Carbon monoxide

Carbon monoxide is toxic, meaning that it can poison people. Its danger is increased by two physical properties: it is colourless and odourless. A person cannot rely on seeing or smelling it, so it may build up without being noticed.

When less oxygen is available, some carbon can therefore be only partly oxidised, producing carbon monoxide rather than carbon dioxide.

A carbon-containing fuel may produce carbon monoxide and soot if oxygen is limited. Sulfur in a fuel can produce sulfur dioxide. At the high temperatures reached during combustion, nitrogen and oxygen from the air can react to form oxides of nitrogen.

Real combustion can produce a mixture containing carbon dioxide, carbon monoxide and other products; “incomplete” does not mean that carbon monoxide must be the only carbon-containing product.