Carbon monoxide, or CO, is a normal by-product of fuel combustion, but high levels of it indoors can be dangerous to your health. Typical heating fuels (oil, coal, wood, gasoline, kerosene, diesel fuel, or natural gas) can create carbon monoxide.
Carbon monoxide can build up to a dangerous level if a fuel-burning appliance isn't operating properly, or is not safely venting fuel combustion by-products.