I understand the toxicity is in the neighborhood of Hydrogen Cyanide, no?Wait. Strike that. Reverse it. Flammable, sure, and also extremely toxic and insidious. Our noses are very sensitive to it because it's so dangerous. And when you're poisoned by it, what's the first thing effected? You guessed it, the sense of smell. It accumulates in small spaces, you go in, you think "That's a really bad smell" but then the bad smell is gone, so you relax, and then you drip dead.
If you've ever worked someplace where you had safety training on entering confined spaces, you can thank hydrogen sulfide.
As to where it comes from, anaerobic bacteria don't eat it unless you'd say aerobic bacteria eat oxygen. No, the anaerobes eat basically the same stuff, and suffer is their oxidizer. So they emit H2S instead of H2O. I don't know where the carbon goes, but it's not CS2 as you might suppose. Some is methane, but that would need more hydrogen than the food has. Some is methane thiol, a.k.a. methyl mercaptan, CH3SH, which is also horribly toxic, but our noses are even more sensitive to it; that's the stuff they put in natural gas at no more than 8 ppm so you can smell when there's a leak. But that too would require excess hydrogen. So I don't know how it works out.
Hans.