CO is common gas poisonous to hemoglobin. CO is a known byproduct of tobacco and narghile smoking. CO originates from the incomplete combustion of carbon-containing fuels. Affinity of CO for hemoglobin (Hb) is about 200-fold that of dioxygen (O2). No cyanosis is present in toxic patients as HbCO is bright red in color. Multivawe Tc measurement or spectrometry is necessary to objectively evidence and quantitate HbCO, since Pa02 and SpO2 stay normal. HbCO values up to 20% or more can be observed in active smokers, especially in narghile users. High HbCO readings should lead to examination of heating system or exposure to burned gasoline in the home or occupational environment even in smokers. A recent trend for suicide is the use of charcoal burning stoves and barbecues (PMID 24912402). See PMID 9828249, 12362005, 19297574, 21896525, 22201610, 23087025