Final answer:
Oxygen is the element least likely to be found in a poorly ventilated gas furnace because it is consumed during combustion, leading to increased levels of carbon monoxide from incomplete combustion.
Step-by-step explanation:
The element least likely to be found in a poorly ventilated gas furnace is oxygen. Without proper ventilation, oxygen levels decrease as it is consumed during combustion to produce carbon dioxide and water. In such environments, byproducts like carbon monoxide (CO) are likely to form due to incomplete combustion, caused by the insufficient presence of oxygen. Carbon monoxide is particularly dangerous as it is a colorless, odorless gas that can cause serious health issues or even death. Ensuring good ventilation in areas with combustion appliances, such as gas furnaces, is crucial for safety and for maintaining a healthy level of oxygen in the air.