Carbon monoxide is a by-product of incomplete combustion.
There is always some produced whenever we burn any carbon-based fuel, such as natural gas, heating oil, wood, reconstituted wood logs, gasoline, charcoal, or any similar product. Carbon monoxide can be fatal. We frequently hear of CO related deaths here in California, usually from use of inappropriate indoor heaters or broken or improperly vented heaters. Carbon monoxide blocks the blood's ability to transport oxygen to the internal organs and can result in death at even very low levels. It is a colorless, odorless gas, making it impossible to detect without instruments.