How heatstroke kills—it can still be deadly after your body cools down

Europe is enduring another devastating heatwave, leading to over 10,000 excess deaths in late June 2026. As climate change accelerates, heatstroke is expected to become more common, raising concerns about its often deadly consequences. When body temperature exceeds 40°C (104°F), the brain can malfunction, causing confusion and seizures, while muscles break down, releasing cellular waste into the bloodstream.

However, even after the body cools down, patients can still die hours or days later. This mysterious post-heatstroke mortality is attributed to the immune system's hyperactive inflammatory response. Overheating diverts blood from vital organs to the skin, starving the gut of blood flow. The compromised gut barrier allows microbial components to leak into the bloodstream, triggering an inflammatory reaction.

Simultaneously, heat-damaged cells release distress signals that further activate the immune system, leading to a runaway chain reaction. White blood cells called neutrophils become overactive, releasing enzymes that damage blood vessels and organs. These enzymes can also activate the blood-clotting system, forming clots that obstruct blood supply to organs like the kidneys, liver, and brain.

Cooling the patient may remove the trigger, but it doesn't stop the inflammatory cascade, which can cause delayed organ failure. Vulnerable groups include older adults, infants, those with heart disease, diabetes, obesity, and those on certain medications. Athletes and outdoor workers also face heightened risks. Current treatment focuses on rapidly cooling the patient, but future therapies may target the immune response to prevent organ damage.