The thing about Carbon Monoxide is that you don't need very much to produce symptoms.
Carbon Monoxide Poisoning. Free information. Patient | Patient
Carbon monoxide competes with oxygen to form carboxyhaemoglobin (HbCO) instead of oxyhaemoglobin but it has 210 times the affinity for haemoglobin. In an atmosphere of 21% oxygen and 0.1% carbon monoxide, therefore, the blood will leave the lungs about 50% saturated with oxyhaemoglobin and 50% saturated with carboxyhaemoglobin.
An ambient carbon monoxide level of 100 parts per million (ppm) produces 16% HbCO at equilibrium, which is sufficient to produce clinical symptoms.