Gas mask
From HellWiki
A piece of gear that you wear on your face. Helps defend against any inhale-able diseases or gases. Its usually a VERY good idea to wear one of these at all times, as some areas such as Phage Lab will kill you otherwise. True, Hyperimmune will protect from diseases, but that wont stop you from being knocked out by a THC grenade.
Where to get
Eschaton Outfitters will sell you one for $210
You can find them sometimes on the bodies of serial killers
