What hazardous gas can be produced if combustion air is inadequately supplied to a gas furnace?

When a gas furnace operates, it requires a sufficient supply of combustion air to convert gas into heat efficiently. If there is an inadequate supply of air, the combustion process can become incomplete. In this scenario, instead of the gas burning fully to produce carbon dioxide, one of the byproducts can be carbon monoxide, which is a hazardous gas.

Carbon monoxide is particularly dangerous because it is colorless, odorless, and tasteless, making it difficult for people to detect its presence until they experience symptoms of poisoning. When combustion is inefficient due to restricted air supply, more carbon monoxide is produced, posing a significant health risk to occupants.

In contrast, while carbon dioxide can result from complete combustion, it is not considered hazardous in low concentrations. Nitrogen oxide and sulfur dioxide are related to different combustion processes, typically involving higher temperatures or specific fuel types, making them less relevant to the context of a gas furnace lacking proper air supply. Therefore, the primary concern in this situation is the production of carbon monoxide, which is why that choice is the correct answer.