Why can a person's sense of smell not be depended upon to detect H2S?

Correct Answer: Option D — the sense of smell is deadened by the gas

H2S causes olfactory fatigue, a condition where the gas rapidly deadens the sense of smell. While it may have a 'rotten egg' odor at low concentrations, this warning sign disappears quickly as the concentration increases. Relying on smell is dangerous; electronic detection equipment is mandatory for monitoring H2S levels.