What is an example of a suffocating gas?

The choice of carbon monoxide as an example of a suffocating gas is justified because it is a colorless, odorless gas that can lead to suffocation by displacing oxygen in the blood. When inhaled, carbon monoxide binds with hemoglobin in red blood cells more effectively than oxygen does, which inhibits the blood's ability to carry oxygen throughout the body. This can result in hypoxia, or an insufficient oxygen supply to tissues, ultimately leading to symptoms such as headaches, dizziness, confusion, and at high levels, even death.

The other gases listed have different characteristics and effects: hydrogen sulfide is toxic and can be lethal in high concentrations but primarily affects the nervous system, chlorine is a respiratory irritant and can cause damage to lung tissues, while ammonia is irritating to the respiratory system as well. However, they do not primarily act by displacing oxygen in the way carbon monoxide does, which is critical in determining what constitutes a suffocating gas.