1. Division of Population Medicine, Cardiff University , Heath Park, Cardiff , UK
2. Division of Epidemiology and Public Health Clinical Sciences, Nottingham City Hospital, University of Nottingham , Nottingham , UK
3. Marie Curie Research Centre, Cardiff University , Cardiff , UK
4. School of Social Sciences, Cardiff University , 1-3 Museum Place, Cardiff , UK
5. Department of Respiratory Medicine, Leeds Teaching Hospitals NHS Trust , Leeds , UK