[1] Univ London Imperial Coll Sci Technol & Med, Natl Heart & Lung Inst, Dept Occupat & Environm Med, London SW3 6LR, England
[2] Royal Brompton Hosp, Dept Paediat Resp Med, London SW3 6LY, England
[3] Univ London Imperial Coll Sci Technol & Med, London SW3 6LR, England