Explanation:

Carboxyhemoglobin or carboxyhaemoglobin (symbol COHb or HbCO) is a stable complex of carbon monoxide and hemoglobin (Hb) that forms in red blood cells upon contact with carbon monoxide (CO). The clinical diagnosis of acute CO poisoning should be confirmed by demonstrating an elevated carboxyhemoglobin level. COHb levels of at least 3–4% in nonsmokers and at least 10% in smokers can be considered outside the expected physiological range (25). The COHb level in smokers is generally in the 3–5% range (25).