1 Department of Pulmonary Diseases, University of Groningen, University Medical Centre Groningen, PO Box 30001, 9700 RB, Groningen, The Netherlands. j.hartman@umcg.nl.
2 Groningen Research Institute for Asthma and COPD, University of Groningen, University Medical Centre Groningen, Groningen, The Netherlands. j.hartman@umcg.nl.
3 Department of Pulmonary Diseases, University of Groningen, University Medical Centre Groningen, PO Box 30001, 9700 RB, Groningen, The Netherlands.
4 Groningen Research Institute for Asthma and COPD, University of Groningen, University Medical Centre Groningen, Groningen, The Netherlands.