Which of the following is a toxic gas?

Hydrogen sulfide (H2S) is considered a toxic gas primarily due to its properties and the effects it can have on human health. It is a colorless gas known for its characteristic smell of rotten eggs, which can serve as a warning at low concentrations. However, it is important to note that at higher concentrations, hydrogen sulfide can be extremely hazardous. It can cause irritation to the eyes and respiratory system, and can lead to more severe effects, including unconsciousness or even death, if inhaled in sufficient amounts.

In wastewater treatment contexts, hydrogen sulfide is often produced through anaerobic decomposition of organic matter. Its presence in confined spaces, such as sewers or wastewater treatment facilities, can create dangerous working conditions if not properly monitored and managed. Understanding the risks associated with hydrogen sulfide is crucial for ensuring safety in environments where wastewater is present.

Other gases listed, such as nitrogen, carbon dioxide, and oxygen, are generally non-toxic under normal atmospheric conditions. While they can be present in high concentrations that may pose certain risks, they do not have the same acute toxic effects as hydrogen sulfide.