1. Department of Allergology, University Hospital Groningen, PO Box 30.001, 9700 RB Groningen, The Netherlands
2. Department of Epidemiology, University Hospital Groningen, PO Box 30.001, 9700 RB Groningen, The Netherlands
3. Imperial College School of Medicine, At the National Heart and Lung Institute, Dovehouse Street, London S3W 6LY, UK
4. Johns Hopkins Asthma and Allergy Center, 5501 Hopkins Bayview Circle, Baltimore, MD 21224, USA
5. Allergy Centre, Sahlgrenska University Hospital, 413 45 Göteborg, Sweden
6. Department of Respiratory and Environmental Epidemiology, Channing Laboratory, Brigham and Women's Hospital, 181 Longwood Avenue, Boston, MA 02115, USA