[1] Univ Nottingham, Div Epidemiol & Publ Hlth, Nottingham, England
[2] Nottingham Univ Hosp, Dept Resp Med, Nottingham, England
[3] Cardiff Univ, Div Populat Med, Heath Pk, Cardiff, Wales
[4] UCL, Inst Epidemiol & Publ Hlth, Dept Behav Sci & Hlth, London, England