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Can carbon monoxide be absorbed from the upper respiratory tract in man?

To investigate the possible absorption of carbon monoxide into the blood from tobacco smoke confined to the mouth and upper airways, an indirect study was made on 12 normal subjects of the recovery of CO from a bolus administered at various levels during a preceding inspiration. No measurable CO uptake was observed unless the bolus was inhaled early enough in inspiration to reach the alveolar region of the lung. Some movement of a tracer gas, argon, was however seen from the upper airways during breath-holding so that some CO might escape into the lungs this way. No detectable CO uptake occurred in one subject who inspired the bolus into his nasal cavities. We conclude that the elevated carboxyhaemoglobin levels seen in smokers, can only occur when smoke is inhaled into the lung, since no appreciable uptake can occur in the upper airways.