If you happen to lose your gas mask after Plague City, you can search the cupboard in Edmond's house to retrieve another one.
Interestingly, the homemade gasmask made by Alrena using dwellberries is the same item (stacking in a bank account, etc.) as the one dropped by mourners. This, plus the fact that each is actually functional in environments such as the smokey well, suggests that they are made from the same design, and that the dwellberries don't add any benefits at all to the apparatus. It could also suggest that the mourners feel a need to comply with folklore about the plague, and so add the berries to their gas masks regardless of the plague's nonexistent nature.
This item has not been proven to protect from other hazards that a real-life gas mask would stop, such as the poison gas released by Mi-Gor during The Great Brain Robbery.
On the 9th of November 2010, the gas mask received a graphical update which didn't appear on the front page but did in the forums under patch notes. The mourner clothing and plague clothing have also been updated but only changed to a darker colour.
The current design of the gas mask is based on those worn by plague doctors in Europe during the Black Death. The beak contained fragrant items believed to ward off the smells that were erroneously thought to carry the disease. Such references have been made through the Plague City quest much like the Black Death.
The old gas mask's design looked like the gas masks worn by civilians and soldiers during World War I to protect against chemical attacks.