The primary DBPs of chlorine dioxide are chloride, chlorite, and chlorate. A provisional GV was recommended for chiorite, while no adequate data were available to recommend a GV for chlorate. No GV was recommended for chlorine dioxide per se because of its rapid breakdown in aqueous solutions. In addition, the chlorite GV is adequately protective for potential toxicity from chlorine dioxide. Furthermore, the taste and odor threshold for chlorine dioxide in water is 0.4 mg/L which constitutes a limiting factor and a signal for its presence at higher concentrations in drinking water.

Other reaction by-products of chlorine dioxide with organics in drinking water have not been well characterized. They include aldehydes, carboxylic acids, haloacids, chlorophenols, quinones, and benzoquinone (Bull and Kopfler 1991). In a recent article, more than 40 organic DBPs were identified in a pilot plant in Indiana that uses chlorine dioxide as a primary disinfectant. The toxicity of these by-products is largely unknown (Richardson et al. 1994).

In a 90-day study in rats at doses of 0, 1, 5, 10, 25, or 50 mg/kg bw/day chlorite in drinking water, the NOAEL was 1 mglkg bw/day, based on decreases in red blood cell glutathione levels. Using an uncertainty factor of 100 and allocating 80% of the TDI to drinking water results in a provisional GV of 0.2 mg/L. This provisional GV is supported by a two-year study in rats and a 12-week clinical study in a small number of human volunteers. IARC has concluded that there is inadequate evidence for the carcinogenicity of chlorite in experimental animals, and no data are available on its carcinogenicity in humans (Group 3).

The GV was designated as provisional because use of chlorine dioxide as a disinfectant may result in the chlorite GV being exceeded, and difficulties in meeting the GV must never be a reason for compromising disinfection.