Recently it came out, that in some parts of Switzerland (where I live), there is Chlorothalonil & metabolites/breakdown-products of it in the drinking water. Yikes.

Problem is, chlorothalonil synthesis frequently result in contamination of it with small amounts of hexachlorobenzene (HCB), which is toxic. Worse, one of the metabolites/breakdown-products is 2,3,7,8-Tetrachlorodibenzodioxin, which is one of the most potent carcinogens known.

Now, according to official sources, the drinking water in my region, which exceeded the maximum permissible value by law (and those are not necessarely low, e.g. the nitrates permissible values is far too high), has been diluted with lake water from Lake Constance, and according to the official narrative, all is fine now. That's what the same official sources said before, too - until someone discovered that this wasn't the case. However, Syngenta, noble originater of the problem-causing fungicide(s), is moving in courts to successfully forbid obligatory measurements, because "it has not yet conclusively been proven that Chlorothalonil causes cancer". That's bullshit, but the scientific proof is indeed still owing, unfortunately, yet. Unfortunately, that little detail also means their court-work sabotage is successful. However, the correlation is clear, and I'm sure it will be proven in the not so distant future, sooner or later (but too late for the courts). Syngenta's behaviour equalling the reprehensible ethics of the tabacco industry.

Now, me being a distrustful person, I have bought a reverse-osmosis table-appliance, to be sure with my cooking and drinking water, just in case this is a lie. I have read that this filters the chemicals in question and more (or at least it should), and is also good for decalcification, which is an additional benefit.

However, assuming the reverse-osmosis-appliance itself is safe, not to mention it working as advertized, there remains the problem of showering and bathing water.

Is that even relevant for concentrations of Chlorothalonil breakdown products of <= 0.1 microgramms per liter, or is the main danger from oral consumption ?
For absorption by skin, can such concentrations be harmful ?

lab blog for reference

In January 2020, Syngenta Agro AG appealed to the Federal Administrative Court against the ban on chlorothalonil. The proceedings are still pending, but two interim rulings have already been issued in which Syngenta Agro AG's applications for precautionary measures have been approved. In the second interim ruling, the FSVO is required not to designate four metabolites of chlorothalonil, including R417888 and R471811, as toxicologically relevant for the time being (this would once again result in a high maximum level of 10 µg / l). According to a media release from the Federal Administrative Court of February 18, 2021, it will only be necessary to assess in the main decision which classification with regard to carcinogenicity (category 2 or 1B) is to be assumed and whether all metabolites are automatically considered relevant with a possible classification in category B1, and which maximum drinking water levels (10 µg / l or 0.1 µg / l) are applicable.

Due to the pending proceedings, Labor Veritas AG has decided to suspend the conformity assessment of water samples in accordance with the TBDV in connection with the chlorothalonil metabolites until the legal situation has been clarified. The following note therefore appears on our test reports: "Due to the currently unclear legal situation with regard to the applicable maximum values for the metabolites of chlorothalonil, the samples are no longer evaluated for conformity". In our Spotlight No. 34 from May 2020 , all metabolites of chlorothalonil are described as relevant due to the initial situation at the time. This statement should be treated with caution for the time being. This spotlight will not be updated for the time being, especially because the statements made in it could become valid again.

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    $\begingroup$ Note that medical/health advices are explicitly off-topic on this site // There is many funny but true evidence even strong correlation does not automatically mean causality. // There are still high level debates and you want us to say the verdict. You will not get it // Generally, questionable concentration levels for drinking/cooking are safe for showering/bathing, with exception of microbiology and radon // BTW what about microbiology and ion composition of the output of the RO unit? $\endgroup$
    – Poutnik
    Jul 21 at 5:17
  • $\begingroup$ @Poutnik: As always, correlation is a necessary condition for causation, but not a sufficient condition. But it also couldn't be a sufficient condition if there were no correlation. Certainly exactly the point of one side of the debate. Generally, if you don't store the water, but use it immediately, the microbiology shouldn't be a problem, as long as you change the filters in the necesary interval as described in product maintenance. The water is deionized/demineralized, which is why it is being remineralized, requiring mineral cartridges in the RO unit. Fair point, nobody really knows, yet. $\endgroup$ Jul 21 at 9:22
  • $\begingroup$ correlation is a necessary condition for causation, but not a sufficient condition. But it also couldn't be a sufficient condition if there were no correlation. Sure, that is the same by other words. The world has enough struggles already with correlation/causality in COVID vaccination. // It can still be replecement of evil by devil, as you do not have operational feedback about water quality, unless you pay regularly for it big money. // I would guess RO is not needed unless required for other reasons. Regularly replaced active-C based filterer in flow through devices should be enough, $\endgroup$
    – Poutnik
    Jul 21 at 9:29
  • $\begingroup$ @Poutnik: A TDS-detector is monitoring water quality at the inlet and outlet of the RO appliance. According to sciencedirect.com/science/article/pii/S0043135420306035 active-C filters have difficulty with TP R471811. Don't know how relevant that is for small-time use. Also, active-C doesn't filter nitrates, and after having seen the legally allowed values for nitrates, i also think filtering them would be a good idea. Because legally allowed or not, values that are well in the range to cause cancer are pointless BS. Also, the RO has an active-C filter 4 VOCs: B0817DCXG5 $\endgroup$ Jul 21 at 11:06
  • $\begingroup$ TDS is too simple check to say all is OK, but it can serve as indicative parameter for maintenance or emergency shutdown. // Well, there is no lower limit under which alcohol does not cause cancer either. Many areas of life work with "acceptable risk level" for potential risks. Nitrates in low scale can be eliminated easily by RO or ionexes, but that is not always the case for other risks, all aspects considered. $\endgroup$
    – Poutnik
    Jul 21 at 11:24