Gas Mask is a Tier Two Perk in Call of Duty: World at War that reduces the effects of a Tabun Gas grenade. This allows a player to attack enemies affected by the grenades without hindrance. This is the only use for this perk, and therefore, it is usually passed over for more useful perks, like Sleight of Hand, Stopping Power, and Overkill, especially since it takes what is arguably the most important perk slot. It is unlocked at Level 1, in the 'Light Gunner' class with a Thompson.
To use this perk effectively, it is almost necessary to utilize it with the Special Grenades x3 perk and, of course, Tabun Gas. This combination makes the player lethal when storming buildings, since they will be unaffected by the gas and can easily kill the poisoned enemies inside. Using a close-range weapon for this purpose is recommended. This perk should be used in a class with a fast-firing or high-damage weapon (such as a shotgun, or sub machine gun) for use in objective based games, especially if there is a point that must be captured, such as in "Domination", "War", or "Headquarters". In "Team Deathmatch", using this perk is normally considered not worth it, as perks such as Stopping Power and Juggernaut are generally more useful.
Another time where this perk can be useful is if one or more members of the enemy team are frequently using gas grenades against the player.