Report No FR0375

G O'Neill

Mar 1993



This report provides water utilities with important information on the possible health risk to consumers of MX, a highly active mutagenic by-product of drinking water disinfection.


To review the current toxicological data on MX and to present the results of a study to determine the disposition of MX in mice. To use these data to determine whether MX in chlorinated drinking water poses a significant health risk to consumers.


Concern about the possible link between drinking water chlorination and cancer has led to increased concern about individual by-products. One of these, M)z has been shown to interact with DNA in short-term bacterial and mammalian cell tests done in culture, and has been estimated to contribute up to 60$0 of the mutagenic activity detected in drinking water. Practical investigations into the uptake and distribution of MX in mice, and a review of available toxicological data from other studies, have been carried out in order to better assess the potential health risks posed by MX in drinking water.


  1. Studies of the disposition of MX in mice do not support previous associations between MX and cancer in the bladder and bowel.

  2. In mice, MX is associated with the stomach wall but is cleared within 24 hours. In other studies, nuclear anomalies have been observed in gut cells but only at very high doses.

  3. All indications are that there is limited risk to drinking water consumers posed by the presence of MX.


Although MX has been shown to interact with DNA in short-term bacterial tests and mammalian cell tests done in culture, the evidence for DNA damage in intact animals is limited. The only positive results have been confined to effects on gastrointestinal cells, following application of very high doses. Studies on the behaviour of radiolabelled MX given orally to mice indicate that it is rapidly absorbed, widely distributed and rapidly eliminated (mostly in the urine). MX was not found to any significant extent in the bowel or bladder, weakening any proposed associations with cancer in these tissues. However, high concentrations of radioactivity observed in the stomach are associated with the stomach wall itself and not simply with the stomach contents. This could explain the DNA damage seen in the stomach cells of mice and rats following administration of high doses of MX. Nevertheless, these effects have only been seen at very high doses, approaching the LD50, and it would therefore appear that the risk to consumers from MX in drinking water is limited.

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