When there is a lack of oxygen to ensure complete combustion of the fuel, each atom of carbon links up with only one atom of oxygen - forming carbon monoxide gas. Carbon monoxide inhibits the blood's capacity to carry oxygen. By replacing oxygen with carbon monoxide in our blood, our bodies poison themselves by cutting off the needed oxygen to our organs and cells, causing various amounts of damage - depending on exposure. Carbon monoxide can escape from any fuel-burning appliance, furnace, water heater, fireplace, woodstove, or space heater. Tight closing replacement windows and doors, as well as additional insulation can cause similar problems in older homes.
Carbon monoxide can spill from vent connections in poorly maintained or blocked chimneys.
If the flue liner is cracked or deteriorated, CO can seep through the liner and into the house - slowly creeping up to dangerous levels. If a nest or other materials restrict or block the flue, CO will mostly spill back into the house. Improperly sized flues connected to new high-efficiency furnaces and water heaters can also contribute to CO spillage. Warming up vehicles in an attached garage, even with the garage door opened, can allow concentrated amounts of CO to enter your home through the car port door or near-by windows. Wind can also blow fumes back into the garage, and temperature differences between the indoors and outside can move CO back into your house or garage.