As a frequent flyer, you know that the air on an aircraft is dryer than on the ground. Depending on where in the world you live, the humidity level is likely to be somewhere between 40 and 50 per cent – on an aircraft, it might be as low as 5 or 10 per cent.

The advent of air conditioning means we can live comfortably in places we previously wouldn’t have considered, using a humidifier alongside it if the climate is dry as well as hot. But when we’re travelling, it’s more complicated. Most modern aircraft take in air from outside via the engines and, although this is filtered, warmed and purified before being piped in, it is still extremely dry, since air at high altitude has a low moisture content. The result is that the cabin air is greatly lacking in H2O.