My, how this thead has wandered! Going back to flying, there was discussion in a consumer column today regarding the wearing of masks in planes.
One person who seemed to know what he was talking about wrote:
"The air is HEPA filtered and half of it is replaced with fresh air from the external environment on each pass. The entire air is replaced several times an hour. It is also zoned to a certain number of rows so you aren't breathing in the entire bioburden shed by the passengers more than a small distance away from your seat. It is probably healthier sitting on an aircraft than in a hospital, or even on a bus or in the pub, to be fair."
I'll leave you to look up HEPA on Wikipedia, as I had to, but the real reason I am posting is to say that my wife has just flown with BA to Greece and back, and the Airbus 321's were completely full, there was no chance of social distancing.