The change in the mutagenicity of water treated with chlorine dioxide was compared with that of chlorinated water to estimate the mutagenicity of drinking water in distribution systems. We carried out chromosomal aberration tests using Chinese hamster lung (CHL) cells to evaluate the mutagenicity. First, the levels of chloroform and TOX produced by chlorine dioxidation were approximately 1% and 5–7%, respectively, of those produced by chlorination. However, in water treated with chlorine dioxide, the activity that induced chromosomal aberrations was stronger than would be expected based on the quantity of by-products. The observed decreasing rate constant of the activity inducing chromosomal aberrations in chlorinated water was 1.4 to 1.9 times greater than that of water treated with chlorine dioxide, indicating that the mutagenicity of water treated with chlorine dioxide is more stable than that of chlorinated water. The mutagenicity of drinking water treated with chlorine dioxide was estimated to be 70–80% of that of chlorinated drinking water. However, these differences in mutagenicity are reduced when drinking water remains in distribution systems for long periods. The use of chlorine dioxide instead of chlorine can drastically reduce the production of trihalomethanes (THMs). However, the results of this study demonstrate that chlorine dioxide does not have much advantage in terms of the mutagenicity of drinking water. There were no disinfection by-products that demonstrated similar tendencies of change compared to the changes in the activity that induced chromosomal aberrations.

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