A theatre nurse said in an interview, that the face masks they wear as surgeons and nurses in theatres are good only for about 20 minutes then they have to be changed, the real use for them in those situations is to prevent blood transfer and splatter into mouth and eyes of the medical staff .. otherwise they're not the best at preventing any kind of viral germs. They also have to be removed from the face in a certain manner to prevent getting the bacteria on the hands ( lots of videos available) ..
There are apparently some much better masks at preventing bacteria from entering the lungs than others... . Most of those paper type that you see Asians wearing , are fairly useless when it comes to anything bacterial... according to what I've read the virus are so small they can penetrate straight through, and that would be exactly the same with a scarf..
There are much better masks at preventing surgeons et al from contracting any type of virus but they involve full face masks as well as as minimum grade FFP3 mask preventing the tiny virus from entering like the paper or cheap cloth masks used by the masses on the streets...