So these gas masks (small box respirator or SBR) are absolutely awful to wear. First off, they don’t form a seal around the wearer’s face with the actual mask. That’s just heavy or treated canvas to reduce gas getting to your eyes. You breathe in filtered air through a snorkel mouthpiece directly connected to the filter via that hose, and exhale through a little check valve flap underneath. Oh and for your nose there’s a clip that pinches it shut so you only breathe through your mouth. Vision is terrible through the eye pieces and you’re supposed to be able to don the mask in seconds when there’s a gas alarm. Then they expect you to fight in a trench with a big long rifle with a bayonet at the end! (Source, I own a historical reproduction SBR)