We obtained all publicly available data from the NHANES website (http://www.cdc.gov/nchs/nhanes.htm). Following the two-stage design as in genome-wide association studies [33], we selected three NHANES cycles, 1999–2000, 2001–2002, and 2005–2006 as stage 1 samples and NHANES 2003–2004 as stage 2 samples, because not all measures of environmental pollutants are available in all cycles and the 2003–2004 cycle had the largest number of shared pollutants. We restricted the sample to adults aged 20 years or older and did not include children in this study.
