This will usually be a small white box on ceiling (or wall) similar in appearance to a smoke alarm but will usually be labelled to identify it as a carbon monoxide alarm. Carbon Monoxide (CO) is a colourless odourless gas produced when fuel is burnt. It is harmful and exposure to a high level of CO can be fatal. CO alarm will sound if the amount of CO in air could be harmful. You do not need a CO alarm if do not you have a gas appliance (cooker, boiler etc.) or an open fire.