Confusion, passing out, hot red skin, or a body temperature of 103°F (39.4°C) or higher point to heat stroke, which needs a 911 call right away (CDC Heat-Related Illnesses).

Signs of heat exhaustion

Heat exhaustion happens when your body loses a lot of water and salt, usually through sweat. The CDC lists these signs (CDC Heat-Related Illnesses):

• Heavy sweating
• Cold, pale and clammy skin
• Fast, weak pulse
• Nausea or vomiting
• Muscle cramps
• Tiredness or weakness
• Dizziness or headache
• Fainting

What to do, per the CDC: move to a cool place, loosen your clothes, put cool wet cloths on your body or take a cool bath, and sip water. Get medical help right away if you're throwing up, your symptoms get worse, or they last longer than 1 hour.

Heat cramps sit at the milder end: muscle pain or spasms with heavy sweating during hard exercise. Stop, cool off and drink water or a sports drink. The CDC adds an important note: get medical help right away for heat cramps if you have heart problems, you're on a low sodium diet, or the cramps last longer than an hour (CDC Heat-Related Illnesses).

Heat exhaustion vs heat stroke

Heat stroke happens when the body can no longer control its temperature. The CDC lists (CDC Heat-Related Illnesses):

• Body temperature of 103°F (39.4°C) or higher
• Hot, red, dry or damp skin
• Fast, strong pulse
• Headache, dizziness, nausea
• Confusion
• Losing consciousness

Here's the quick comparison:

| | Heat exhaustion | Heat stroke | |---|---|---| | Skin | Cold, pale, clammy | Hot, red, dry or damp | | Pulse | Fast, weak | Fast, strong | | Mind | Tired, weak, dizzy | Confused, may pass out | | Body temperature | Can run high | 103°F (39.4°C) or higher | | First move | Cool down, sip water | Call 911 |

What to do for heat stroke, per the CDC: call 911, move the person somewhere cooler, and help lower their temperature with cool cloths or a cool bath. Don't give the person anything to drink (CDC Heat-Related Illnesses).

Notice that sweating doesn't rule out heat stroke. The skin can be dry or damp. Confusion is the bigger tell.

Why heat makes your heart work harder

When you get hot, your body cools itself by sending blood to the skin, where heat can escape. That takes a lot of blood. During passive heat stress, skin blood flow can rise from about 300 mL a minute to as much as 7,500 mL a minute. Cardiac output (the blood your heart pumps each minute) can reach about 13 liters a minute, with roughly half of it headed to the skin. The heart gets there mostly by beating faster (Crandall 2010).

A healthy heart handles that extra work. A heart weakened by heart failure or other heart disease may struggle to raise its output enough to keep skin blood flow up, so body heat builds. Dehydration makes it harder still, and heat can also thicken the blood and raise platelet counts, which matters for people with vascular disease (Kenny 2010). During the 1995 Chicago heat wave, a large share of the excess deaths occurred in people with a prior heart condition (Crandall 2010).

That's the part of heat I talk about with patients every summer. Prevent it, don't just treat it.

Who's at higher risk

Heat can hurt anyone, but some groups run into trouble sooner:

• Older adults. Aging lowers the skin blood flow response to heat and the sweat each gland produces. Older adults also tend to feel less thirsty when they're dehydrated (Kenny 2010).
• People with heart disease. For the reasons above (Kenny 2010; Crandall 2010).
• People taking certain medicines. The CDC flags several common classes (CDC Heat and Medications):
  • Diuretics ("water pills" such as furosemide or hydrochlorothiazide) can cause dehydration, electrolyte imbalance, fainting and falls.
  • Beta blockers can reduce blood flow to the skin and decrease sweating.
  • ACE inhibitors and ARBs can lower blood pressure enough to raise the risk of fainting, and can blunt thirst.
  • Anticholinergics and some antipsychotics can impair sweating and the brain's temperature control.
• Young children, especially anywhere near a parked car (below).

If you take any of these, keep taking them. The CDC specifically warns against stopping medicines abruptly without a plan (CDC Heat and Medications). Ask your doctor or pharmacist what to watch for on hot days. For how weather numbers translate into real heat load, see our guide on what the feels like temperature means.

Hot car dangers for children and pets

Parked cars heat up fast, even on mild days. In a Stanford study, a dark sedan parked on clear days of 72 to 96°F (22 to 36°C) warmed an average of 3.2°F every 5 minutes, with about 80% of the rise in the first 30 minutes. Within an hour, the cabin ran roughly 40°F (22°C) hotter than outside. Even on the coolest day tested, the inside reached 117°F (47°C). Cracking the windows 1.5 inches didn't meaningfully help (McLaren 2005).

The toll stays stubbornly high. NoHeatStroke.org counts 1,070 US children who have died of vehicular heatstroke since 1998, about 37 a year, including 35 in 2025 (NoHeatStroke.org).

Pets sit in the same hot cabin, so the same rules apply:

• Never leave a child or pet alone in a parked car, even with the windows cracked or the engine off for "just a minute."
• If you see a child or pet alone in a hot car, call 911.

Some cars can keep the cabin from overheating while parked. Our guide to Tesla Cabin Overheat Protection explains what that feature does and doesn't cover. It doesn't replace taking your kid or dog with you.

How to lower your risk on hot days

A few habits go a long way, and you can read more about why the perfect temperature changes with the weather and your body:

• Check the heat index along with the temperature, and remember that full sun can push it higher.
• Do hard outdoor work or exercise early or late in the day.
• Drink water through the day, before you feel thirsty, unless your doctor has limited your fluids.
• Check on older relatives and neighbors, especially anyone with heart disease or on the medicines above.
• Give yourself time to adjust when the heat arrives. Your body adapts over days, not hours.

When to see a doctor

Call 911 right away for anyone with signs of heat stroke: confusion, passing out, hot red skin, or a temperature of 103°F (39.4°C) or higher. Call 911 for any child or pet left alone in a hot car.

Get medical help promptly if heat exhaustion symptoms get worse, last longer than an hour, or come with vomiting, or if you get heat cramps and have heart problems (CDC Heat-Related Illnesses).

Talk with your doctor before summer if you have heart disease or take diuretics, beta blockers, ACE inhibitors, ARBs or other medicines that affect heat tolerance. Start before you have to. This guide shares general information and can't diagnose you or replace care from your own clinician.