Which of the following is an example of a harmful sewer gas?

Hydrogen sulfide (H2S) is indeed an example of a harmful sewer gas. It is a colorless gas known for its characteristic odor of rotten eggs, and it can be produced during the decomposition of organic matter in wastewater systems. At low concentrations, it is dangerous because it can be harmful to human health. Prolonged exposure can lead to serious health issues, and at higher concentrations, it can be fatal. This makes it a significant concern in wastewater management, as it can pose serious risks to workers and the environment.

In contrast, while ammonia (NH3) can be harmful and is often present in wastewater, it is not classified in the same category of severe toxicity as hydrogen sulfide. Ozone (O3) and carbon monoxide (CO) have very different origins and risks associated with their presence, with ozone typically being involved in air quality issues outside of sewer systems and carbon monoxide being a product of incomplete combustion rather than sewage processes.