Flow diagram of the study selection process. Summary of inclusion and exclusion criteria used in this systematic review of studies investigating the association between environmental chemicals and DNA methylation levels, 10 April 2014. *17 references include the following studies with multiple environmental toxicants evaluated in unique study populations: Hanna et al. (2012)  examined in SMART population urine cadmium, blood lead and mercury, and serum BPA. Tajuddin et al. (2013)  examined in EPICURO population toenail cadmium, nickel, and lead. Tellez-Plaza et al. (2014)  examined in the SHS populations urine tungsten, antimony, and cadmium. Abbreviations: BPA, bisphenol A; PCF, perfluorinated compounds.