I believe (?) it's important for the mask to be changed regularly. A warm, wet being an ideal germ breeding ground. In China I always smile at seeing people in marks and people hoiking and spitting within feet of each other.

A couple of flights ago a sniffer dog jumped up at me at Beijing. I ended up covered in dog hairs and coughing and sneezing all the way home in WT+. I have to say I felt awful for the guy sat next to me. I did explain it was an allergy, but I don't think he believed me (I wouldn't).