Project methodologyIn the baseline cohort of The Telemark study (2013), we have collected data from 16099 participants. A nested case-control study that included 1318 persons was performed in 2014/15. Four PhD-candidates are currently involved in these projects (Fig. 1).The five-year follow-up in 2018 was the first of three planed follow-ups of the baseline cohort. It will provide us with new and updated knowledge regarding the incidence and causes of respiratory disease from an area, which includes both rural and urban areas ensuring exposure contrasts. A full-time principal investigator/project manager, a project nurse, a statistician, and a post-doc student are also working on the project. Together with a large national and international network of collaborators, they will assure feasibility and project implementation.Project arrangements, method selection and analysesThe Telemark study is a prospective study designed to identify causes and risk factors for obstructive lung disease [17]. The study includes 16 099 subjects who were between 16 and 50 years at baseline. For the whole cohort we have register based data regarding, age, gender, and residential address. All participants enrolled at baseline were invited to participate in the 5-year follow-up. We used the services of the Norwegian National Inhabitant Registry (Folkeregisteret) to identify additional subjects (Approximately 24 000) in the same age group (born 1963-97). This was done because it is well known from similar prospective studies that a substantial proportion is lost to follow-up. We will continue to use validated, self-administered questionnaires for registration of respiratory symptoms, comorbidity, other possible confounders and exposure at follow-up. Two reminders will be mailed to increase the response rate. The Dep. of Occupational and Environmental Medicine at Telemark Hospital has the scanning equipment needed for complex questionnaires and trained personnel with experience from collection andanalysis of large regional surveys (The South-Eastern Health Authority employee survey N=55 000). The regional ethics committee has approved the use of register data (details below). Further, comprehensive and unique JEM-data for the participant’s occupational exposure are available. The N-JEM is based on the International Classification of Occupations 88 (ISCO-88) codes of occupation and defines an exposed occupation on the basis of assumption that at least half of the participants with the specific code should have a high probability of being exposed to the critical agents. The N-JEM includes six main exposure groups: high-molecular-weight (HMW) agents, low-molecular-weight agents, irritating agents, accidental peak exposure to irritants, uncertain or low exposure, and an unexposed reference group in addition to occupational codes [22]. Non-response was assessed at baseline and few differences were found in the prevalence of respiratory symptoms and asthma [23]. Non-response will be assessed again at follow-up.