Which byproduct is most commonly produced during smoldering?

During smoldering, the combustion process occurs in a way that does not supply enough oxygen to enable complete combustion of the fuel. As a result, carbon monoxide becomes the predominant byproduct. In smoldering fires, where flames may not be present, the incomplete combustion of carbon-containing materials leads to a greater production of carbon monoxide, which is colorless, odorless, and highly toxic.

In contrast, when sufficient oxygen is available for combustion, carbon dioxide is produced instead. Ozone is a different type of gas that does not typically form directly from smoldering processes, and hydrogen is not a major byproduct of this type of fire behavior. Therefore, carbon monoxide is the correct answer, as it is directly linked to the mechanisms of incomplete combustion that define smoldering.