Which of the following gases is considered lethal?

Hydrogen cyanide is considered lethal because it is a highly toxic chemical compound that can interfere with the body's ability to use oxygen at the cellular level. It acts as an inhibitor of cytochrome c oxidase, an essential enzyme in the electron transport chain, which ultimately disrupts aerobic metabolism. Exposure can lead to severe health consequences, including respiratory failure and death, particularly with high levels or prolonged exposure. Its potential for harm makes it particularly dangerous in industrial environments, as well as in scenarios involving smoke inhalation from fires where it can be released.

The other gases listed, such as carbon dioxide, nitrogen, and helium, do not share the same level of toxicity when encountered under normal conditions. While carbon dioxide can be harmful in high concentrations by displacing oxygen, it is not lethal in the same immediate way hydrogen cyanide is. Nitrogen is an inert gas that makes up a substantial portion of the atmosphere and is not toxic. Similarly, helium is also non-toxic and primarily acts as a simple asphyxiant, presenting no chemical toxicity on its own.