Carbon monoxide (CO) has over two hundred times the binding affinity with hemoglobin compared to oxygen. When CO binds with hemoglobin it forms carboxyhemoglobin. A pulse oximeter does not differentiate between oxygen-bound hemoglobin and CO-bound hemoglobin, so will give a false reading of normal or elevated oxygen saturation (SpO2) levels. It take a long time of breathing normal air... https://www.quora.com/How-can-carbon-monoxide-poisoning-affect-my-oxygen-saturation-readings-on-a-pulse-oximeter