You can't see, smell or taste it.
Carbon monoxide is slightly lighter than air and quickly spreads throughout an entire house.
Carbon monoxide gas is produced when fossil fuel burns incompletely because of insufficient oxygen. During incomplete combustion, the carbon and hydrogen combine to form carbon dioxide, water, heat, and deadly carbon monoxide. In properly installed and maintained appliances gas burns clean and produces only small amounts of carbon monoxide. Anything which disrupts the burning process or results in a shortage of oxygen can increase carbon monoxide production. Wood, coal, and charcoal fires always produce carbon monoxide, as do gasoline engines.