Germany has a practice called "Luft" ("Air"), where tenants are _required_ to throw open the windows for X minutes each day. Seems smart. The room & furnishings retain heat long enough while the air is changed in toto. Can someone who knows about this practice comment ?

Seems smart why? If we check the air two hours later, is it meaningfully better than before we started?

A daily changeover seems much worse than constant exchange, and both of those pale in comparison to filtering.

Guessing they live in a city or by a road