[1] Univ Nottingham, Div Pulm Care, Nottingham NG7 2RD, England
[2] Univ W England, UK Ctr Child & Adolescent Hlth, Bristol BS6 6JS, Avon, England
[3] Univ British Columbia, Dept Pediat, Vancouver, BC V6H 3V4, Canada
[4] Child Accid Prevent Trust, London EC1R 3AJ, England
[5] Univ W England, Sch Hlth & Social Care, Bristol BS16 1DD, Avon, England