In-depth analysis of time trends in adult asthma and rhinitis

Introduction: Lower airway symptoms (LAS) are closely related to rhinitis. In Sweden 1990-2008 the prevalence of LAS was unchanged, while rhinitis increased from 22% to 28%, leading to the hypothesis that their association had changed. The aim of this study was to characterize the relationship between rhinitis and LAS over time. Methods: In 1990 10,345 (86% participation) Swedish adults aged 20-44 participated in the ECRHS study. In 2008 the same questions were completed by 26,527 (60% participation) Swedish adults in the GA2LEN study. Of these, the 9,637 subjects aged 20-44 and living in the areas surveyed both years were compared to the 1990 data. Results: The prevalence of rhinitis was linearly related to the number of LAS, and all LAS were associated with rhinitis. From 1990 to 2008 any wheeze decreased in subjects with (36%-30%) and without rhinitis (16%-10%), each p<0.001. Most other LAS decreased in subjects with rhinitis, also when trends were adjusted for age, sex and area. Adjustment for smoking however dissipated all negative time trends except any wheeze, OR 0.86 (0.76-0.98). In never-smokers, the associations of rhinitis with any wheeze, asthma-related symptoms and current asthma were unchanged at OR 4, OR 6 and OR 8-10 (all p>0.2). Current smoking decreased from 35% to 12%, p<0.001. Discussion: All LAS were positively associated with rhinitis, although at very different magnitudes. The different time trends in rhinitis and LAS were not explained by a decreased strength of association. This points towards other causative factors. One plausible explanation is that the large decrease in smoking and the increase in rhinitis cancelled each other out, thus resulting in a level prevalence of lower airway symptoms.