Finding a mask which offers you a high protection level is crucial to filtering(过滤) out any particles for a long time. However, after a few hours, the mask becomes contaminated(污染) by the particles and you need to replace it with a new one. See, you have to get a mask offering active protection in addition to passive filtration.
The first rule of a mask offering you any kind of protection is that it needs to fit well. The mask also needs to be comfortable and allow you to breathe easily for you to be able to keep it on for as many hours as you need.
During long hours of wear, food and dirt may remain on the inside of your mask. All the hours spent in a place with less-than-ideal conditions for human health are wonderful news for bacteria that thrive on food particles and muck left behind on the inside of your mask. This growth opportunity for bacteria could not just be dangerous but could also cause a bad smell. So you'd better have a kind that kills off the smell-causing bacteria, leaving your mask smelling fresh.
You should look for cloth masks that have multiple layers of breathable fabric that is tightly woven, include a robust(结实的) nose wire for you to adjust the mask to the shape of your face and prevent air escaping from around your nose, do not have gaps around the nose or sides of the face and that block out the light if you hold the mask up to a bright light source. It is good for the mask to come with your choice of either head straps or ear straps, both adjustable. It is also good if the mask has different sizes ranging from kids to adults to ensure everyone stays protected.