Which gas is produced in sewer systems and can be hazardous with a rotten egg odor?

• A. Carbon Dioxide
• B. Hydrogen Sulfide
• C. Methane Gas
• D. Chlorides

Answer: (B)

Gas with the rotten-egg odor that shows up in sewer systems is hydrogen sulfide. It forms when sewer bacteria break down sulfur-containing compounds in anaerobic conditions. Hydrogen sulfide is heavier than air and can accumulate in confined spaces, making it a serious safety concern. At low concentrations you notice the rotten egg smell, but as the level increases your sense of smell can fade, which is dangerous because you might not realize you’re exposed. It’s toxic and can cause irritation, dizziness, or even loss of consciousness at higher doses. The other options don’t fit: carbon dioxide is odorless and not associated with that distinct smell; methane is also odorless and mainly a fire hazard; chlorides are not gases (and chlorine gas, while hazardous, has a different odor). So the gas responsible is hydrogen sulfide.