Gas mask

A gas mask is a portable air filter designed to be worn by a human. Strapped onto the face, or stretched over the entire head, military-grade gas masks (as opposed to some civilian industrial dust masks) protect the eyes, nose, and mouth of the user, and filter out both particulate debris and unwanted gases.

Uses in Wasteland
The gas mask is useful if you decide to meet Fat Freddy and decline his offer. If no one in your party has a gas mask, your party will wake up in jail (with all of your equipment, oddly enough). Any member of your party that has a gas mask will be attacked by Freddy - the rest of the party will be rendered Unconscious.

Location
You can purchase a gas mask for $75 at most general stores, or you can find four of them in the Mine Shaft near Quartz, and a few more later in the Guardian Citadel.

Bug (How to Resurrect a Dead Party Member)!
Strangely enough, it seems that if you confront Freddy with a dead member in your party and refuse his offer, and the dead member doesn't have a gas mask, Freddy's gas will bring the dead party member back to life at Unconscious status!