Introduction
The prevalence of asthma is frequently studied and a consistent pattern of a worldwide increase during the latter half of the 20th century has been reported both among children and adults, although the increase may have leveled off in some areas [1,2]. The reason for the increase is unclear, but changes in lifestyle, environments, and rapid urbanization may have contributed [1,3-5]. Prevalence is mainly a function of incidence and remission. In adulthood, asthma is regarded as a chronic disease due to low remission [6,7], why the prevalence among adults is a result of the high incidence in childhood and adolescence [8-11] with persistence into adulthood, but also due to the relatively high incidence in adulthood [4,12-24]. However, incidence studies of asthma among adults are scarce, particularly population-based studies where cohorts are followed prospectively.
Comparing incidence rates of asthma is complex as the results are highly sensitive to the age of the study population and the methods used. Reported incidence estimates vary substantially but are higher among children and teenagers, 10-30/1000/year [8-11,20], than among adults, 0.2-5/1000/year [4,12-24]. Results from retrospective [11,13,22,24,25] and register studies [13] tend to generate lower incidence rates than prospective studies [11-21,25]. The definitions of both the outcome and the population at risk significantly influences the incidence rate [12,15,18,19], which highlights the importance of using the same methods and definitions to enable comparisons of results from different studies and over time. Three studies in Sweden or the neighboring countries, whereof one in the same area as the current study, based on identical methods reported similar incidence rates of physician-diagnosed asthma among adults from the 1980’s up until 2006, 2-3/1000/year [15,16,18], but it is unknown if the incidence has changed thereafter. As the prevalence of asthma in adults has increased in Sweden during the last decades [2,26] we hypothesized that the incidence of asthma in adulthood still is high or even has increased during the same period. Thus, the main aim of the current study was to estimate the incidence of asthma among adults during the period 1996 to 2016 by surveys of two cohorts each followed for ten years, one from 1996 to 2006 and the other from 2006 to 2016. A further aim was to identify risk factors for adult onset asthma.