Sunstroke, more commonly called heatstroke in medical settings, is the most dangerous form of heat illness. It occurs when your body’s core temperature reaches 104°F (40°C) or higher and your brain starts to malfunction, causing confusion, seizures, or loss of consciousness. Unlike milder heat problems that resolve with rest and fluids, heatstroke is a medical emergency that can damage your brain, kidneys, and other organs if cooling doesn’t begin immediately.
How Heatstroke Differs From Heat Exhaustion
Heat illness exists on a spectrum, and the jump from heat exhaustion to heatstroke is significant. Heat exhaustion causes fatigue, headache, nausea, dizziness, and heavy sweating, but your core temperature stays below 104°F and your thinking remains clear. You feel terrible, but your brain is working normally.
Heatstroke crosses two critical lines at once: your temperature hits 104°F or above, and your central nervous system stops functioning properly. That second part is what makes it dangerous. Confusion, agitation, slurred speech, seizures, or coma are the hallmark signs. If someone seems disoriented or “not right” after being in the heat, that’s the red flag that separates a bad day from a life-threatening situation.
Two Types: Classic and Exertional
Classic heatstroke typically affects older adults and people with chronic health conditions. It builds up over days of heat exposure, often during heat waves, and these patients frequently have hot, dry skin because their sweating mechanism has failed entirely.
Exertional heatstroke hits otherwise healthy people, usually during intense physical activity in hot or humid conditions. Athletes, military personnel, and outdoor workers are the most common cases. Unlike classic heatstroke, people with the exertional form are often still sweating heavily. This is an important distinction: the presence of sweat does not mean someone is safe. If they’re confused or acting strangely after exertion in the heat, their body temperature may already be in the danger zone.
What Happens Inside Your Body
Your body cools itself primarily by sweating and by pushing blood toward the skin’s surface, where heat can radiate away. When this system gets overwhelmed, whether from extreme external heat, intense exercise, or both, core temperature climbs unchecked. At 104°F and above, the heat itself starts to injure cells throughout the body.
The brain is particularly vulnerable. The cerebellum (which controls coordination and balance), the hippocampus (involved in memory), and other deep brain structures can sustain direct cellular damage. This is why neurological symptoms define the condition. But the damage isn’t limited to the brain. Heatstroke can trigger a cascade of problems across multiple organ systems: kidney failure, liver damage, muscle breakdown, blood clotting abnormalities, and respiratory distress. The longer the body stays at extreme temperatures, the more widespread the damage becomes.
Symptoms to Recognize
The early signs of heatstroke often overlap with heat exhaustion, which is why the neurological symptoms matter so much for telling them apart. Watch for:
- Confusion, agitation, or bizarre behavior (the most important warning sign)
- Slurred speech or difficulty understanding conversation
- Hot skin, which may be dry or still sweaty depending on the type
- Rapid, pounding heart rate
- Headache that worsens suddenly
- Nausea or vomiting
- Seizures or loss of consciousness
Because the affected person’s brain isn’t working properly, they often can’t recognize or report their own symptoms. This makes it critical for the people around them to notice the change.
What to Do Immediately
Cooling needs to start before professional help arrives. Every minute at an extreme temperature increases the risk of permanent organ damage. Call emergency services first, then focus entirely on bringing the person’s temperature down.
The most effective first aid method is water immersion. Submerging the person in cold water (anywhere from ice water to around 63°F) cools the body dramatically faster than any other technique. A systematic review of cooling methods found that water immersion at any temperature between 34°F and 63°F lowered core temperature significantly faster than passive cooling alone, with no single water temperature proving clearly superior to another. A bathtub, a stock tank, even a kiddie pool will work.
When full immersion isn’t possible, apply ice packs to the neck, armpits, and groin, where large blood vessels run close to the skin. Wetting the skin and fanning aggressively creates evaporative cooling, which helps but works more slowly than immersion. Remove excess clothing. Move the person to shade or air conditioning. Do not give fluids by mouth if the person is confused or unconscious, as they could choke.
Who Is Most at Risk
Age is a major factor on both ends. Adults over 65 are more vulnerable because their bodies regulate temperature less efficiently and they’re more likely to have chronic conditions or take medications that interfere with cooling. Young children are at risk because their bodies produce more heat relative to their size and they depend on adults to keep them hydrated and cool.
Several common medications increase heat vulnerability in ways people don’t expect. Antihistamines like diphenhydramine (Benadryl) reduce sweating. Antipsychotic medications impair both sweating and the brain’s temperature regulation. Some antidepressants, including SSRIs and tricyclics, alter sweating patterns in different directions, both of which can be dangerous. Even the anti-seizure medication topiramate decreases sweat production. If you take any of these medications, you’re starting at a disadvantage on hot days and need to be more aggressive about staying cool.
Other risk factors include obesity, dehydration, lack of acclimatization to heat (such as the first hot week of summer), previous heatstroke, and working or exercising outdoors without adequate breaks.
Long-Term Effects for Survivors
Many people recover fully from heatstroke with rapid treatment, but the condition can leave lasting damage. Brain imaging studies conducted months or even years after a heatstroke episode have revealed persistent cellular damage in the cerebellum, hippocampus, and other brain regions. Survivors may experience ongoing problems with coordination, difficulty swallowing, trouble with speech, and cognitive impairment that doesn’t fully resolve.
Kidney injury from heatstroke sometimes progresses to chronic kidney problems. The severity of long-term complications correlates strongly with how long the body remained at dangerous temperatures before cooling began, which is why the speed of that first response matters so much.
Prevention That Actually Works
The basics are straightforward but easy to neglect. Drink water before you feel thirsty, especially during activity in the heat. Take breaks in shade or air conditioning. Wear lightweight, light-colored clothing. Avoid intense outdoor exercise during the hottest parts of the day, typically between 10 a.m. and 4 p.m.
If you’re not used to the heat, give your body time to adjust. Acclimatization takes about 10 to 14 days of gradually increasing heat exposure. This is why heatstroke cases spike at the beginning of summer or during the first heat wave of the year, before people’s bodies have adapted. Alcohol increases dehydration and impairs your ability to notice early warning signs, so it’s worth limiting intake on extremely hot days. And if you take any of the medications mentioned above, plan ahead on high-heat days: stay indoors during peak temperatures, drink more than you think you need, and don’t push through early symptoms like dizziness or nausea.