Which substance is toxic, corrosive, and explosive?

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

Answer: (A)

Gases in wastewater can pose multiple kinds of hazards, and hydrogen sulfide is notable because it can be toxic, corrosive, and explosive all at once. It’s produced in anaerobic conditions when microbes break down organic matter, so it’s a common risk in sewers and treatment plants.

Hydrogen sulfide is highly toxic to humans; inhaling it can irritate and damage the respiratory system and, at higher concentrations, lead to loss of consciousness or death. It’s also corrosive to materials in contact with it—over time it can contribute to the formation of sulfuric acid by bacterial action on concrete, leading to infrastructure damage. In addition, it can form flammable mixtures with air and become explosive in certain concentrations, especially in enclosed spaces where ventilation is poor.

The other substances don’t fit all three hazards together. Carbon dioxide isn’t typically explosive and isn’t corrosive under normal exposure conditions, though very high levels can cause asphyxiation. Nitrogen is inert and not toxic or corrosive in ordinary circumstances. Methane is flammable and explosive but not inherently toxic or corrosive in the same sense as hydrogen sulfide.