When the United States entered the war in April 1917, the U.S. Army was unprepared for chemical warfare and had to use borrowed foreign equipment. Soldiers were issued a British Small Box Respirator (S.B.R.) for the highest level of protection and a French M2 Mask for long-term wear comfort. Unfortunately, the untrained troops had a tendency to initially put on the S.B.R. following a gas attack and then switch to the M2 when it appeared they would have to wear a mask for an extended period of time. Of course, during the mask switching, many soldiers inhaled toxic chemicals and became casualties.

Training Mask

Despite the uncomfortable mouthpiece and nose clip of the British S.B.R., the U.S. Army decided this type of mask provided the best protection. The initial attempt in June 1917 to copy the mask was completed by the Bureau of Mines of the Department of the Interior and was not a success. The next attempt was the Training Mask in July 1917, which used an extra large canister of charcoal and soda-lime. Over 600,000 of the masks were manufactured and used only for training in the United States.

Corrected English (C.E.) Mask

An improved version of the British S.B.R. was developed in October 1917. This mask was slightly more comfortable to wear and used a canister containing activated coconut charcoal, soda-lime, and cotton pads to protect against toxic smokes. The facepiece was more impermeable to all known chemical warfare agents. Approximately 1.6 million of the masks were produced during the war.

Richardson Flory Kops (R.F.K.) Mask

Continued improvements to the C.E. Mask resulted in the R.F.K. Mask introduced in early 1918. The facepiece was cotton fabric coated with rubber. The canister was reduced in size, which created less breathing resistance. In addition to being more comfortable to wear, the mask was also much easier to manufacture. Over 3 million of these masks were produced during the war.

Akron Tissot (A.T.) Mask

This mask was designed by the Akron Rubber Company in June 1918 and used a French design that eliminated the uncomfortable mouthpiece and nose clip of the S.B.R.-type masks. Instead, the incoming air was directed over the eyepieces, which helped prevent fogging. The facepiece was made of molded rubber covered with stockinet. The canister was the same as that used on the R.F.K. Mask. The mask provided far more comfort, but was more difficult to manufacture, so only 197,000 were produced during the war.

Kops Tissot (K.T.) Mask

This mask was similar to the Akron Tissot Mask and copied the French design of directing incoming air over the eyepieces. The facepiece was made of vulcanized fabric and used the same canister as the R.F.K. Mask. Because it also was difficult to manufacture, only 337,000 were made starting in September 1918.

Kops Tissot Monro (K.T.M.) Mask

In October 1918, the best aspects of the Akron Tissot and Kops Tissot Masks were combined to create the K.T.M. Mask. It had a stockinet-covered rubber facepiece and used a new canister that added felt as a filter against toxic smoke particles. This was perhaps the best mask developed during the war although only 2,000 were produced prior to the armistice on November 11, 1918.