[1] Univ Cincinnati, Coll Med, Dept Environm Hlth, Div Epidemiol, POB 670056, Cincinnati, OH 45267 USA
[2] Brown Univ, Sch Publ Hlth, Dept Epidemiol, Providence, RI 02912 USA
[3] Ctr Dis Control & Prevent, Div Sci Lab, Natl Ctr Environm Hlth, Atlanta, GA USA
[4] Simon Fraser Univ, BC Childrens Hosp Res Inst, Vancouver, BC, Canada
[5] Cincinnati Childrens Hosp Med Ctr, Dept Pediat, Div Gen & Community Pediat, Cincinnati, OH 45229 USA