1. Medical DivisionAkershus University HospitalLørenskogNorway
2. Faculty of MedicineUniversity of OsloOsloNorway
3. Division of Respiratory Disease StudiesNational Institute for Occupational Safety and HealthMorgantownWest Virginia
4. Department of Occupational Health SurveilanceNational Institute of Occupational HealthOsloNorway
5. Department of Respiratory MedicineRikshospitaletOslo University HospitalOsloNorway