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Some sources claim chlorinated tap water is harmful.

  • Global Healing Center, for example, claims links to cancer, heart disease, asthma and atherosclerosis.

    The risk of developing cancer is 93% higher in people who drink or are otherwise exposed to chlorinated water

  • This Your Health Keys article has the headline:

    3 Ways Chlorine in Drinking Water is Harming Your Health

    and claims chlorine cause weight gain, is an oestrogen mimic and kills good gut bacteria.

While these are not the most credible of sources, the idea that the chlorine in domestic water supplies is harmful appears to be widespread. What do reputable sources say: is chlorine in water harmful or not?

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    Both added links are from sites selling supplements; not notable claims IMO
    – user22865
    Commented Jun 13, 2018 at 13:40
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    While this question has been improved, it still suffers from being too broad: See more here about "harmful" foodstuffs. Perhaps we can limit to one claim - e.g. the cancer question.
    – Oddthinking ♦
    Commented Jun 13, 2018 at 14:21
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    (The cancer question is kind of boring, because it is a correlation claim, not a causation claim. It is far less interesting to say "People who live in cities with complex sanitation infrastructure see more diseases related to old age and obesity than people who don't have access to such resources.")
    – Oddthinking ♦
    Commented Jun 13, 2018 at 14:23
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    @matt_black: The counter-argument is that if I can show that chlorine (at the specified doses) does NOT cause bladder cancer, weight gain and AIDS, but I have no evidence about whether it causes diabetes, itchy scalps and forgetting where you put your keys, I can't post a definitive answer to the Gish gallop in the claim. I don't think there can every be an answer that shows "Chlorine is not harmful in any conceivable way", but there can be plenty "Chlorine does not cause X." answers.
    – Oddthinking ♦
    Commented Jun 13, 2018 at 16:53
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    I wonder how much a higher incidence of heart disease, cancer, etc would be due to not being killed off by water-borne pathogens, allowing someone to survive long enough to be afflicted with those other issues.
    – PoloHoleSet
    Commented Jun 13, 2018 at 19:35

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Quoted from this article on chlorine in water

The EPA warns that levels that exceed the regulated amounts will be harmful. The agency’s conventions state that the allowable chlorine levels in drinking water may be up to 4 parts per million. This stipulation has a wide margin of error, so if higher levels of chlorine are mistakenly added, most drinkers will still be safe while action is taken to correct the error.

So the environmental protection agency states we should not exceed 4 parts per million, Link to the full report on contaminants in drinking water. The article you posted is about chlorine levels being lowered from 1 to 0.5 parts per million. So they would have to increase the old concentration by 4x or the new concentration by 8x to be at risk.

Assuming other towns in the US work on a similar concentrations (The uk stays below 1ppm) these levels are not generally harmful to humans.

Doing some more digging I would say that there does seem to be a correlation between consumption of chlorinated water and bladder cancer but claims of other adverse effects are not that strong.WHO paper, Study on Carcinogenic Potential , Study on bladder cancer, Study on delivery . These studies also point to adverse effects not being present when using Chlorine dioxide which is claimed to be in widespread use but is lacking citations on wikipedia.

It's important to remember water chlorination has been vital in combating waterborne diseases. I can't find figures of deaths before water chlorination became common in modern culture but even now waterborne disease account for 3.4 million deaths annually in places with unclean water, things like typhoid, polio, cholera, malaria

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    While I tend to trust regulators standards, I can't help thinking that to solidly refute this idea you might have to also show some of the supporting evidence for them. Or even show something about the harms of not having chlorine in the water (given that contagion-free water might have delivered the biggest improvement in public health in the last century or possibly the last millennium).
    – matt_black
    Commented Jun 13, 2018 at 13:33
  • When you say "links" to bladder cancer do you mean correlation or causation? The deaths figure is really important while the question remains vaguely about "harmful", because if chlorination kills one in a million people due to bladder cancer, but saves 100 in a million, calling it "harmful" is misleading.
    – Oddthinking ♦
    Commented Jun 13, 2018 at 14:27

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