Quantification of the fraction of nitric oxide in the total exhalation volume, used as a non-invasive biomarker of eosinophilic airway inflammation. Higher Feno values are associated with childhood asthma symptoms, exacerbations, physician-diagnosed asthma and atopy. The trait is Experimental Factor Ontology entry EFO_0005536 (nitric oxide exhalation measurement). Also known as: FENO, fraction of exhaled nitric oxide.