A method and an apparatus for measuring the concentration of a specific gas component of a gas mixture including another gas whose concentration is independently known using light absorption spectroscopy are provided. A method and an apparatus for assessing human airway inflammation by measuring the concentration of exhaled NO and CO.sub.2 present in orally exhaled breath using light absorption spectroscopy are also provided. NO concentration is determined at the time during breath sampling corresponding to a known exhaled CO.sub.2 concentration. Methods and apparatus are further provided for measuring NO concentration in orally exhaled human breath that analyze breath emanating from the lower airways and lungs, while excluding breath from the nasal cavity. They include steps and apparatus for discarding initially exhaled breath, flowing breath through an analysis chamber using a vacuum pump and flowing breath through an analysis chamber using a vacuum pump at an initial flow rate and later at a flow rate higher than the initial flow rate.