Heatstroke can remain life-threatening even after a person's body temperature has been brought back to normal, as the immune response triggered by extreme heat may continue damaging vital organs for days.
The issue is becoming increasingly important as heat waves grow more frequent and intense. More than 10,000 excess deaths were recorded across Europe during the final days of June 2026 alone, and researchers warn that climate change is likely to increase the prevalence of heatstroke.
Heatstroke occurs when the body's core temperature rises above 40°C (104°F). Severe overheating can impair brain function, causing confusion and seizures, while also damaging muscles and releasing harmful substances into the bloodstream.
However, the direct effects of heat are only part of the danger. The immune system can respond to heatstroke with a powerful inflammatory reaction that resembles the response to a severe infection—even though no infection is present.
The chain reaction may begin in the gutScientists believe one of the first problems develops in the intestines. When the body overheats, blood flow is redirected toward the skin to help release excess heat. As a result, the gut can become deprived of blood and oxygen.
The intestinal lining normally acts as a barrier, keeping trillions of bacteria inside the digestive tract. Severe heat stress can weaken this barrier, allowing bacterial components to enter the bloodstream and activate the immune system.
At the same time, heat-damaged cells throughout the body release molecular signals that indicate tissue injury. These two sources of danger signals can trigger an excessive inflammatory response.
Research into this process has focused in part on neutrophils—white blood cells that normally provide an early defense against infections. During heatstroke, however, they can become excessively activated.
Hyperactive neutrophils release enzymes that can damage blood vessels and organs. Research suggests that this state may persist for several days, even after the person's temperature has returned to normal.
Neutrophils may also interfere with the body's blood-clotting system. Their activity can contribute to the formation of tiny clots in small blood vessels, potentially reducing blood flow to organs such as the kidneys, liver and brain.
Animal studies have suggested that preventing inflammation-related clotting may improve survival, raising the possibility that future treatments could target the immune response rather than focusing solely on lowering body temperature.
Why cooling alone may not be enoughRapid cooling remains the most important treatment for heatstroke and can save lives. But by the time a patient reaches a hospital, the inflammatory process may already have gained momentum.
Lowering body temperature removes the original trigger, but it does not necessarily switch off the immune response immediately. As a result, some patients can develop serious complications, including kidney failure or neurological damage, hours or days after their temperature has returned to normal.
The risk is particularly high among older adults and young children, whose bodies have more difficulty regulating temperature. People with cardiovascular disease, diabetes or obesity may also be more vulnerable, as may those taking certain medications. Dehydration, alcohol consumption and social isolation can further increase the risk.
Athletes and people who work outdoors face another danger: prolonged physical exertion in extreme heat can overwhelm the body's ability to cool itself.
For now, the priority in treating heatstroke remains rapid cooling. Researchers are increasingly looking at what happens afterward, however, including ways to control excessive inflammation, prevent dangerous blood clots and protect the intestinal barrier.
Such approaches could eventually help reduce the organ damage that can continue even after the body has been cooled.
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