[LucyT]

Wikipedia

Hydrogen sulfide is the chemical compound with the formula H
2S.

It is a colorless chalcogen hydride gas with the characteristic foul odor of rotten eggs. It is very poisonous, corrosive, and flammable.[9]

Hydrogen sulfide is often produced from the microbial breakdown of organic matter in the absence of oxygen gas, such as in swamps and sewers; this process is commonly known as anaerobic digestion which is done by sulfate-reducing microorganisms.

H 2S also occurs in volcanic gases, natural gas, and in some sources of well water.

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And as a special bonus, H₂S deadens the sense of smell.

Typically the victim smells a whiff of it *gag* takes another sniff, the perceived smell is much less intense, by the third sniff, huh, the smell is gone, I wonder what that was? *croak*