Well, he did mention that bit about how a few rain storms would clear out the sewage (assuming the storm drains connect to the sewer system). So, without sewage, gases like H2S persumably wouldn't be a problem (at least according to the show's logic), although I think it would've been smart for them to mention toxic gasses, just in case any viewers get any "bright ideas" about exploring their local sewer system in the near future.