|The following is based on Van Buren and has not been confirmed by canon sources.|
It's the sulfur-like smell that tipped you off, but you have a horrible feeling the gas is something called "Hydrox-Dioxide," a hazardous substance you've read about in the computer library in your vault. If you're smelling it now, you know it's only a matter of time before you all keel over - the best protection is to get some covering over your face and get out of the contaminated area as quickly as possible.”
Hydrox-Dioxide was used by several pre-War companies in treating animal wastes and was outlawed by the EPA in 2066. The chemical was used in recycling certain nutrients and extracting moisture from raw sewage, but it required careful storage measures and was both toxic and flammable. If inhaled, it can cause extreme nausea, and if you have the misfortune of being exposed to a large dose, it can be lethal.