We read with interest the article by Olin et al1 on the influence of age and height on fractional exhaled nitric oxide (Feno) in a sample of lifelong never-smoking adults randomly selected by a postal questionnaire from the general population. According to these authors,,1the upper limits of Feno range from 24.0 to 54.0 parts per billion (ppb), and the geometric mean of Feno for the whole population is 16.6 ppb (95% confidence interval [CI], 5.87 to 47.14 ppb). Age and height would account for 9 to 11% of the variance of reference values. We think that these statements deserve a comment: in fact, such upper Feno values are definitely higher than those reported in healthy subjects, in both adults and children in American Thoracic Society/European Respiratory Society guidelines2 and in our experience.3 The “normal” range for Feno is influenced by constitutional as much as by environmental and pathophysiologic factors.,2 We do agree that only studies in the general population could detect associations between “abnormal” Feno values and known and supposed risk factors. Recently, Rolla et al,,4investigating 108 of 590 consecutive patients referred in 1 year for rhinitis, reported that Feno is significantly higher in patients with allergic rhinitis and chronic rhinosinusitis compared to patients with nonallergic rhinitis (44.3 ppb [95% CI, 34 to 54 ppb] and 53 ppb [95% CI, 42 to 64 ppb] vs 22 ppb [95% CI, 18 to 27 ppb], respectively), reinforcing and extending the Allergic Rhinitis and Its Impact on Asthma guidelines5 of testing for asthma patients with allergic rhinitis. Seasonal variations of Feno values due to fluctuations of exposure to allergens have also been reported in patients with allergic asthma and also seasonal allergic rhinitis. According to this hypothesis, whether the measurements were performed during or outside the pollen season should have been reported,6 with rhinitis and asthma having a similar weight in Feno changes as recently reported by Travers et al7 in a random community survey of adults. The high prevalence of allergic rhinitis and/or nasal symptoms, probably missing in replies to the questionnaire8 or in statistical analysis,1 could represent an important confounder for reference values. Moreover, a positive correlation between Feno and dietary consumption of fats in children with asthma assuming low levels of antioxidants has recently been reported,,9 thus suggesting the need of further studies might aimed at investigating the relationship between Feno levels and dietary habits.