What is a mask fitter? A soft, flexible and adjustable “frame” that significantly improves the outer seal of a mask. Why use it? Adding the Badger Seal to a 3-ply disposable mask reduces the effective particle penetration by typically 15x (see the Performance section below). What makes the Badger Seal unique? It’s cheap (< $1 in materials), easy to assemble, made from readily available materials and tools, comfortable, quickly customizable and open source. There are other fitters out there, though none seem to excel in all of these areas.