Method of feNO measurement will utilize the offline options for preschool children & infants appropriate for age as described in the 2005 Joint Statement of the American Thoracic Society & the European Respiratory Society when discussing tidal breathing techniques with uncontrolled flow rate Offline exhaled air can be collected via a mouthpiece or a face mask connected to a non-re-breathing valve that allows inspiration of NO-free air from an NO-inert reservoir to avoid contamination by ambient NO. Exhaled breath samples are collected into an NO-inert bag fitted with the expiratory port once a stable breathing pattern is present.