Dirty outdoor air versus indoor heat
Poor air quality is a reason to keep windows mostly closed. Indoor heat during a heatwave is also a health risk. Temperature still leads when the room is already very hot.
Poor outdoor air is a real reason to keep windows mostly closed. Indoor heat during a heatwave is also a health risk. Those two facts pull in opposite directions.
The instinct, when the air-quality map turns orange or red, is to shut everything. In a mild room that is the right call. In a flat that is already 28 or 30 °C, shutting the last cooler outdoor air out can make the heat worse.
Temperature still leads
The app does not close everything the moment air quality looks bad.
- Poor outdoor air (European AQI around 60 or more): prefer a short opening rather than wide open, unless the room is already very hot
- Very poor air (around 80 or more): close, unless indoor heat is severe (about 28 °C or more) and outdoor air is still cooler. Then a short opening still dumps heat
- Cooler outdoor air can still help a hot room. The overlay never opens a window that temperature and sun had already closed
PM2.5 is the fine-particle number behind a lot of AQI colour. It is useful context, not a medical diagnosis.
In a dense street the same AQI often sits on top of a heat island: nights stay warmer, so you have fewer hours to dump heat. That is when this trade-off is sharpest.
The air-and-heat checklist
- Temperature still leads: cooler outdoor air can help a hot room
- Prefer a short burst when air is poor
- Keep closed when air is very poor and the room is not dangerously hot
This is not medical advice. Follow your local health agency if you have asthma, heart or lung conditions, or if someone is showing heat illness.