Iodinated chemicals safely used as contrast agents for medical imaging of soft tissues form toxic compounds when they go through drinking water disinfection at treatment plants, according to a study (Environ. Sci. Technol., DOI: 10.1021/es200983f). The research explains the presence of iodine-containing disinfection by-products in drinking water where there is no known natural source of iodide. In drinking water treatment plants, iodide can react with chlorine or chloramine, two common disinfectant chemicals, to form iodoacids and iodotrihalomethane by-products, which are among the most toxic disinfection by-products known. Looking for an iodide source, Susan D. Richardson of EPA’s National Exposure Research Laboratory, in Athens, Ga., and colleagues discovered low concentrations of iodine-containing medical contrast agents such as iopamidol in the drinking water of six of 10 U.S. cities they sampled. The researchers also combined the cities’ untreated water, iodinated contrast agents, and chlorine or chloramine, as would happen in a water treatment plant. They confirmed that the chemicals reacted to produce iodoacids and iodotrihalomethanes. The toxic compounds didn’t form when any one of the three ingredients was absent.