The German way to air a room is Stoßlüften, burst airing. All windows are opened wide for a few minutes, and if possible windows on opposite sides at once. The Federal Environment Agency publishes the method. Its poster for schools says three to five minutes in winter and ten to twenty minutes in summer. In October 2020, for schools during the pandemic, it gave a rule: Alle 20 Minuten fünf Minuten lüften. Air for five minutes every 20 minutes. To air, all windows should be opened wide. A tilted window is not enough.