Wikipedia dit ceci sur le sujet :

Hydrogen sulfide is a highly toxic and flammable gas (flammable range: 4.3–46%
). Being heavier than air, it tends to accumulate at the bottom of poorly ventilated spaces. Although very pungent at first, it quickly deadens the sense of smell, so victims may be unaware of its presence until it is too late.

Et la fiche de sécurité le confirme :

Hydrogen Sulfide has a distinct “rotten-egg” smell. The odor cannot be relied on as an adequate warning of the presence of Hydrogen Sulfide because at high concentrations olfactory fatigue occurs.

Edit : cross