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.
Fire occurs and the player must save a Gas Mask, a First Aid Kit or a Boy Scout Handbook. The unselected items will be damaged. This event tends to happen early in the game, usually the first two weeks.
- Curiously, the gas mask is much more flexible once it enters the shelter, as when its on a hook at the house, it retains its shape, but when it enters the shelter it seems to sag downward on its hook.