When carboxyheamoglobin reaches 3−4% in blood, it leads to headache resulting in cardiovascular problem. Carboxyheamoglobin ( COHb) is a stable complex of carbon monoxide that forms in red blood cells when carbon monoxide is inhaled. The reference range of COHb differs among individuals. For a normal person, it is up to 3%. Individuals with hemolytic anemia and neonates may have COHb levels of up to 5%.