Actually... Hydrogen Sulfide is the hazardous gas commonly associated with sewers... But either way, it's still hazardous. I deal with hazardous materials as part of my job so I figured I would chime in. Since I can't send you a material safety data sheet for hydrogen sulfide... here is a fact sheet about it that I've used in HazMat classes before.

http://www.dhfs.state.wi.us/eh/Air/pdf/sewergas.pdf