I understand that completely that carbon monoxide is a waste gas. I stand by my prior post.
methyl mercaptan is the chemical added to propane and natural gas for odor. It's generally described as a rotton "cabbage" type smell, rather than the rotton "egg" smell generally associated with sulfer.
This may interest you. From the methyl mercaptan MSDS:
"Irritating to the eyes, mucous membranes and respiratory system. Inhalation may result in pulmonary paralysis, sudden collapse and death. Exposure to high concentrations may result in acute hemolytic anemia. Extremely flammable. Poison gas."
I hadn't realized how nasty that smell additive can be.