Which gas has a rotten egg odor?

• A. Sulfur dioxide
• B. Hydrogen sulfide
• C. Carbon dioxide
• D. Methane

Answer: (B)

Rotten egg smell comes from hydrogen sulfide (H2S). In wastewater and other anaerobic environments, sulfur-containing compounds are reduced to H2S, which has that distinctive rotten-egg odor even at very low concentrations. It’s a useful early warning in facilities, but the sense of smell can quickly fade with exposure, and the gas is toxic and denser than air, so it can accumulate in low-lying spaces. The other gases don’t share this odor: sulfur dioxide has a sharp, irritating smell, carbon dioxide is odorless, and methane is also odorless (unless the gas has been deliberately odorized for leak detection).