Masks rely on a tight seal with the face to work. This is so that only air going through the filter is breathed. If the mask does not fit properly the dust can slip through any gap between the mask and the face and into the airways. Dust particles can be much smaller than the width of a hair so the face seal needs to be very good.
There are many designs of masks. Not all will fit you well enough to form a good seal with your face. To find one that does you therefore need to be face fit tested. There are two types of test: qualitative and quantitative. Both are acceptable for testing masks used for construction dusts. For more information on face fit testing see the [Fit2Fit site ](http://www.fit2fit.org/).