The defining symptom of heat stroke is a core body temperature of 104°F (40°C) or higher, combined with changes in mental function like confusion, slurred speech, or loss of consciousness. Unlike milder heat illnesses, heat stroke affects the brain and can damage multiple organs if cooling doesn’t begin immediately.
The Key Symptoms
Heat stroke produces a specific cluster of warning signs that set it apart from less dangerous heat-related conditions. The most recognizable symptoms include:
- High body temperature: 104°F (40°C) or above
- Confusion or altered mental state: difficulty thinking clearly, acting disoriented, or becoming agitated
- Slurred speech
- Loss of consciousness
- Seizures
- Hot skin that may be dry or damp depending on the type of heat stroke
- Rapid heart rate
- Nausea or vomiting
- Headache
The neurological symptoms are the clearest red flag. Muscle cramps, heavy sweating, and dizziness can all happen with heat exhaustion, a less severe condition. But once confusion, slurred speech, or seizures appear, the situation has crossed into heat stroke territory and requires emergency treatment.
How Heat Stroke Differs From Heat Exhaustion
Heat exhaustion and heat stroke exist on the same spectrum, but heat stroke is a medical emergency while heat exhaustion typically is not. With heat exhaustion, you feel weak, dizzy, and nauseous, and you may sweat heavily. Your body is struggling but still managing to cool itself. You can usually recover by getting to a cool place, resting, and drinking fluids.
Heat stroke means your body’s cooling system has failed. The critical difference is what happens to your brain. If someone is confused, speaking incoherently, stumbling, having seizures, or losing consciousness, that points to heat stroke rather than exhaustion. A person with heat exhaustion may feel terrible, but they can still think and communicate clearly. Someone with heat stroke often cannot.
Two Types With Different Patterns
Heat stroke comes in two forms, and they don’t look exactly the same.
Exertional heat stroke hits healthy, active people during intense physical effort. Athletes, military recruits, and outdoor workers are the typical cases. It develops fast, often within hours, and the skin is usually damp with sweat because the person has been physically exerting themselves. Lack of acclimatization to hot conditions is a major risk factor.
Classic (non-exertional) heat stroke builds over two to three days of heat exposure. It primarily affects older adults, young children, and people with chronic medical conditions. The skin tends to be hot and dry, though it can sometimes be moist. This type often occurs during heat waves, especially when people lack air conditioning or adequate access to fluids. Because it develops gradually, the early warning signs can be easy to miss until confusion or collapse makes the situation obvious.
What Happens Inside the Body
Heat stroke isn’t just about feeling overheated. At 104°F and above, sustained high temperature starts damaging organs directly. The brain is the most immediately affected, which is why confusion and seizures are hallmark symptoms. Prolonged heat can cause swelling, small bleeds, and tiny blood clots in the brain, and lower levels of consciousness are associated with a higher risk of lasting neurological problems.
The gut is one of the first organs hit. Heat damages the lining of the intestinal wall, increasing its permeability and allowing bacteria to leak into the bloodstream. The liver takes significant damage as well, with impaired ability to clear waste products from the blood. The kidneys can fail, particularly when muscle tissue breaks down (a condition called rhabdomyolysis) and floods the kidneys with proteins they can’t process. The heart works harder as it tries to pump blood to the skin’s surface for cooling, and a sustained elevated heart rate increases the risk of cardiac injury.
In severe cases, multiple organs can begin failing simultaneously. This cascade is what makes heat stroke potentially fatal even after the person has been cooled down, because organ damage may continue to unfold over the following hours and days.
Warning Signs That Come Before
Heat stroke rarely strikes without any lead-up. Your body sends progressively louder signals as it overheats. Early symptoms of heat illness include muscle cramping, unusually heavy sweating, shortness of breath, dizziness, headache, weakness, and nausea. These are signs of heat exhaustion, and acting on them can prevent progression to heat stroke.
Pay particular attention if sweating suddenly stops despite continued heat exposure, or if mild dizziness progresses to confusion. The transition from heat exhaustion to heat stroke can happen quickly, especially during intense exercise or in extreme temperatures. Children and older adults may not recognize or communicate their symptoms effectively, so watching for behavioral changes like irritability, unusual lethargy, or disorientation is important in those groups.
What to Do Immediately
Heat stroke requires emergency medical care, but cooling should start before help arrives. Every minute at a dangerously high body temperature increases the risk of organ damage.
Move the person to the coolest environment available. Then cool them aggressively using whatever you have: a cold bath or shower, a garden hose, cool wet towels, or ice packs placed on the neck, armpits, and groin where large blood vessels sit close to the skin. Fanning while misting the skin with water helps accelerate evaporative cooling. Covering the person with cool, damp sheets is another option.
If the person is conscious and able to drink, offer chilled water or a sports drink with electrolytes. Avoid alcohol and caffeine. Do not try to give fluids to someone who is unconscious or seizing. The priority is getting body temperature down as fast as possible while waiting for emergency responders.