In a small, airless room, two people sit and breathe. There is no breeze, no fan, no open window. The humidity climbs, steadily, until the air is saturated—100 percent. This thought experiment, posed by the German meteorologist Jörg Kachelmann in an interview with Bild, was not a hypothetical. It was a blunt illustration of what happens when a heatwave meets the instinct to seal every gap against the sun. “Alles verrammeln – das ist passive Sterbehilfe,” he said: barricading everything is passive euthanasia. The remark landed in the middle of a long-running domestic dispute that, as temperatures again push past 30°C across much of Europe, has spilled into public view: when the heat arrives, should a window be open or shut?

In the German-speaking world, the answer is a timetable. Bild’s “heat clock” instructs readers to fling windows wide before 7 a.m., then shutter them the moment sunlight strikes the glass. External blinds, not interior curtains, are the gold standard; they stop heat before it enters. A family doctor from North Rhine-Westphalia, Thomas Aßmann, prescribes thirty-minute bursts of ventilation in the early morning and late evening, while the federal physician of the German Red Cross, Bernd Böttiger, adds a caveat: if a small flat is occupied all day, intermittent airing is essential to keep humidity down, but an empty home should stay sealed. The Fraunhofer Institute for Building Physics offers a simpler rule—open the window only when it is cooler outside than in—and notes that well-insulated new builds, once heated through, cool down more slowly, making night-time ventilation critical. Beneath the precise scheduling lies a cultural reflex: the German Stoßlüften, a brief, vigorous air exchange, is as much a daily ritual as a thermal strategy.

Viewed from Britain, the debate takes a different shape. A survey of 1,600 UK households, conducted after the 40°C heatwave of 2022, found that 80 per cent of homes had overheated that summer—four times the figure a decade earlier—and that air-conditioning ownership had risen sevenfold in the same period. Yet researchers warn that a rush to mechanical cooling risks an energy-hungry, unequal future. Their prescription, drawn from southern European practice, is “passive cooling first”: shutters, reflective surfaces, trees, and buildings oriented to minimise heat gain. The cultural hurdle is considerable. In Spain, the hottest hours are for siestas; curtains stay drawn. In Britain, a 30°C day is still widely framed as “good weather”, a cue for beach trips and barbecues. Public-health messaging, the researchers argue, has not caught up with the danger.

Then there is the fan—a device that, according to both Swiss engineering and US public-health guidance, is widely misused. Beat Ribi, a professor of thermo- and fluid-engineering at the University of Applied Sciences Northwestern Switzerland, explains that a fan does not cool the air; it accelerates the body’s own convective cooling by sweeping away the warm, moist envelope that clings to the skin. For that to work, the airflow must reach the person, not merely recirculate around an empty room. The US Centers for Disease Control and Prevention draw a harder line: if the indoor temperature exceeds 90°F (32°C), a fan can actually raise body temperature by blowing hot air over the skin, and they urge a switch to air conditioning or a cooling shelter. Consumer groups in Britain add that a fan placed beside an open window, when the outside air is cooler, can act as a poor man’s intake vent, while a second window on the far side of the room lets the hot air escape.

Beyond the physics, a parallel universe of folk remedies is gaining scientific attention. At Teesside University, a lecturer in exercise physiology, Nicolas Berger, suggests sleeping in merino wool: the fibre’s layered structure wicks sweat away from the skin, allowing it to evaporate rather than pool. In India, the traditional practice of rubbing onion juice onto the skin has been noted by Oxford neuroscientist Russell Foster; the sulphur-based essential oils evaporate on contact with air, drawing heat from the body. And from yogic tradition comes the tongue-roll—curling the tongue into a U-shape and breathing slowly through it—which practitioners describe as a miniature, personal air conditioner for the head and neck. None of these replaces a well-placed shutter or a timed burst of morning air, but together they sketch a portrait of a warming world in which the search for coolness is becoming at once more scientific and more improvisational.