It is used in the second part of the level "Rebirth" and, when the player controls Jason Hudson. When the Nova gas is released, and all the US soldiers put on their gas masks, the player's current weapon will disappear, and the mask will appear in the player's hand, placed on the player's face and then taken off once the player reaches the bio facility. Note that when it is placed on the players face, the bottom right-hand corner of the player's screen will label "Gas mask" as a weapon. The mask will start to develop cracks depending on how much damage the player takes and will eventually break, if the player dies. They will restart in the same place they died, but with a new Gas Mask.
The Gas Mask appears in the campaign level "Struck Down", where is put on right after the player uses the Remote Sniper for the second time. The player then is able to use Tear Gas against enemies, with the Gas Mask protecting the player and other Ghosts from its effects. It affects the player's HUD similarly to past games. The gas mask is removed after the Ghosts locate Ajax. In addition, gas masks are seen at the start of "Sin City", where Federation soldiers can be seen wearing them. In multiplayer, a gas mask is worn by the Maniac.
A gas mask worn by a Federation soldier in Homecoming.
The Gas Mask is a buildable featured in the zombies map Zetsubou No Shima. Once equipped, players will not be able to cough when entering a cloud of gas left by a spore. It will break once it has been used enough, although a fresh Gas Mask can be equipped once the old one is destroyed. Players can also use the gas mask to increase the durability of the player's breath underwater.