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17.2F: Chlorination and the Law of Unintended Consequences

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    Chlorination is the adding of chlorine to water in order to kill any dangerous bacteria that might be present. Most municipal water supplies are chlorinated with chlorine gas, Cl2. Swimming pools, hot tubs, and the like are usually chlorinated with chlorine-containing substances like

    • calcium hypochlorite, Ca(HClO)2
    • sodium hypochlorite, NaHClO (bleach)
    • trichloro-s-triazinetrione

    In every case, the effectiveness of chlorination as a germicide is a result of chlorine's powerful oxidizing action. The widespread chlorination of municipal water supplies has been one of the public health triumphs of the past century. However, chlorine also reacts with any organic matter present in the water. Among the products formed are chloroform and a variety of other trihalomethanes (THMs). Although these substances are normally present only in the range of parts per billion (ppb), they nevertheless have caused considerable anxiety because several of them are known or suspected carcinogens.

    The U.S. Environmental Protection Agency (EPA) sets a limit of <1 ppb of THMs in major water systems and an absolute limit of 100 ppb in any water system.


    • that laboratory animals respond the same as humans when fed these chemicals (they may not; when methylene chloride, a THM, is fed to mice, it increases their incidence of cancer, but it is not carcinogenic when fed to rats),
    • that we know how to scale up from doses in rats and mice to the equivalent dose in humans (there is still controversy about how best to do this),
    • that there is no threshold below which doses of THMs are safe and thus
    • that the concept of collective dose applies,

    the EPA estimates that if everyone in the U.S. drank water containing 100 ppb of THMs for their entire lives, their chance of developing cancer (currently about 25%, representing some 500,000 cancer deaths per year) would increase by some 700 cases per year.

    Turn off the chlorinators?

    Claiming that they were responding to the questions raised by the U.S. EPA over the safety of THMs, officials in Peru began, in the late 80s, shutting down some of the chlorinators in the capital city, Lima, as well as in other cities and towns.

    In January 1991, an outbreak of cholera began in several towns just north of Lima. Within weeks the epidemic of this dangerous disease (the first epidemic of cholera in the Western hemisphere in a century) spread throughout Peru and eventually through much of South and Central America. Once introduced into a city, town, or village, the disease spread rapidly through contaminated, but now unchlorinated, water supplies.

    By Dec. 31, 1992 — 23 months after the epidemic began, a total of 731,312 cases had been recorded with 6,323 deaths. Worst hit was Peru itself. Barely 10 months into the epidemic (Nov. 13, 1991), 2,720 of its people had died of cholera. With a population of 22 million, that works out to 140 deaths per million people. Even taking the EPA's gloomiest prediction, a lifetime of drinking water containing 100 ppb of THMs would increase the rate of cancer deaths each year in Peru by less than 3 deaths per million.

    What's to be done?

    After the appalling devastation caused in Central and South America by misguided risk analysis, one might have hoped that the choices would be clear.

    • Certainly don't suddenly stop disinfecting municipal water supplies!
    • Continue to explore alternatives to chlorination.
      • For example, many water systems in France and some in the U.S. use ozone as the disinfectant. However, this strong oxidant also interacts with organic matter to produce undesirable contaminants.
      • Adding ammonia (NH3) as well as chlorine to water produces chloramine, which is an effective disinfectant but has the disadvantage of producing carcinogenic nitrosamines and leaching lead from ancient water pipes.
      • Irradiation with ultraviolet light is least likely to produce unwanted contaminants.
    • Whatever method(s) used, treat the water to reduce the amount of organic matter in it.
    • Keep a cool head and try to evaluate the size of the risks involved before taking action.

    This page titled 17.2F: Chlorination and the Law of Unintended Consequences is shared under a CC BY 3.0 license and was authored, remixed, and/or curated by John W. Kimball via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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