Only recently has it become known that, at very low concentrations, CO participates in many physiological reactions. Where a CO exposure of 10,000 parts per million (ppm) (1% by volume CO in air) is toxic, 100-250 ppm (one hundredth to one fortieth as much) will stimulate the physiological effects without apparent toxicity [4] . The majority of endogenous CO production originates from active heme metabolism (>86%), though a portion may be produced in lipid peroxidation and drug metabolism reactions [46] . Cigarette smoking, still practiced by many lung patients, represents a major source of chronic lowlevel exposure to CO. Inhaled CO initially targets alveolar macrophages and respiratory epithelial cells.