Abstract: A method and apparatus for analyzing trace levels of CO in human breath for the purpose of, among other things, assessing the severity of pulmonary diseases and monitoring the patient's response to a prescribed treatment. The apparatus measures in situ and in real time the CO concentration at sensitivity levels at least as low as parts per billion. A laser of the apparatus has a wavelength in mid-infrared (MIR) spectrum. The optical is constructed and arranged to perform cavity enhanced absorption spectroscopy (CEAS) the breath sample. The cavity includes highly reflective mirrors mounted to each side of the optical cavity to cause the light received from the laser to bounce back and forth within the optical cavity to increase effective path length of the light. A breath intake tube is connected to the optical cavity for collecting a sample of the patient's exhaled breath and transferring it to the optical cavity. A photo detector measures parameters of the light exiting the optical cavity.