Carbon monoxide is pollutant as it :

Combines with haemoglobin

Carbon monoxide (CO)—a colourless, odourless, tasteless, and toxic air pollutant—is produced in the incomplete combustion of carbon-containing fuels, such as gasoline, natural gas, oil, coal, and wood. CO affects human health by impairing the ability of the blood to bring O2​ to body tissues. When CO is inhaled, it rapidly crosses the alveolar epithelium to reach the blood, where it binds to haemoglobin to form carboxyhemoglobin (COHb), a useful marker for predicting the health effects of CO. Because CO has an affinity for haemoglobin more than 200 times greater than does O2​, the presence of CO in the lung will displace O2​ from the haemoglobin.