Carbon monoxide is a colourless, odourless gas that is produced in most fires. Breathing in carbon monoxide will lead to it becoming attached to haemoglobin in preference to oxygen, so it is only necessary to breath in a small amount of carbon monoxide to have a large amount of haemoglobin taken up by it and therefore not available to carry oxygen. For instance if 25% of someone's haemoglobin is taken up by carbon monoxide then only 75% is available to carry oxygen, and so their oxygen saturation could, at best, be only 75%.
Pulse oximeters will display an oxygen saturation which is approximately equal to the percentage of haemoglobin combined with oxygen plus the percentage of haemoglobin combined with carbon monoxide 1. So if someone has 25% of their haemoglobin saturated with carbon monoxide and a true oxygen saturation of 70% a pulse oximeter will display an oxygen saturation of about 95%. This is obviously extremely dangerous and for this reason pulse oximeters should not be used with people who may have inhaled smoke, ie anyone who has been involved with any sort of fire, unless you are certain that they do not have any significant level of carbon monoxide in their blood.
page last modifed: 04/09/2005