BackgroundAsthma and chronic rhinosinusitis with nasal polyps (CRSwNPs) are coexisting diseases that are multifactorial. The rural environment seems to protect from atopy, but its relation with nonatopic airway inflammations has been less investigated. Indoleamine 2,3-dioxygenase (IDO) is an enzyme involved in the catabolism of the essential amino acid tryptophan (Trp) to kynurenine (Kyn). Low IDO activity has been previously observed in atopy and asthma. The objective was to investigate the relationships of IDO activity, eosinophils, and cofactors during asthma and/or CRSwNPs.MethodsA Finnish population-based cohort of adult asthmatic patients (n = 245) and nonasthmatic patients (n = 405) was used. The presence of asthma and atopy were based on patient history and standardized diagnostic tests. The presence of acetyl salicylic acid intolerance, doctor-diagnosed NPs, and countryside environment during childhood were based on a questionnaire report. Serum IDO activity was evaluated by assessing the Kyn/Trp ratio by liquid chromatography.ResultsLow IDO activity was associated significantly with atopy, CRSwNPs, and an urban background. IDO activity did not correlate with pulmonary function. As expected, CRSwNPs was more frequent among asthmatic patients. A rural background has a protective effect from atopy and atopic asthma but it did not affect the prevalence of CRSwNPs or nonatopic asthma.ConclusionLow IDO activity might result from the urban environment and influence the development of the atopic phenotype. On the other hand, low IDO activity, found in CRSwNPs, does not seem to be related to the urban background an us may result from other, still unknown, factors.