Sunstroke is a life-threatening condition in which your body’s cooling system fails and your core temperature rises to 104°F (40°C) or higher. It’s the most severe form of heat illness, and without treatment, the mortality rate can reach 80%. Sunstroke and heatstroke are the same condition; “sunstroke” is simply the older, more colloquial term.
How Your Body’s Cooling System Fails
Your body normally keeps its temperature in check through two main strategies: pushing blood toward the skin’s surface so heat can radiate away, and evaporating sweat. Both of these depend heavily on your cardiovascular system. During intense heat exposure, your heart has to simultaneously send blood to your muscles, your organs, and your skin. When the demand outpaces what your heart can deliver, skin blood flow drops, sweating becomes less effective, and heat starts building up faster than your body can shed it.
Humidity makes everything worse. Sweat only cools you when it evaporates, and evaporation depends on the moisture difference between your skin and the surrounding air. In humid conditions, that gap shrinks, so your most powerful cooling mechanism stalls. During vigorous exercise, your muscles can produce heat at 15 to 18 times your resting metabolic rate, which is why athletes and outdoor laborers are at particular risk even on days that don’t feel unbearable.
Symptoms to Recognize
The hallmark of sunstroke is a change in mental state. Confusion, agitation, slurred speech, delirium, and seizures all signal that the brain is overheating. Some people become irritable or combative before progressing to unresponsiveness. If someone who has been in the heat suddenly seems “off” mentally, treat it as an emergency.
Other signs include:
- Hot, dry skin in cases caused by prolonged heat exposure (though people who collapse during exercise may still be sweating heavily)
- Nausea and vomiting
- Rapid, pounding pulse
- Flushed skin
The skin clue trips people up. Many assume that someone with sunstroke will have stopped sweating entirely, but exercise-related cases often involve profuse sweat. The mental confusion is the more reliable red flag.
Who Is Most at Risk
Older adults, especially those over 65 with chronic conditions like diabetes, heart disease, kidney disease, or dementia, face the highest risk. In one large study of Medicare beneficiaries, the average age of patients hospitalized during heat waves was over 80, and nearly two-thirds were women.
Several common medications also interfere with the body’s ability to cool itself. Diuretics (water pills) and blood pressure medications like ACE inhibitors can contribute to dehydration. Beta blockers limit how much the heart can ramp up output to push blood toward the skin. Medications with anticholinergic effects, which include many allergy drugs, antipsychotics, and some antidepressants, reduce sweating directly. If you take any of these, you have a narrower margin of safety in the heat.
Young, healthy people are not immune. Athletes, military recruits, and outdoor workers account for most exertional sunstroke cases. The combination of high physical output, direct sun, and inadequate hydration can overwhelm even a fit cardiovascular system.
When the Heat Index Gets Dangerous
The National Weather Service uses the heat index, a combination of air temperature and humidity, to estimate the actual stress your body faces. At a heat index of 90 to 105°F, sunstroke becomes possible with prolonged activity. Between 105 and 130°F, it becomes likely. Above 130°F, it’s considered highly likely with any continued exposure.
Those numbers assume shade and light wind. Direct sunshine can add up to 15 degrees to the effective heat index, meaning a day with a listed heat index of 95°F can feel like 110°F in full sun. This is why the same temperature that feels manageable under a tree can become dangerous on an unshaded playing field or construction site.
What to Do Immediately
Speed of cooling is the single most important factor in survival. Rapid cooling, defined as dropping core temperature by more than 0.15°C per minute, significantly reduces organ damage and death. Every minute counts.
Cold water immersion is the most effective cooling method available. If you can get the person into a tub, kiddie pool, or any container of cold water, do it. Ice water at around 2°C (35°F) can lower core temperature at a rate of at least 0.2°C per minute, which is fast enough to bring someone below the danger zone in roughly 10 to 20 minutes. If a tub isn’t available, pouring cold water or cold beverages continuously over the body while fanning is the next best approach. Emergency dispatchers now sometimes guide bystanders to use 10 to 20 bottles of cold drinks from nearby stores or coolers for this purpose.
Methods that seem intuitive but perform poorly include placing ice packs on the neck, armpits, and groin; using cooling blankets; and relying on a single fan. Studies consistently show these techniques cool too slowly to prevent organ damage.
Organ Damage and Mortality
Sunstroke is not just about getting too hot. The extreme temperature triggers a cascade of inflammation throughout the body that can damage multiple organs simultaneously. The kidneys are especially vulnerable: in one study, acute kidney injury occurred in over 90% of non-exertional sunstroke patients. The liver, brain, heart, and blood clotting system are all at risk as well. Problems with blood clotting, measured by a scoring system used in hospitals, were directly linked to higher odds of dying during the hospital stay.
Even with modern critical care, mortality for severe cases requiring intensive treatment can exceed 60%. Liver failure secondary to sunstroke accounts for a meaningful share of acute liver failure cases in the United States. These numbers reflect the most serious end of the spectrum, but they illustrate why sunstroke is not something to “wait out” or treat casually.
Recovery and Lasting Effects
With prompt cooling and treatment, many people recover with little or no permanent organ damage. But “prompt” is doing a lot of work in that sentence. The longer the body stays above 104°F, the greater the chance of lasting harm.
Some people experience lingering effects for weeks or months after an episode. These can include difficulty coordinating movements, slurred speech, and trouble forming new memories. In more severe cases, organ function problems can persist for years, raising the risk of chronic kidney disease, heart issues, or other conditions that require ongoing medical follow-up.
Perhaps the least well-known consequence: a single episode of sunstroke can make you more susceptible to heat illness in the future. Your body’s thermoregulatory system may not fully reset, meaning activities and temperatures you tolerated before may become risky. People who have had sunstroke once often need to be more cautious about heat exposure for the rest of their lives.