How much heat can a human take?

In cool air it is easy. Skin sits at around 35 degrees, and air cooler than skin takes heat away by itself: warmth radiates off you and the air carries it off. Top right shows how much the air is taking. At 22 °C it takes more than you make. That is why you reach for a jumper.

As the air nears skin temperature, that route closes. At 35 degrees, in this simple model, the air and your surroundings take nothing. Above 35 the flow reverses, and the air heats you. From here on there is one way out.

Sweat. It cools you by leaving: water that evaporates takes heat with it. Each litre that evaporates off the skin carries away about 2.4 megajoules, roughly 670 watts for an hour. Sweat that drips onto the floor does almost nothing. And to leave, it needs air with room left for water.

A July afternoon like Bucharest's: 36 °C, but dry air at 28 % humidity. The wet bulb reads 22.4 °C. Sweat leaves easily.

A summer evening by the sea: only 30 °C, but 70 % humidity. The wet bulb climbs to 25.6 °C, higher than on the 36-degree afternoon. On a cooler day, the body has less room to shed heat.

33 °C at 80 % humidity, an ordinary monsoon day in South Asia or on the Persian Gulf coast. Wet bulb: 30.0 °C. Try it yourself: move the sliders or pick a place.

Theory assumed an ideal body. A team at Penn State put young, healthy volunteers in a climate chamber, doing minimal activity, and slowly raised the heat and humidity until their core temperature could no longer level off. None reached 35. In humid heat the average was 30.55 °C, and in hot, dry air it was lower still. The study was published in 2022.

The same lab then tested 49 people with an average age of 71. At rest, at 34 °C and 66.5 % humidity, their core temperature stopped levelling off. That is a wet bulb of about 28.7 °C.

These lines are not death lines. They mark where the body stops keeping up and core temperature starts to climb. How long someone lasts beyond them depends on the hours, the shade, the water and their health. But they move the danger zone several degrees below the number you have heard. The dashed line is Bucharest's hardest hour of summer 2022, and the next chapter is about what happened below it.

Heat can kill through heatstroke, but much of the toll falls on people whose hearts were already weak and which heat overloads. To shed heat the body sends blood to the skin, and the heart's output can double. A diseased heart, blood thickened by dehydration, medicines that blunt thirst or sweating: that is how a hot week can end in a heart attack or a stroke. After Chicago's 1995 heat wave, the odds of dying were almost five times higher on a top floor, more than twice as high for people living alone, and almost seven times as high for those who did not leave home every day.

Each dot here is one of 2022's 61,672 estimated deaths. Of those whose age is known, nearly three in four were 80 or older.

The white dots are Romania: 2,455 deaths, 122 per million people. Almost half were 80 or older. For the summer of 2024, a revised method using daily data came to more than 4,900 deaths in Romania, or 259 per million; the numbers are not directly comparable with 2022's.