The box respirator equipment, the general principle of which was finally adopted by all the nations at war, fell into two classes. In a single-protection mask the wearer breathed air from inside of the face piece, so that any leakage around the edges of the face piece would result in a casualty when the wearer was in a strong concentration of gas. The other sort, known as the double-protection mask, consisted of a gas-tight face piece, similar to that of the single-protection mask. In this type, to guard against any possible leakage around the edges between the mask and the wearer's skin, the breathing system was sealed away from the air inside the face piece by means of a rubber mouthpiece and a nose clip, the wearer inhaling through the mouthpiece.
VARIOUS TYPES OF GAS MASKS.
Top row, left to right.—First type U. S. Navy mask, now obsolete; U. S. Navy mask as finally developed; U. S. C. E. respirator (production started October, 1917); U. S. R. F. K. respirator (production started February, 1918); U. S. A. T. respirator (production started August, 1918); U. S. K. T. mask (production started August, 1918); U. S. model 1919 mask (ready for production when armistice was signed).
Middle row, left to right—British black veil mask (first mask used after initial gas attack in April, 1915); British P. H. helmet (stops phosgene but not tear gases); standard British box respirator used by all British forces after 1916; French M-2 mask used by the French; French Tissot mask used by artillerymen; French A. R. S. mask.
Bottom row, left to right.—Late type of German mask; Experimental mask; Italian mask (similar to French M-2 mask); British Motor Corps respirator; U.S. rear area emergency respirator; U. S. Connell mask (never passed the experimental stage).
AMERICAN C. E. TYPE OF BOX RESPIRATOR.
This side view shows face piece, harness, hose, flutter valve, and knapsack. This is the mask most used by our troops.