Right now you are breathing through mostly one nostril

Cover one nostril and breathe. Now the other. Almost always one of the two runs freer: at that moment you are using mainly that side.

The German ear, nose and throat doctor Richard Kayser described it in 1895 and gave it a name: the nasal cycle. The turbinates —three shelves of bone and mucous membrane jutting into each nostril— swell with blood on one side while they shrink on the other, and they take turns every one to four hours, day and night, without you noticing. Between 70 and 80 per cent of healthy adults have it.

It is not a fault. The congested side rests and rehydrates while the other one works, and the air, moving more slowly through the narrow side, gives odour molecules time to stick to the membrane. That is why some smells are caught better through one nostril than the other.

The home check takes ten seconds: hold a mirror up to your nose and breathe out. The two patches of mist almost never come out the same size.