(1) They act as reducing agent
ZnO(s)+ CO(g) → CO2(g) + Zn(s)
(2) They react with oxygen to form
2CO(g) + O2(g) → 2CO2(g)
NB — They attack the hemoglobin of the human blood to form deoxyhemoglobin, which deprives human beings of oxygen, so it is very poisonous.