Why might the absence of an unpleasant odor in a manhole or lift station be misleading?

The absence of an unpleasant odor in a manhole or lift station can be misleading because certain dangerous gases, such as carbon monoxide or methane, do not have any smell. This means that even if the air appears to be free from any odors, it does not guarantee safety. In these environments, it is crucial to recognize that just because there is no odor, it does not mean there are no harmful gases present. Proper monitoring and detection equipment should always be used in these scenarios to ensure safety, as relying solely on smell can lead to misjudgments about the air quality and potential hazards.

Other options suggest that some gases have odors or that all gases can be smelled, which is not accurate, while the statement about odorless gases being less harmful is also incorrect, as many odorless gases can be extremely dangerous regardless of their lack of scent.