Which gas is not considered malodorous?

Carbon monoxide is not considered malodorous because it is a colorless, odorless gas. While it can pose significant health risks due to its ability to interfere with the blood’s capacity to carry oxygen, it does not have a discernible smell, which distinguishes it from the other gases listed.

On the other hand, sulfur dioxide has a sharp, acrid smell that can be noticeable even at low concentrations. Hydrogen sulfide is famously known for its rotten egg odor, making it easily identifiable. Phenol, although not as commonly recognized as a gas, also has a distinct and unpleasant odor that can be noticeable when present in sufficient quantities. All these other substances have characteristic odors, which makes carbon monoxide unique in this context.