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Background: The cause of nonallergic rhinitis (NAR) and its relation to lower airway disease remains unclear. The purpose of this study was to perform a descriptive analysis of the occurrence of rhinitis in a Danish general population with focus on NAR and its association with smoking and lower airway disease. Methods: A population-based, cross-sectional study conducted in Copenhagen, Denmark was performed. A random sample from the general population (n = 7931; age, 18-69 years) was invited to a general health examination including measurements of serum-specific immunoglobulin E (IgE) to common aeroallergens; 3471 (44%) persons were accepted. For further analysis, we divided the population into the following groups: (I) negative specific IgE and no rhinitis (controls); (II) negative specific IgE and rhinitis (NAR); (III) positive specific IgE and rhinitis (allergic rhinitis [AR]); and (IV) positive specific IgE but no rhinitis (sensitized). Results: We found that NAR was associated with asthma (odds ratio [OR] = 2.51 [1.87-3.37]); chronic bronchitis (OR = 2.27 [1.85-2.79]); current smoking (>15 g/day; OR = 1.57 [1.18-2.08]); lower forced expiratory volume in 1 second/forced vital capacity (FEV1/FVC) ratios and reduced FEV1 values. The association with chronic bronchitis was stronger in NAR than in AR, whereas the opposite was true for asthma. FEV1/FVC of <70% was not significantly associated to any group. Conclusion: This epidemiological study indicates that both asthma and chronic bronchitis are important comorbidities in NAR confirming the 'united airway' hypothesis, and that smoking might be a significant modulator of disease. Although NAR was significantly associated with poor lung function, no significant association with chronic obstructive pulmonary disease was shown. [ABSTRACT FROM AUTHOR]