Water Disinfectant Turns Medical Compounds Toxic

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DYEING TO SEE Contrast media, some containing iodine, enable X-ray images of soft tissues, such as the kidneys and bladder.

A group of benign iodinated chemicals used for medical tests forms highly toxic compounds when they go through drinking water disinfection, according to a new study (Environ. Sci. Technol., DOI: 10.1021/es200983f). The research solves the mystery of the presence of the disinfection by-products in drinking water in areas in which there is no natural source of iodine.

Iodide naturally occurs in drinking water supplies when the fresh water comes in contact with salt water or with underground deposits of salts. In drinking water treatment plants, this iodide can react with chlorine or chloramine, two disinfectant chemicals, to form iodo-acid and iodo-trihalomethane by-products. Researchers have previously shown that both families of by-products kill living cells and damage genes; they are among the most toxic disinfection by-products known.

Susan D. Richardson, a chemist with the Environmental Protection Agency’s National Exposure Research Laboratory, in Athens, Ga., and her colleagues first found iodo-acids during a 2000-2002 survey of drinking water disinfection by-products (Environ. Sci. Technol., DOI: 10.1021/es060353j). In that study, the researchers were puzzled to find iodinated by-products in some drinking water sources that didn’t have an iodide source.

Environ. Sci. Technol.

Iopamidol is one of several medical contrast agents that can react with disinfectants.

But after seeing a 2007 presentation about the structures of iodine-containing dyes used in medical tests, she wondered whether those compounds could be the iodide source. People receive injections of these dyes to make soft tissues visible on X-ray images. Iodinated X-ray contrast media are inert – people excrete them unchanged, and they pass through wastewater treatment plant filters untouched.

So she and her colleagues looked for the contrast media in the drinking water of 10 U.S. cities (which the researchers won’t identify). They found four different iodinated contrast agents in six of the 10 cities’ water samples, including samples from four cities without a natural iodide source. Richardson points out that natural iodide is still a more significant source for the toxic by-products because natural iodide occurs in much higher concentrations than do the contrast agents.

The researchers also combined the cities’ untreated water, iodinated contrast agents, and chlorine or chloramine, as would happen in a drinking water plant. They confirmed that the chemicals reacted to produce iodo-acids and iodo-trihalomethanes. The resulting aqueous mixtures had a cell-killing effect 4.5 to 7 times greater than did a mixture of the contrast agents and water alone. The researchers also found that the toxic compounds didn’t form when any one of the three ingredients was absent.

Still, the concentrations of iodinated by-products in drinking water are very low. In a 2008 study of their incidence in treated drinking water from 23 North American cities, her team found that the levels were far lower than those that the Environmental Protection Agency allows for halogenated disinfection by-products (Environ. Sci. Technol., DOI: 10.1021/es801169k).

The new research suggests that measuring the toxicity of a chemical in water is not sufficient, says David Sedlak, a professor of civil and environmental engineering at the University of California, Berkeley, who served as the journal editor for the paper. “This will lead to more research on the disinfection by-products, and undoubtedly we’re going to find other man-made chemicals that react with disinfectants to form toxic by-products.”

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