A mathematical model is formulated for computing alveolar partial pressure of carbon monoxide (PACO) from that in the atmospheric air. The model takes into account parameters like inspired/expired air flow rates, diffusion capacity of the lung, concentration of CO in the atmospheric air, blood flow rate and the non-linear CO dissociation curve. The effect due to the presence of O2 in the blood on CO dissociation curve is also incorporated. It is shown that for a given atmospheric CO concentration, PACO increases exponentialy with time and attains asymptotic value. Alveolar PCO increases further with the increase in the atmospheric CO concentration. The model can also be used to compute carboxyhaemoglobin levels in the blood as a function of exposure time and the results are comparable with the CFK equation and the values measured experimentally.