What gas is likely to accumulate in the air during the smoldering phase?

During the smoldering phase of combustion, the fire burns without flames, often in a low-oxygen environment, which leads to incomplete combustion. This incomplete combustion produces carbon monoxide, a colorless, odorless gas that can be highly toxic and is a common byproduct when there is not enough oxygen present to convert carbon to carbon dioxide. In smoldering fires, oxygen levels are typically reduced, which maximizes the production of carbon monoxide instead of carbon dioxide and other gases. The presence of carbon monoxide indicates that combustion is taking place inefficiently, which is characteristic of the smoldering phase.