The high level of air pollution is an important problem. One of these major pollutants is carbon monoxide (CO), which can block normal physiology. Hematologic alteration is another important side effect of exposure to CO. Adverse effects on red blood cells, formation of nonfunctional hemoglobin called carboxyhemoglobin (COHb), have been well documented. The effect of CO on platelets is not well documented. Therefore, the correlation between the level of blood COHb and the platelet count was evaluated in 28 subjects exposed to CO. There is no statistical significance; however, decreased platelet count, platelet distribution width, and plateletcrit, and an increase in blood COHb were observed.