CO usually forms when fuel-burning appliances and equipment are malfunctioning or improperly maintained. Normally, when adequate combustion air is available and the appliance is properly installed and maintained, all gases and other combustion byproducts will be harmlessly vented to the outdoors.

Homes and other buildings that are tightly sea;ed or have large exhaust systems, such as kitchen exhaust fans, need a system that provides air to replace the air that is pulled out by the exhaust. Without adequate make-up air, air from the outside can be pulled down a chimney and cause CO to form.