40°C heat and your body: The kidneys, heart and other organs that suffer the most (and how to stay safe)

For most of human history, temperatures this extreme meant seeking shade and slowing down. But in our modern world, temperatures are rising faster than our bodies can adapt, and the damage happening inside us is more serious than we realize.
When your core body temperature hits 40 degrees Celsius—which is 104 Fahrenheit—something fundamental changes. "Organs start shutting down and cells deteriorate," Jason Kai Wei Lee, an expert on heat's impact on the human body at Singapore's NUS Yong Loo Lin School of Medicine told TIME. "An overtaxed heart can go into cardiac arrest." At that threshold, you're no longer just uncomfortable. You're in heat stroke territory. And if your temperature keeps climbing even slightly higher, multiple organ systems begin to fail simultaneously.

The kidneys are almost always first to go. That's not an exaggeration—that's what the research shows. When extreme heat hits, your body makes a desperate choice. It diverts blood away from your internal organs, including the kidneys, and pushes that blood toward your skin surface to try to cool you down through sweating. Kidneys need consistent, reliable blood flow. They're metabolically demanding organs. When that blood flow gets cut off, they start dying. "During periods of extreme heat, physiologic adaptations redirect blood flow from visceral organs, including the kidneys, to the skin surface to facilitate heat dissipation," researchers explain in a 2024 study. "This redistribution, compounded by dehydration and volume loss, leads to renal hypoperfusion, ischemia, and heightened susceptibility to acute kidney injury."

Then there's the heart. It's essentially a muscle that never stops working, and in extreme heat it's forced into overdrive. Your cardiovascular system has to maintain blood pressure while your blood vessels are dilating everywhere to cool you down. That's a physiological impossibility at a certain point. The heart's workload becomes unsustainable. A 2024 studyon heat stress and cardiac mortality in migrant workers found correlations between heat exposure and heart-related deaths, but the mechanism is simple enough: push any muscle hard enough for long enough, and it will fail.

What happens to your brain might be the scariest part. Heat stroke isn't just a physical crisis. It's a neurological emergency. At 40°C and above, the blood-brain barrier—the protective wall that guards your brain from toxins and pathogens—becomes compromised. Immune cells infiltrate brain tissue. Inflammation accelerates. Your memory, attention, and decision-making abilities collapse. In acute heat stroke, confusion and delirium set in quickly. Patients become disoriented. They don't know where they are. They don't know what day it is. Some have seizures.
But the brain damage doesn't stop when the crisis passes. Recent research shows that heat stroke causes lasting changes. "Heat-related conditions encompass an array of symptoms, including cognitive dysfunction, emotional agitation, epilepsy, lethargy, and coma," according to research published in 2024. The study notes that "heat stroke stands as the most severe, characterized by body overheating under high temperature, often involving dysfunction of the central nervous system and multiple organs." Long-term studies have found cellular damage in the cerebellum, hippocampus, midbrain, and thalamus—the regions controlling memory, balance, and basic survival functions.

The liver also takes a beating. During extreme heat, blood flow to the liver decreases while inflammatory signals increase. The organ that's responsible for detoxifying your blood and regulating metabolism gets stressed into dysfunction. Your ability to process medications, filter waste, and maintain metabolic balance degrades. Heatstroke-induced liver failure is uncommon but not rare—and when it happens, it's often fatal.

What makes this worse is that heat waves are becoming more frequent, more intense, and lasting longer. India recorded more than 40,000 suspected cases of heat stroke and 100 deaths in June 2024 alone during one of its longest recorded heat waves. Iran essentially shut down its government for two days in August 2023 because the heat was too dangerous. Europe keeps breaking temperature records year after year. And as temperatures rise, the recovery time between heat waves shrinks.

Older adults are more vulnerable because their bodies have lost some capacity to sweat and to redirect blood flow efficiently. People taking certain medications, blood pressure drugs, diuretics, some antidepressants, have their thermoregulation impaired. Outdoor workers, athletes, military personnel. People who are homeless. Anyone who can't escape the heat or access cold water. And because heat waves hit harder where there are fewer trees, less air conditioning, and more heat-absorbing pavement, it's often the poorest and most vulnerable who suffer first and worst.

Get inside and stay cool
Drink water before you feel thirsty
Cool your home at night
Protect your skin and cover up
Check on vulnerable people around you
Know the warning signs: Heat cramps, heat exhaustion, and heat stroke are different stages of the same problem. Early signs are headache, sweating, fatigue, dizziness, nausea, and lightheadedness. If someone is confused, has a very high body temperature, hot dry skin, rapid weak pulse, rapid shallow breathing, or is vomiting—that's heat stroke. Call emergency services immediately. Don't wait. Heat stroke causes brain damage and death if not treated in minutes, not hours.
Monitor official heat warnings from your local health authorities