If a gas-fired furnace has incomplete combustion, what dangerous substance is produced?

When a gas-fired furnace experiences incomplete combustion, it predominantly produces carbon monoxide. This occurs because insufficient oxygen is available to fully oxidize the fuel, which in a gas furnace is typically natural gas. As a result, rather than converting the carbon in the gas completely into carbon dioxide, the combustion process yields carbon monoxide, which is a colorless, odorless, and highly toxic gas.

In terms of potential dangers, carbon monoxide can bind to hemoglobin in the blood, significantly reducing the blood's oxygen-carrying capacity, leading to symptoms like headaches, dizziness, confusion, and in severe cases, fatality.

The other choices represent substances that can occur in different contexts. Carbon dioxide is a normal byproduct of complete combustion, ozone is generated from photochemical reactions and not directly from furnaces, and nitrogen dioxide is a product of combustion processes, particularly those involving high temperatures, but is not the primary concern with incomplete combustion in this scenario.