[1] Univ London Imperial Coll Sci Technol & Med, NHLI, Dept Occupat & Environm Med, London SW3 6LR, England
[2] Sahlgrens Univ Hosp, Dept Internal Med, SE-41266 Gothenburg, Sweden
[3] Univ London Imperial Coll Sci Technol & Med, Dept Environm Sci & Technol, London SW7 2BP, England