[1] Imperial Coll, Natl Heart & Lung Inst, Resp Epidemiol Occupat Med & Publ Hlth, Emmanuel Kaye Bldg,1B Manresa Rd, London SW3 6LR, England
[2] MRC PHE Ctr Environm & Hlth, London, England
[3] St Georges Univ London, Populat Hlth Res Inst, London, England
[4] Univ Bergen, Dept Clin Sci, Bergen, Norway
[5] Haukeland Hosp, Dept Gynecol & Obstet, Bergen, Norway