The main result of our study is the decrease in prevalence of most respiratory symptoms, particularly symptoms of bronchitis, from 1996 to 2006. This decrease parallels a 30% decrease in smoking prevalence, a decrease which was significant in all age groups, in both sexes, and in the proportion of smokers consuming >14 cigarettes/day. All respiratory symptoms were associated with smoking, and the proportion of respiratory symptoms attributed to smoking (PAR of smoking) varied from 10 to 25%. The decrease in smoking prevalence was greater than the decrease in prevalence of symptoms, and a tendency of a decrease in PAR of smoking from 1996 to 2006 was observed for most respiratory symptoms. In contrast to respiratory symptoms, the prevalence of physician-diagnosed asthma increased and asthma was not consistently associated with smoking.
It is of great importance for preventive decisions to identify effects of changes in smoking habits on respiratory health in the population. In 2005, legislation was implemented in Sweden to reduce smoking in restaurants, which probably contributed to the decrease in smoking. More women than men smoked already in 1996, and the decrease in smoking prevalence was more pronounced among men compared to women. Thus, the gender difference in smoking prevalence increased. Worryingly, a recent study indicates an increase in prevalence of smoking and symptoms of bronchitis among teen-age girls in Sweden . It is of great importance to establish a decrease in cigarette consumption also among women; in particular since women are more susceptible to cigarette smoke than men .
Few studies have reported PAR of smoking for respiratory symptoms and asthma. Our results imply that if smoking had been eliminated, 10-25% of the symptom prevalence in the population would have been eliminated. A study comparing the Swedish part of the Global Allergy and Asthma European Network (GA2LEN) study performed in 2008 with the Swedish part of the European Community Respiratory Health Survey (ECHRS) from 1990  also found a decrease in the prevalence of smoking, and a decrease in PAR of smoking for any wheeze. However, the methods for estimating PAR differed somewhat between the present study and the cited study.
Comparisons of prevalence estimates between studies are difficult, since populations, questionnaires, and classification of symptoms and diseases often differ. One major advantage of our study is the use of identical methods in both surveys. The same methods as in our study were used in Helsinki, Finland, and that study also found a decrease in symptoms of bronchitis parallel to a decrease in smoking prevalence from 1996 to 2006 . In west Sweden, a decrease was seen in the prevalence of most respiratory symptoms from 1990 to 2008 parallel to a 50% decrease in smoking prevalence . In contrast to our results and the studies cited above, a study in Stockholm using identical methods found the prevalence of symptoms to be more or less level from 1996 to 2006, despite a decrease in smoking prevalence by 43% .
The decrease in the prevalence of bronchitis symptoms is probably mainly a result of the decrease in smoking prevalence, but also of some other factor since we found a decrease among non-smokers. The decrease among non-smokers could be related to a reduction of environmental tobacco smoke (ETS), which followed the stricter Swedish smoking legislation. The increase in use of asthma medication, which can be prescribed also for bronchitis symptoms, may also contribute to a decrease in symptoms.
Wheeze is a common symptom both in asthma and bronchitis and is strongly related to smoking [6, 22, 25]. In contrast to bronchitis symptoms, the prevalence of any wheeze and asthmatic wheeze remained at a similar level in 2006 compared to 1996, also stratified by smoking habits, while recurrent wheeze decreased slightly but significantly. Since PAR of smoking was largest for recurrent wheeze, with smoking explaining about one fifth of the symptom prevalence, a decrease in the prevalence of recurrent wheeze was expected due to the decrease in smoking prevalence. Actually, a pronounced decrease in the prevalence of all wheeze symptoms was expected due to the decrease in smoking prevalence. One hypothesis that would explain the lack of an overall decrease in wheeze symptoms is that a parallel increase in asthma prevalence levels the prevalence of wheeze. Smoking has some anti-inflammatory effects and may be inversely related to allergic sensitisation [33, 34]. Regarding asthma however, there is no data indicating a similar effect of smoking. Instead several studies suggest no effect or an increased risk of smoking on the incidence of asthma among adults [8–11]. Further, parental smoking increases the risk for childhood asthma [35, 36]. Thus, it is not likely that the increase in asthma is related to the decrease in smoking.
