Closing Windows
Every heatwave, the same advice circulates. Close your windows. Pull the shutters. Seal the house like a tomb and wait for the sun to go down. It is common wisdom, repeated by authorities and passed along by neighbors who mean well.
But Jorg Kachelmann, a meteorologist known for refusing to sugarcoat things, says the advice is incomplete and sometimes dangerous[1].
His argument is about humidity. Temperature alone does not kill people. It is the combination of heat and moisture that turns a room into a health hazard. When you shut windows all day with people inside, the air grows damp. Cooking, breathing, showering, all of it adds water to the air. In a small apartment with no ventilation, humidity climbs fast. The body cannot cool itself through evaporation when the air is already saturated.
"The killer is not the temperature, it is the humidity," Kachelmann argues.
The standard advice works fine for people who leave the house in the morning and return in the evening. An empty, sealed house stays cooler. But for elderly residents, people working from home, families in small flats, the sealed-tomb strategy backfires. Air needs to circulate when people are present.
Kachelmann also addressed the forest fire narrative that spreads every summer. High temperatures alone do not ignite forests. You need two things: dry conditions and an ignition source of 250 to 300 degrees. Lightning can do it. Humans usually do it, intentionally or not. A hot day without those two factors is just a hot day.
This matters because Luxembourg is currently dealing with the consequences. Smoke from a cross-border wildfire in Belgium's Hautes Fagnes drifted into the Grand Duchy this week, and residents received LU-Alert warnings to stay indoors[2]. The advice to close windows was correct in that specific case, because the danger was particulate matter in the air, not heat. But the broader instinct to seal the house every time it gets hot deserves more nuance.
The heatwave will end. The advice will not. Next time someone tells you to close all the windows, ask them what they think happens to the air inside.