[Consultant32452]

The virus is carried in tiny droplets in the air, usually from a cough or sneeze. Any mask that catches those helps prevent virus spread to and from you. So literally a bandana covering your face is going to prevent some. Of course it won't be as effective as a legit N95 or N99 mask, maybe it's only 10% as effective, but that is not the same as being totally ineffective.

Further, any form of masks provides a psychological aid to help people stop touching their face.

[SirLuxuryYacht]

The main goal is minimizing leaks and making sure air flows through material. While a bandana would not be totally ineffective, there's room for innovation:

> A Hanes Heavyweight 100% preshrunk cotton T-shirt (made in Honduras) [...] was boiled for 10 minutes and air-dried

> A commercially available N95 respirator requires a fit factor of 100

> The prototype mask achieved a fit factor of 67

> We showed that a hand-fashioned mask can provide a good fit and a measurable level of protection from a challenge aerosol. [...] When made by naive users, this mask may be less effective because of variations in material, assembly, facial structure, cultural practices, and handling

From "Simple Respiratory Mask" (2006) http://dx.doi.org/10.3201/eid1206.051468

edit: formatting