1. From the Department of Respiratory Medicine, Bispebjerg Hospital, Copenhagen, Denmark,
2. Department of Cardiology and Respiratory Medicine, Hvidovre Hospital, Copenhagen, Denmark,
3. The Danish Twin Registry, University of Southern Denmark, Odense, Denmark,
4. Department of Occupational and Environmental Medicine, Haderslev Hospital, Haderslev, Denmark, and
5. Department of Respiratory Medicine, Gentofte Hospital, Copenhagen, Denmark