Compared to similar studies from other countries [13, 14, 37] as well as in other Swedish studies using the same questionnaire as in our study [6, 21, 28], a higher prevalence of asthma was found in Norrbotten. This finding confirms previous indications of a north–south gradient in asthma prevalence in Sweden [38, 39]. The prevalence of physician-diagnosed asthma increased from 1996 to 2006, also when stratified by smoking. The highest prevalence of asthma was found among ex-smokers, a result confirming other cross-sectional studies [6, 12]. It is known that diagnosis of COPD significantly impacts smokers to quit smoking  and therefore it could be suggested that smokers who get an asthma diagnosis also are likely to quit smoking, as several studies have found an association between current smoking and incident asthma [8, 10, 11]. The prevalence of physician-diagnosed asthma decreased by age in both surveys, in line with results from other studies [6, 22]. In 1996 however, in contrast to 2006, there was a higher prevalence of asthma in the oldest age group (60–69 years) compared to those aged 30–59 years. This result might reflect misclassification of COPD as asthma among the elderly in 1996.
Does the estimated increase in the prevalence of asthma reflect a “true” increase in asthma? The proportion reporting symptoms common in asthma decreased significantly from 1996 to 2006 among subjects with physician-diagnosed asthma. This decrease occurred despite the fact that the proportion of subjects using asthma medicines also decreased significantly among the asthmatics. These data suggest an increased awareness of asthma in the society, improved recognition of asthma in primary care and an increasing diagnostic activity, in particular of mild asthma, which probably have contributed to the observed increase in physician-diagnosed asthma; a result in line with studies in other areas [6, 21, 22]. On the other hand, the lack of decrease in asthmatic wheeze and any wheeze despite the major decrease in smoking prevalence may indicate a true increase in asthma in Northern Sweden. Also, both asthma and use of asthma medication increased most in the youngest age group, which imply a true increase of asthma since misclassification of asthma as e.g. COPD is unlikely among young subjects. One might argue that the observed increase in asthma could be due to both a true increase in asthma as well as an increased diagnostic activity, which would be a result in line with studies from other countries . This hypothesis requires further studies including clinical examinations. In our study, the majority of subjects with physician-diagnosed asthma used asthma medicines in both surveys, although somewhat less so in 2006 compared to 1996. In a study from Australia the authors argue that improved guidelines on how to implement asthma treatment are required , but as we cannot provide detailed information about use of inhaled corticosteroids we cannot evaluate if the adherence to maintenance treatment is appropriate.
The validity and reliability of the present study was high due to the population-based samples, the high participation rates in both surveys and the use of a validated questionnaire. The somewhat lower participation rate in 2006 compared to 1996 could possibly lead to a slight overestimation of symptom prevalence in 2006 . An overestimation in 2006 would not alter the findings of decreased prevalence in symptoms, but could possibly affect the observed increase in asthma prevalence. However, the close to identical prevalence estimates after adjustment to the age and gender distribution in Norrbotten each year verify the representativeness of the study sample. A weakness is the lack of clinical data when discussing asthma, and measures of cotinine for validating data about smoking. Further, data of exposure to ETS, body weight, gastro-oesophageal reflux, obstructive sleep apnoea and occupational dust exposure would have been valuable, since the presence of these factors may have changed between surveys, which in turn could have contributed to a change in both respiratory symptom and asthma prevalence. The cross-sectional study design weakens the discussion of causality. However, the strong and consistent association between smoking and respiratory symptoms in both surveys as well as the decrease of both smoking and bronchitis symptoms imply an improvement in respiratory health in the population at least partly due to the decrease in smoking.