carbon monoxide

Colorless, odourless, tasteless, non-corrosive, highly poisonous gas of about the same density as that of air. Very flammable, burning in air with bright blue flame. Although each molecule of CO has one carbon atom and one oxygen atom, it has a shape similar to that of an oxygen molecule (two atoms of oxygen), which is important with regard to it's lethality. (Source: PHYMAC)