Co and $\mathrm{CO}_{2}$ gas are released by the buring of various types of fuel. Carbon monoxide is poisonous where as carbon dioxide is not poisonus in its nature.

Carbon monoxide reacts with hemoglobin and produce carboxyhemoglobin complex. This complex is more stable than oxyhemoglobin.

Carboxyhemoglobin that carries $3-4 \%$ concentration reduces the ability of blood to absorb oxygen.

This oxygen deficiency, results into headache, weak eyesight, nervousness and cardiovascular disorder. Due to the increase in its proportion, the death rate is high. Carbon dioxide is nonveritable. Its high concentration is very harmful.