Perchlorate is an environmental contaminant associated with the manufacture or use of aerospace fuels, explosives, and hypochlorite water disinfection products. Perchlorate inhibits iodide transport into the thyroid gland, which may reduce the synthesis of thyroid hormones critical to normal neurologic development. Exposure occurs in utero and through intake of contaminated water, food, and breast milk. In addition, iodide may be reduced in contaminated breast milk, particularly when mothers are iodine deficient. Zewdie et al. review the information and methods used by the Massachusetts Department of Environmental Protection (MA DEP) to derive the MA perchlorate reference dose (RfD) and drinking water standard (DWS).