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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?

Andrew Grimm
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cham
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  • I've added in a couple of commonly seen sources for this (apparently a little extreme but also apparently relatively common) claim. @cham this is the sort of thing that will help get a better reception for questions like this here. Feel free to modify if I haven't kept the spirit of your question. – matt_black Jun 13 '18 at 13:30
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    Both added links are from sites selling supplements; not notable claims IMO –  Jun 13 '18 at 13:40
  • @JanDoggen: The consumer protection angle of skepticism is a key one. We tackle marketing claims all the time. If these are widely believed claims, they are worth tackling. – Oddthinking Jun 13 '18 at 14:17
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    While this question has been improved, it still suffers from being too broad: See more here about ["harmful" foodstuffs](https://skeptics.meta.stackexchange.com/questions/3480/is-is-this-foodstuff-harmful-harmful). Perhaps we can limit to one claim - e.g. the cancer question. – Oddthinking Jun 13 '18 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 Jun 13 '18 at 14:23
  • @Oddthinking The trouble with ruling out this specific question is that you will end up with a whole bunch of questions that are too specific and also too repetitive. The benefits of water chlorination are always the same but the *claimed* harms are many: every answer will have to repeat the benefits side to be a good answer. And we are not arguing about a vague *dose* which is one of the issues that normally makes these questions too vague: chlorine in domestic water is a *specific* dose. I think it would be better to keep this as one question. – matt_black Jun 13 '18 at 15:05
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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 Jun 13 '18 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 Jun 13 '18 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

Jon.G
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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 Jun 13 '18 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 Jun 13 '18 at 14:27