A furnace burns natural gas to create heat and the creation of this heat creates combustion fumes. These combustion fumes contain high levels of carbon monoxide. The heart of the furnace is called the heat exchanger and this is the metal body that is heated to create warm air. The combustion fumes travel through the body of the heat exchanger where they are ultimately exhausted out of the furnace exhaust. When a furnace heat exchanger is found to be cracked or severely degraded eventually the fumes that are supposed to be exhausted out of the furnace will leak out into your furnace and those fumes are then blown throughout the home. This can lead to carbon monoxide poisoning with headaches, vomiting, or death. It is critical especially when a furnace reaches beyond 10 years old that the heat exchanger is inspected for flaws or cracks to ensure the product is safe.