The back of the fan sucks air, then blows air out the front. You put the filter on the back, it sucks the dust into the filter and blows out filtered air,, hopefully. It works pretty good, at least I have to clean or replace my filter fairly often. Doesn't catch everything, not full proof, but a lot better than nothing. I also use a Resp-o-rator which works real well (espically if you have a beard). The only drawbacks with it, you have to wear a nose plug which is a little uncomfortable and because you breath through you mouth, my throat get a little dry. A sip of wine er, I mean water, fixes that.