Abstract: A method whereby a patient inhales a fixed dose of a test agent, followed by collection of a blood or urine sample, and measurement of the test agent in blood or urine. This “Leaky Lung Test” is useful in the diagnosis and management of subjects with asthma, cystic fibrosis, COPD, lung cancer and other related lung diseases. In addition, the Leaky Lung Test is useful in identifying subjects with dysfunctional epithelial barriers caused by exposure to noxious inhaled irritants, or after respiratory tract viral infections.