1. A mask construction, comprising: a head encompassing protective hood having a front portion adapted to overlie a wearer's face and having a viewing window adapted to overlie the wearer's eyes and a hood opening below the viewing window; a respiratory gas filter frame including a first front frame portion having a front inlet opening, a second rear frame portion connected to said hood, said second frame portion having an opening opposite to and aligned with said hood opening, said first frame portion and said second frame portion defining a chamber therebetween which extends through said filter frame and is sized to hold a filter cartridge; a filter cartridge adapted to be positioned in said cartridge chamber, held by said first and second frame portions and define with said first frame portion opening and said second frame portion opening a gas flow passage, resilient means connected to said first front frame portion and said second rear frame portion at the opposite ends of said cartridge chamber for sealing between the filter cartridge and the respiratory frame, said filter cartridge having a lower most edge defining a cutting edge; a sealed package containing said filter cartridge, said package including retaining collar means for engagement on said frame when said package is positioned with the cutting edge of said filter cartridge over an adjacent one of the ends of the cartridge chamber of said frame so as to retain the package at said one of the ends of the cartridge chamber when said cartridge is inserted into the cartridge chamber thereby, allowing replacement of a depleted filter cartridge already within the cartridge chamber with said filter cartridge while the gas mask is in use.