- I assume you are talking about the chain-mail masks and metal goggles that British tank crews wore in WWI. These masks were supposed to protect the crews' faces from red hot shrapnel thrown off by the tank as enemy anti-tank bullets and even artillery shells hit it. They didn't have any respiratory function. The Mark I tank's engine was in the crew compartment and produced heat and noxious fumes that overwhelmed many tank crews. Fumes from the firing of the guns added to the horribly hot and noisy environment inside the tank. A breathing mask probably would have been a better choice considering that a crew can become disabled just driving to the battle!
Tank design improved greatly from WWI to WWII, and virtually all tanks in use in WWII had the engine completely separate from where the crew was. Most tanks now had one or more powerful ventilators that sucked noxious air out of the crew compartment and especially the turret.
So in short, the "masks" were just a measure used to protect against shrapnel because of the Mark I's very thin armor (6-12 mm)