Nitric oxide (NO) has been known as a gas in the environment polluted by vehicle exhaust and cigarette smoke. However, recently NO in the breath has become more known thanks to its association with the FeNO test - a highly reliable test.

NO is a ubiquitous intracellular and intercellular messenger and has many physiological roles in the body, such as assessing airway inflammation in asthma, predicting response to corticosteroids, or supporting respiration in pediatric patients with aspiration pneumonia and premature birth.