Dry air in planes causes colds

From a 2004 study on the increased likelihood of catching a cold after
flying on a commercial plane.

Depending on three different flight scenarios, Hocking and Foster found
that airline passengers in three different scenarios were 5, 23, or 113
times more likely to catch a cold than if they had not flown at all!

...

The most logical reason for infections would seem to be the limited
amount of cabin air shared by the passengers. But Hocking, Foster and
other scientists have found this is only one factor. The very low
humidity in an airplane seems to be much more important.

...

Very dry air dries up the mucous system that captures and expels
bacteria and viruses from our noses. This may be a key reason why
airplane passengers catch more colds

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