The article focuses on delayed offline measurement of exhaled nitric oxide (eNO), which is although useful in environmental and clinical research, but is limited by the instability of stored breath samples. Measurement of eNO by chemiluminescence analysis is valuable for the rapid, noninvasive assessment of airway inflammation and other disease states. Increases or decreases in NO output from the bronchial passages, or from the alveolar region of the lungs, may have health significance. Routine eNO measurements applicable in environmental health screening cannot distinguish between bronchial and alveolar-region abnormalities. The article demonstrated both positive and negative changes in NO concentration during eNO sample storage, which were partly predictable on the basis of temperature, initial concentration, and expected reaction with oxygen. Concentration increases were observed in some previous investigations, but the present study provides the first clear evidence of decreases in NO concentrations during storage.