Carbon monoxide is a colorless, odorless gas, produced when a fire or other type of combustion proceeds with insufficient oxygen. Prior to introducing motor vehicle exhaust controls, it used to occur in concentrations of about 1% in gases escaping in motor vehicle exhaust. It is a very important contributor to the hazards of cigarette smoke, and many types of heating and cooking fires may produce carbon monoxide contamination indoors, especially during cold weather when the doors and windows are tightly closed. 1