Anaerobic activity results in which gas that is extremely toxic even at low concentrations?

• A. Carbon monoxide
• B. Methane
• C. Hydrogen sulfide
• D. Oxygen

Answer: (C)

Under anaerobic conditions, certain microbes release gases as they break down organic waste. The gas that is extremely toxic even at low concentrations is hydrogen sulfide. It is produced when sulfate in the waste is reduced by sulfate-reducing bacteria. Hydrogen sulfide is dangerous because it inhibits cellular respiration by blocking the enzyme cytochrome oxidase, effectively stopping cells from using oxygen. That makes it more hazardous at low levels than the other gases listed in typical wastewater scenarios. A rotten-egg smell can be present at very low concentrations, but at higher exposures the sense of smell is impaired, so you can’t rely on odor to gauge danger. It is also heavier than air and can accumulate in confined spaces like manholes or digester rooms, highlighting the need for proper ventilation and gas monitoring. By contrast, carbon monoxide comes from incomplete combustion, methane is not highly toxic (though it can displace oxygen at high concentrations), and oxygen is not produced by anaerobic activity.