Okay, here's my understanding; if you want a mask to protect yourself, go for a full face visor. It'll even stop spittle and stuff getting into your eyes. NHS front line staff use these because they work close up to covid-19 patients.
But otherwise even a scarf would do to stop you sneezing/coughing the Chinese plague around the shop if/when you're asymptomatic, which means you're infected but show no symptoms. You can be asymptomatic for around three days and not know it. The scarf would be more effective than using your elbow, and defo better than using your hands which you'd then touch other stuff with.
Yes, the virus is extremely small and can pass though a scarf, but most of it is attached to your snot/cough droplets, which would be caught by the weave of the scarf.
The scarf/mask isn't there to protect the wearer, but to protect everyone else.
That's my understanding; happy to be wrong.