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Nitrogen oxides (NOx), regarded in the past primarily as toxic air pollutants, have recently been shown to be bioactive species formed endogenously in the human lung. The relationship between the toxicities and the bioactivities of NOx must be understood in the context of their chemical interactions in the pulmonary microenvironment. Nitric oxide synthase (NOS) is a newly identified enzyme system active in airway epithelial cells, macrophages, neutrophils, mast cells, autonomic neurons, smooth muscle cells, fibroblasts, and endothelial cells. The chemical products of NOS in the lung vary with disease states, and are involved in pulmonary neurotransmission, host defense, and airway and vascular smooth muscle relaxation. Further, certain patients with pulmonary hypertension, adult respiratory distress syndrome and asthma may experience physiologic improvement with NOx therapy, including inhalation of nitric …