Patients with CO poisoning usually have pale or cyanotic, not cherry-red skin. However, red retinal veins are a sensitive early finding. Headache is the most common complaint, followed by dizziness and weakness. A high index of suspicion must be maintained for CO poisoning in the winter months to avoid misdiagnosis of CO poisoning as the flu. With higher blood levels of COHb, visual disturbances, dyspnea, confusion, and finally coma and respiratory failure occur. Patients exposed to high levels of CO may collapse without progressing through this sequence. CO not only combines with hemoglobin more avidly than oxygen, but it shifts the oxygen–hemoglobin dissociation curve to the left and prevents any remaining oxygen from being released in the tissues. CO combines with other heme-structure molecules, including cytochromes and myoglobin. Considerable debate over the relative importance of each of these sites of poisoning exists, but the effect can be important clinically, since rebound increases in the blood CO may occur as these tissue stores are mobilized and cause confusion in interpreting COHb results. The COHb level at the scene correlates best with the clinical presentation. Administering 100% oxygen can lead to underestimating the degree of toxicity; however is necessary as this is the treatment. Cigarette smokers have COHb levels of 5–10%.