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 in the fuel combine to form carbon dioxide, water, heat, and deadly carbon monoxide. In properly installed and maintained appliances gas burns clean and produces little carbon monoxide. Anything that disrupts the burning process burners or results in a shortage of oxygen or burners, can increase carbon monoxide production. Wood, coal, and charcoal fires always produce carbon monoxide, as do gasoline engines.