The gas mask is used for protection against the radioactive fallout whenever the player exits the shelter via an expedition or certain events.
Before the "What's in the Suitcase?" DLC, the gas mask was used passively when the player sends a character on an expedition; however, the game now asks a player to take an item outside when starting an expedition, and the gas mask is an absolute must before the radio informs the player that the fallout outside is mostly gone. If the player goes outside without a gas mask before this message is received, the character has a far higher chance of getting Sick or not returning from the expedition.
Sometimes it is used in events to get supplies. It is also one of the items required for a military event. If the gas mask is damaged, it can be repaired with the Boy Scout Handbook.
The military will send a box of supplies, containing 2 food and 2 water. The Gas Mask may be damaged if choosing to get the supply drop.
The Military will order to send someone with a Gas Mask to meet them on a bus stop.
The family starts to smell bad and the player can use the gas mask. If not, most of the family could get sick. The gas mask can also cure sickness, in the event someone is sick.
- Curiously, the gas mask is much more flexible once it enters the shelter, as when it's on a hook at the house, it retains its shape, but when it enters the shelter, it seems to sag downward on its hook.
- In 60 Parsecs!, if the player uses the mask on the party event, the player will argue with a mysterious "Timmy" and break the gas mask.
- In one of his insane events, Ted boxes the Gas Mask and destroys it.