Increased levels of nitric oxide (NO) in exhaled air of asthmatics were investigated. Using chemiluminescence, NO levels were measured in exhaled air during normal breathing through the nose or mouth. In control subjects, most NO in exhaled air originated from the nasal airways, with minor contributions from the lower airways and oral cavity. However, in mild asthmatics, NO levels during oral breathing were 2-3 times higher, indicating involvement of the lower airways. This suggests that NO may be used to monitor bronchial inflammation, particularly involving inducible NO synthase. The study also examined the presence of nitrogen dioxide (NO₂) in exhaled air. In healthy subjects, NO levels were higher during nasal breathing compared to oral breathing. In asthmatics, NO levels during oral breathing were significantly higher than in controls, while nasal breathing did not show significant elevation. This suggests that NO production in the lower airways is increased in asthmatics. The study found that NO production in normal human airways is mainly in the nasal mucosa, possibly from endothelial cells or parasympathetic nerves. During oral breathing, higher NO levels may reflect NO from the nasopharyngeal mucosa. The presence of NO₂ in exhaled air may be due to clearance of NO₂ absorbed from ambient air. In asthmatics, increased NO levels in exhaled air during oral breathing suggest involvement of macrophages, which produce high levels of NO in bronchial airways. However, the involvement of neutrophils or other cell types cannot be ruled out. The study also found that NO levels in asthmatics with allergic rhinitis did not increase during nasal breathing, possibly due to reduced inducible NO synthase in nasal airways or reduced permeability due to inflammation. The study suggests that exhaled NO levels, measured by chemiluminescence, may be used to monitor ongoing inflammation in the lower airways. The findings have implications for the diagnosis and monitoring of asthma and other inflammatory lung diseases.Increased levels of nitric oxide (NO) in exhaled air of asthmatics were investigated. Using chemiluminescence, NO levels were measured in exhaled air during normal breathing through the nose or mouth. In control subjects, most NO in exhaled air originated from the nasal airways, with minor contributions from the lower airways and oral cavity. However, in mild asthmatics, NO levels during oral breathing were 2-3 times higher, indicating involvement of the lower airways. This suggests that NO may be used to monitor bronchial inflammation, particularly involving inducible NO synthase. The study also examined the presence of nitrogen dioxide (NO₂) in exhaled air. In healthy subjects, NO levels were higher during nasal breathing compared to oral breathing. In asthmatics, NO levels during oral breathing were significantly higher than in controls, while nasal breathing did not show significant elevation. This suggests that NO production in the lower airways is increased in asthmatics. The study found that NO production in normal human airways is mainly in the nasal mucosa, possibly from endothelial cells or parasympathetic nerves. During oral breathing, higher NO levels may reflect NO from the nasopharyngeal mucosa. The presence of NO₂ in exhaled air may be due to clearance of NO₂ absorbed from ambient air. In asthmatics, increased NO levels in exhaled air during oral breathing suggest involvement of macrophages, which produce high levels of NO in bronchial airways. However, the involvement of neutrophils or other cell types cannot be ruled out. The study also found that NO levels in asthmatics with allergic rhinitis did not increase during nasal breathing, possibly due to reduced inducible NO synthase in nasal airways or reduced permeability due to inflammation. The study suggests that exhaled NO levels, measured by chemiluminescence, may be used to monitor ongoing inflammation in the lower airways. The findings have implications for the diagnosis and monitoring of asthma and other inflammatory lung diseases.