“To protect oneself from infectious particles, the air should be changed three times per hour,” the UBA said in a four-page instruction manual for ventilation. “This means that the air is completely swapped for fresh air from outside three times an hour.”

The recommended practice of opening windows wide for a period of three to five minutes before closing them again “makes the temperature in the room drop by only a few degrees,” the UBA said.

“After closing the windows, it quickly rises again,” is added.

Ventilating rooms – something of a national obsession in Germany – has been added to the German government’s formula for tackling the coronavirus. Chancellor Angela Merkel has lauded the practice as a cheap and effective way of preventing the virus from spreading.

Parents and teachers have been discussing in recent days whether to recommend that pupils wear hats and scarves inside classrooms as wide-open windows become the norm.

The UBA said in its manual that windows should also remain wide open during break times, and that teachers should refrain from opening doors to avoid the spread of infectious particles through corridors into other parts of the school.

For classrooms in which windows cannot be opened for security reasons, the UBA recommends installing ventilation units.