1. Centre for Research in Environmental Epidemiology (CREAL); Barcelona Spain
2. IMIM (Hospital del Mar Medical Research Institute); Barcelona Spain
3. CIBER Epidemiología y Salud Pública (CIBERESP); Barcelona Spain
4. Department of Biostatistics; Harvard School of Public Health; Boston MA 02115 U.S.A.
5. Department of Epidemiology; Harvard School of Public Health; Boston MA 02115 U.S.A.