Heat and the human body # How much heat can a human take? You are a small furnace. Sitting still, you give off about as much heat as an old 100-watt bulb, and over the day you have to get rid of all of it to stay around 37 degrees. The thermometer at the edge of the screen measures the air's heat and humidity together, and tells you whether you still can. Scroll, and the air around the body will warm up. Scroll I ## A 100-watt furnace Every working cell makes heat. To avoid overheating, the body has to push out exactly as much as it makes. The figure is drawn the way a thermal camera would see it: the paler the colour, the warmer. The air around it is at 22 °C, with 35 % humidity. 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. II ## The number your body reads So what you can take depends on how easily sweat can leave, and that depends on the air's humidity as well as its heat. Meteorologists have measured this for almost two centuries with a simple trick: wrap a thermometer's bulb in wet cloth and let air flow past it. Evaporating water cools it, and the drier the air, the further it falls. What it reads is called the wet-bulb temperature. 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. Wet bulb: **. III ## The limit is lower than the famous number In 2010 two climate scientists, Steven Sherwood and Matthew Huber, worked out a ceiling. For heat to leave the body, skin has to stay cooler than the 37-degree core, at around 35. Once the wet bulb reaches 35, even an ideal person, in shade and in wind, can no longer shed heat. They put the limit, for heat sustained over hours, close to 35 °C, allowing that they could be a degree or two off. 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. - 80 and over - 65–79 - under 65 - no age group IV ## The deaths of 2022 came far below those lines In the summer of 2022, none of the 36 European cities on this page went past 26.4 °C wet-bulb. Even so, researchers estimate that between 30 May and 4 September heat killed 61,672 people in 35 European countries. 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. V ## Where is your city on the thermometer? Pick a city. The blue arrow on the thermometer shows its most humid, hottest hour since 2015, computed from the ERA5 weather reanalysis. City ERA5 averages over squares about 30 kilometres wide, so it smooths the peaks: weather stations and city centres run hotter. ## What actually cools you - **Water on skin, moving air.**Wet skin cools by evaporation, as sweat does. A lukewarm shower, a wet towel on the shoulders, feet in a basin. - **A fan has a limit.**In very hot air a fan can bring more heat than the extra evaporation takes away. A 2021 modelling study put the threshold at about 39 °C for young adults and 38 °C for people over 65, lower for some medicines. - **Nights matter.**The body recovers at night. When the nights stay hot, the risk of dying rises. Open windows when it is cooler outside than in, and draw the blinds by day. - **Call your older neighbours.**Most people heat kills are old, alone, in hot flats, and often feel thirst less. A daily call or visit while the heat lasts costs nothing. - **Don't stop medicines on your own.**Diuretics, some heart and blood-pressure drugs and medicines that dry the mouth change how the body handles heat. Ask a doctor what to do in summer. The lines on the thermometer stay put; it is the air that climbs. The human body has not changed, but hot, humid hours come more often and Europe is getting older. The limit that matters is not 35 degrees. It is that of the frailest person in your building. ## Method and sources The wet-bulb gauge at the edge of the screen is computed with Stull's (2011) formula from air temperature and relative humidity; its error is under a degree in the range used here. The 35, 30.55 and 28.7 °C lines come from the studies cited. The last is our own conversion, with the same formula, of the 34 °C and 66.5 % humidity condition reported by Wolf and colleagues (2023), who do not give a wet-bulb value. The figure in the first chapter is a round-number model, not a measurement: skin held at 35 °C, 1.8 m² of skin, still air, a combined radiation and convection coefficient of 8 W/m²·K and 100 W of heat produced. The air rises from 22 to 42 °C at 35 % humidity. The numbers in the corner show the order of magnitude. The 2022 deaths are statistical estimates of heat-related excess mortality (Ballester et al., 2023), not death certificates. The age breakdown excludes Germany, Ireland and the United Kingdom, and Romania's age groups add up to 2,423 of its 2,455 deaths because each group is estimated separately; the grey dots are deaths with no age group. A later re-analysis with daily data raised the European total for 2022. Cities: the hour with the highest wet-bulb temperature in each year, 2015–2026 (2026 to 21 September), from the ERA5 reanalysis (Open-Meteo), computed with the same formula from hourly temperature and humidity. ### Sources - Sherwood, S. C. & Huber, M. (2010). An adaptability limit to climate change due to heat stress. PNAS 107(21). pmc.ncbi.nlm.nih.gov/articles/PMC2906879/ - Vecellio, D. J. et al. (2022). Evaluating the 35 °C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project). J Appl Physiol 132. journals.physiology.org/doi/full/10.1152/japplphysiol.00738. - Wolf, S. T. et al. (2023). Heat stress vulnerability and critical environmental limits for older adults. Communications Earth & Environment. pmc.ncbi.nlm.nih.gov/articles/PMC10826365/ - Vanos, J. et al. (2023). A physiological approach for assessing human survivability and liveability to heat. Nature Communications. pmc.ncbi.nlm.nih.gov/articles/PMC10687011/ - Ballester, J. et al. (2023). Heat-related mortality in Europe during the summer of 2022. Nature Medicine. europepmc.org/article/PMC/PMC10353926 - ISGlobal (2025). Summer 2024 heat-related deaths in Europe. isglobal.org/en/-/62.700-muertes-asociadas-con-el-calor-del- - Semenza, J. C. et al. (1996). Heat-related deaths during the July 1995 heat wave in Chicago. NEJM 335. pubmed.ncbi.nlm.nih.gov/8649494/ - Kenny, G. P. et al. (2010). Heat stress in older individuals and patients with common chronic diseases. CMAJ 182(10). cmaj.ca/content/182/10/1053 - Crandall, C. G. & Wilson, T. E. (2015). Human cardiovascular responses to passive heat stress. Comprehensive Physiology. onlinelibrary.wiley.com/doi/abs/10.1002/cphy.c140015 - Morris, N. B. et al. (2021). Electric fan use for cooling during hot weather: a biophysical modelling study. Lancet Planetary Health. thelancet.com/journals/lanplh/article/PIIS2542-5196(21)00136 - Royé, D. et al. (2021). Effects of hot nights on mortality in Southern Europe. Epidemiology. researchonline.lshtm.ac.uk/4658453/7/Roye-etal_2021_Effects- - Raymond, C., Matthews, T. & Horton, R. M. (2020). The emergence of heat and humidity too severe for human tolerance. Science Advances. pmc.ncbi.nlm.nih.gov/articles/PMC7209987/ - Stull, R. (2011). Wet-bulb temperature from relative humidity and air temperature. J Appl Meteor Climatol (formula as documented by NCAR). ncl.ucar.edu/Document/Functions/Contributed/wetbulb_stull.sh - Hersbach, H. et al. ERA5 hourly reanalysis, via the Open-Meteo historical weather API. open-meteo.com/en/docs/historical-weather-api ### More on the site - When humans vanish - Your legs are older than your brain - Their last breath Published on 28 September 2026. In the lab, bodies stopped coping at about 31 °C wet-bulb in young adults and 29 in older ones, well below the famous 35. Climb the thermometer, see who heat kills, and where your city's hottest hour lands. Canonical: https://mariuscomper.uk/cat-de-cald-poate-duce/en/