The concentration of exhaled nitric oxide (FeNO) in various airway & respiratory disorders, particularly asthma, has been deeply studied. Its use for noninvasive monitoring of asthma control (reflecting airway eosinophilic inflammation) is being closely examined and put into clinical practice. FeNO is significantly elevated in asthma. Elevated FeNO levels have also been noted in patients hospitalized with a COPD exacerbation, acidosis, rhinitis, bronchiectasis, active pulmonary sarcoidosis, active fibrosing alveolitis, and acute lung allograft rejection. Decreased FeNO levels have been seen in patients with primary ciliary dyskinesia, cystic fibrosis, PiZZ phenotype-related alpha-1 antitrypsin deficiency, and pulmonary hypertension