Extreme Heat: How Your Body Responds and How to Stay Safe

Your body has a narrow comfort zone—roughly 36 to 37.2 degrees Celsius—and it works hard to stay there. When your brain senses the temperature climbing, it triggers a cascade of responses meant to cool you down. You start to sweat. Your blood vessels dilate. Your heart rate climbs. These are ancient, reliable mechanisms that have kept humans alive through countless summers. But when the heat becomes extreme, especially when humidity prevents sweat from evaporating efficiently, your body's defenses begin to fail.

Dr. Judith Linden, who leads emergency medicine at Boston Medical Center, explains that sweating is the body's primary cooling strategy. As perspiration evaporates from your skin, it carries heat away. But in humid conditions, that evaporation slows dramatically. "Cuanto mayor es la humedad, menores temperaturas se necesitan para un calor extremo," Linden notes—the more moisture in the air, the lower the threshold for dangerous heat. When your body can't shed heat fast enough, your core temperature rises, and that excess heat begins damaging your brain and vital organs.

Heat-related illness progresses in stages, each more serious than the last. The mildest forms—heat cramps and heat rash—are common and usually manageable. Heat cramps develop when heavy sweating depletes your body's salt and water reserves, causing painful muscle spasms in the abdomen, arms, or legs. Heat rash is a skin irritation that appears as red bumps or blisters, most common in young children in the folds of skin where sweat collects. These conditions are uncomfortable but not dangerous.

Heat exhaustion marks a turning point. Your body is now struggling visibly to cool itself. You sweat profusely, feel dizzy or faint, experience nausea and headaches. Your skin turns pale and clammy. Your pulse races. Linden describes this as "the last attempt of the body to cool itself before entering a point of no return." If someone shows these signs—dizziness, nausea, headache—the response is immediate: move them out of the heat into a cool space, apply water, give them fluids to drink.

Heat stroke is the final, life-threatening stage. Your body temperature climbs above 40 to 40.5 degrees Celsius, and your cooling mechanisms begin to shut down entirely. The warning signs are severe: extremely high body temperature, dry flushed skin, rapid pulse, severe headache, dizziness, nausea, or loss of consciousness. But the hallmark symptom, Linden emphasizes, is confusion and agitation. "When someone is in the heat and becomes confused and agitated, that is heat stroke until proven otherwise," she says. This is a medical emergency. Call 911. Get the person to emergency care immediately.

Certain groups face far greater risk. The elderly, people with chronic medical conditions, and young children are most vulnerable. Older adults may not sense temperature changes as acutely and often take medications that worsen heat's effects. Young children may not recognize they're overheating and won't remove themselves from danger. Student athletes and pets are also at high risk—and Linden is emphatic on one point: never, under any circumstances, leave a child or pet in a car, even for a minute.

Protecting yourself requires vigilance and planning. During extreme heat, avoid outdoor activity between 11 a.m. and 3 p.m., when the sun is strongest. If you must go outside, wear light-colored clothing, cover your head, and drink water constantly—don't wait until you're thirsty, as thirst itself is a sign of dehydration. Linden recommends at least one glass of water per hour. Seek out air conditioning whenever possible: a shopping mall, a library, anywhere with cool air. At home, fans provide some comfort but won't prevent heat illness; they're useful only if you can also spray yourself with water. Most importantly, watch out for others—check on elderly neighbors, isolated friends, and family members who may be struggling silently with the heat. The moment someone shows confusion or dizziness, that's the moment to act.