Firstly the N95 refers to a respirator. A respirator is a fitted mask that is cup-shaped and designed to encapsulate your face. It sits firmly onto the face, forming a tight seal between the skin and the mask. The tight fit, as well as the filtration materials used, prevents the majority of air borne and inert particles from penetrating the mask area. (The picture to the left, features a typical N95 respirator).
N95 respirators generally have filtration efficiencies for most penetrating particles of size 0.1 – 0.3 microns. These respirators are at least 95% efficient in filtering those tiny particles. BTW 1 micron is a unit of length equal to one millionth of a meter. You can’t see it with naked eye.
If you make a mask and use filter material (such as those found in vacuum cleaners or air filters), be aware that these can have differing filtering capabilities. Depending on the filter medium you can source and use, it may be possible to be relatively efficient in blocking particles up 0.1 microns in size. e.g. HEPA (High Efficiency Particulate Arresting) air filters are 99.97% effective in capturing particles as small as 0.3 microns.
However, this is important to note: blocking of tiny particles also depends on the face leakage, that is to say, if your mask has gaping holes in it, or it isn’t a good fit, this may reduce the face seal effectiveness and therefore the efficiency of particle blocking.
Using your mask in conjunction with preventative measures recommended by the CDC, can help reduce the spread of the virus.