Carbon dioxide $\left( CO _{2}\right)$ and carbon monoxide $(CO)$ gases are emitted during the combustion of various fuels. Carbon monoxide is poisonous, whereas carbon-dioxide is non-toxic in nature.

Carbon monoxide is poisonous because it is capable of forming a complex with haemoglobin (carboxyhaemoglobin), which is more stable than the oxygen-haemoglobin complex. The concentration range of $3-4 \%$ of carboxyhaemoglobin decreases the oxygen- carrying capacity of blood. This results in headaches, weak eyesight, nervousness, and cardiovascular disorders. A more increased concentration may even lead to death. Carbon dioxide is not poisonous. It proves harmful only at very high concentrations.