Hydrogen sulfide is as lethal, molecule for molecule, as cyanide. This may explain why humans evolved such exquisite sensitivity to its smell. Repellent odors are unpleasant but often helpful in terms of not dying. As with any poison, dosage makes the killer. The concentration of hydrogen sulfide in offensive human flatus is around 1 to 3 parts per million. Harmless. Ramp it up to 1,000 parts per million—as can exist in manure pits and sewage tanks—and a couple breaths can cause respiratory paralysis and
