ObjectivesDeclining participation in epidemiological studies has been reported in recent decades and may lead to biased prevalence estimates and selection bias. The aim of the study was to identify possible causes and effects of non-response in a population-based study of respiratory health in Norway.DesignThe Telemark study is a longitudinal study that began with a cross-sectional survey in 2013.SettingIn 2013, a random sample of 50 000 inhabitants aged 16–50 years, living in Telemark county, received a validated postal questionnaire. The response rate was 33%. In this study, a random sample of 700 non-responders was contacted first by telephone and then by mail.Outcome measuresResponse rates, prevalence and OR of asthma and respiratory symptoms based on exposure to vapours, gas, dust or fumes (VGDF) and smoking. Causes of non-response.ResultsA total of 260 non-responders (37%) participated. Non-response was associated with younger age, male sex, living in a rural area and past smoking. The prevalence was similar for responders and non-responders for physician-diagnosed asthma and several respiratory symptoms. The prevalence of chronic cough and use of asthma medication was overestimated in the Telemark study, and adjusted prevalence estimates were 17.4% and 5%, respectively. Current smoking was identified as a risk factor for respiratory symptoms among responders and non-responders, while occupational VGDF exposure was a risk factor only among responders. The Breslow-Day test detected heterogeneity between productive cough and occupational VGDF exposure among responders.ConclusionsThe Telemark study provided valid estimates for physician-diagnosed asthma and several respiratory symptoms, while it was necessary to adjust prevalence estimates for chronic cough and use of asthma medication. Reminder letters had little effect on risk factor associations. Selection bias should be considered in future investigations of the relationship between respiratory outcomes and exposures.

ObjectivesThe aim of this study was to estimate the prevalence of respiratory symptoms and physician-diagnosed asthma and assess the impact of current occupational exposure.DesignCross-sectional analyses of the prevalence of self-reported respiratory health and association with current occupational exposure in a random sample of the general population in Telemark County, Norway.SettingsIn 2013, a self-administered questionnaire was mailed to a random sample of the general population, aged 16–50, in Telemark, Norway. The overall response rate was 33%, comprising 16 099 responders.Outcome measuresThe prevalence for respiratory symptoms and asthma, and OR of respiratory symptoms and asthma for occupational groups and exposures were calculated. Occupational exposures were assessed using self-reported exposure and an asthma-specific job-exposure matrix (JEM).ResultsThe prevalence of physician-diagnosed asthma was 11.5%. For the occupational groups, the category with agriculture/fishery workers and craft/related trade workers was associated with wheezing and asthma attack in the past 12 months, showing OR 1.3 (1.1 to 1.6) and 1.9 (1.2 to 2.8), respectively. The group including technicians and associated professionals was also associated with wheezing OR 1.2 (1.0 to 1.3) and asthma attack OR 1.4 (1.1 to 1.9). The JEM data show that exposure to flour was associated with wheezing OR 3.2 (1.4 to 7.3) and woken with dyspnoea OR 3.5 (1.3 to 9.5), whereas exposures to diisocyanates, welding/soldering fumes and exposure to vehicle/motor exhaust were associated with dyspnoea OR 2.9 (1.5 to 5.7), 3.2 (1.6 to 6.4) and 1.4 (1.0 to 1.8), respectively.ConclusionsThe observed prevalence of physician-diagnosed asthma was 11.5%. The ‘manual’ occupations were associated with respiratory symptoms. Occupational exposure to flour, diisocyanates, welding/soldering fumes and vehicle/motor exhaust was associated with respiratory symptoms in the past 12 months and use of asthma medication. However, prospective data are needed to confirm the observed associations.

This cross-sectional study of the general population of Telemark County, Norway, aimed to identify risk factors associated with poor asthma control as defined by the Asthma Control Test (ACT), and to determine the proportions of patients with poorly controlled asthma who had undergone spirometry, used asthma medication, or been examined by a pulmonary physician. In 2014-2015, the study recruited 326 subjects aged 16-50 years who had selfreported physician-diagnosed asthma and presence of respiratory symptoms during the previous 12 months. The clinical outcome measures were body mass index (BMI), forced vital capacity (FVC) and forced expiratory volume in one second (FEV 1), fractional exhaled nitric oxide (FeNO), immunoglobulin E (IgE) in serum and serum C-reactive protein (CRP). An ACT score � 19 was defined as poorly controlled asthma. Overall, 113 subjects (35%) reported poor asthma control. The odds ratios (ORs) and 95% confidence intervals (CIs) for factors associated with poorly controlled asthma were: self-reported occupational exposure to vapor, gas, dust, or fumes during the previous 12 months (OR 2.0; 95% CI 1.1-3.6), body mass index � 30 kg/m 2 (OR 2.2; 95% CI 1.2-4.1), female sex (OR 2.6; 95% CI 1.5-4.7), current smoking (OR 2.8; 95% CI 1.5-5.3), and past smoking (OR 2.3; 95% CI 1.3-4.0). Poor asthma control was also associated with reduced FEV 1 after bronchodilation (β-3.6; 95% CI-7.0 to-0.2). Moreover, 13% of the participants with poor asthma control reported no use of asthma medication, 51% had not been assessed by a pulmonary physician, and 20% had never undergone spirometry. Because these data are cross-sectional, further studies assessing possible risk factors in general and objectively measured occupational exposure in particular are needed. However, our results suggest that there is room for improvement with regards to use of spirometry and pulmonary physician referrals when a patient's asthma is inadequately controlled.