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: >

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 http://dx.doi.org/10.3201/eid1206.051468 edit: formatting