Q.1125: Can coffee filter be used in home made mask?
Yes it can be used in home made mask. It is food grade paper. It has some type of resin we call wet strength additive to make it strong enough to withstand when wet.
Coffee filter may/will have high porosity to allow faster filtration. I am not sure of pore size but it must provide good protection. It is better than cloth mask
You may try to use 2 or more layers.