Abstract: An apparatus and method is disclosed which allows the non-invasive monitoring of a subject's carboxyhemoglobin level, thereby allowing the detection of possible carbon monoxide poisoning. The subject breathes oxygen to saturate his blood hemoglobin and eliminate reduced hemoglobin, thus allowing the detection and differentiation between oxy- and carboxyhemoglobin by modification of a conventional pulse oximeter. Preferably the device works in two modes. The first mode is a conventional pulse oximeter capable of determining the level of oxy-hemoglobin in the subject's arterial blood. Upon the achievement of certain criteria the preferred embodiment of the inventive device would switch to a second mode, in which mode the device would be capable of determining carboxyhemoglobin levels. Alternatively, carboxyhemoglobin can be measured using two devices and two different pairs of wavelengths transilluminating two different body parts.