CO - what is it

Carbon Monoxide is just that - the monoxide of carbon. It is therefore combustible; what it really wants to be is carbon dioxide.
It is produced from the partial combustion of any kind of 'fossil' fuel, and also carbohydrate fuels like wood and paper.
It is colourless and odourless (although the Victorian gas experts say it had an odour of garlic about it.) It is about the same density as air, but since it is usually associated with hot combustion gases, tends to rise in an enclosed environment.