FENO levels can be increased by: • Airway viral infection [Murphy1998] • Allergic rhinitis [Henriksen 1999] • Nitrate rich food [Olin 2001]   FENO levels can be decreased by: • Repeated spirometric manoeuvres [Silkhoff 1999] • Exercise [Terada 2001] • Bronchoconstriction [Terada 2001, Piacentini 2002] • Smoking [Verleden 1999]

Smoking and FENO levels
As smoking can affect NO levels,  test results in smokers need to be interpreted with caution. Smokers can be seen to have near-normal levels of between 2 and 10 ppb despite the presence of inflammation.[Verleden 1999, Hotvath  2004] But it is important to note that increasing FENO  levels in smokers should still be interpreted as indicative of on-going eosinophilic inflammation. To aid interpretation, the “personal best” for patients who smoke should be obtained by repeating the FENO test on several occasions.