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This page Bleach claims

Bleach is more effective at killing germs when diluted than when used straight out of the bottle.

this other page (Spanish) Alcohol and Bleach are more effective against Coronavirus diluted claims the same

Disinfectants like bleach and alcohol alter the virus structure and prevent it from infecting us. Their effects will depend in the right concentration

But this other page, Does liquid bleach has to be diluted to be effective as a virus killer?

claims

So, all things being equal, undiluted bleach is more effective at killing stubborn bacteria.

I know, he says bacteria and not virus but,

The data of viruses is a little more straight forward � viruses are simply not cut out to defend against the bleach onslaught! A group of scientists in New York studied the effects of very dilute concentrations of bleach on the respiratory syncytial virus (RSV). A tenfold dilution of bleach, which subsequently mixed with an equal volume of RSV-containing medium (so in fact a twentyfold dilution) eradicated all of the virus. A 100-fold dilution of bleach killed 100% of the virus half of the time, and decreased the number of live viral particles by greater than three logs in the other half of the tests. This was all after five minutes of treatment.

So at least for viruses, you can probably dilute the bleach tenfold without worrying too much about decreased antimicrobial activity.

Personally, I don't find any logic why bleach would be more effective diluted, the opposite makes more sense to me. Is there any explanation for this to be so? Or it's just a wrong idea?

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    Interestingly, I think ethanol and isopropyl alcohol are more effective when diluted (IIRC due to some polar vs. non-polar solvent effects), but that would be a topic for another question. – jpa Mar 27 at 13:00
  • Note: Not all bleach is sold at the same concentration. It is important to understand the concentrations which were reached in each paper/test and what you are starting with, rather than just assuming you should be using a "10-fold", "20-fold", "100-fold", or "1000-fold" dilution, because that may or may not reach anywhere close to the same concentration as was tested. – Makyen Mar 27 at 16:13
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The text does not say that more diluted bleach is more effective at killing viruses (or bacteria, or yeast, for that matter).

I'm quoting:

...A tenfold dilution of bleach, which subsequently mixed with an equal volume of RSV-containing medium (so in fact a twentyfold dilution) eradicated all of the virus. A 100-fold dilution of bleach killed 100% of the virus half of the time, and decreased the number of live viral particles by greater than three logs in the other half of the tests. This was all after five minutes of treatment...

So a 20-fold dilution kills all virus always; a 100-fold dilution kills 100% of the virus 50% of the time and kills most of them in the other cases.

I think form this we can conclude that more highly concentrated bleach is more deadly to viruses than more diluted bleach.

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  • But this paragraph is contradictory with what's stated in the other 2 articles, which includes government resources, that's why I'm asking – Pablo Mar 27 at 13:04
  • Only the first link claims that undiluted bleach is more effective. You are right that it comes from a research institute, but I couldn't find any citation for this claim on their website. – PejoPhylo Mar 27 at 13:57
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Household bleach is usually at 5% concentration of sodium hypochlorite. Data from disinfection of other coronaviruses indicates that dilutions of 1:100 are still effective. Higher concentrations of bleach are difficult to manage as they release chlorine gas and are irritant to the mucous membranes, lungs and eyes.

The bleach works as a potent oxidizer of the viral capsule and its contents. The oxidation relies on the amount of free chlorine molecules available which is why bleach solutions need to be made up and used in 24 hours as potency drops with time. Normal contact time is 30 seconds to 10 minutes for most microorganisms.

So, this clearly means the higher the concentration the greater the efficacy, but also the greater the risk to the user on account of off gassing of chlorine gas.

Alcohol is different. It requires water to enter the virus so that is why more than 95% concentrations are not recommended. But chlorine just needs to oxidize the outside of the virus to disrupt the viral capsule, and then that opens up the virus RNA which can then be oxidized.

https://www.ncbi.nlm.nih.gov/books/NBK214356/

Use of 10% Ultra Chlorox regular bleach mixed with liquids containing virus to give 5% final concentration to treat coronavirus waste.

https://cen.acs.org/biological-chemistry/infectious-disease/How-we-know-disinfectants-should-kill-the-COVID-19-coronavirus/98/web/2020/03

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  • 3
    This doesn't answer the question at all – PejoPhylo Mar 27 at 6:57
  • so does bleach have to be diluted to be more effective to kill virus? – PejoPhylo Mar 27 at 9:17
  • Answer says relies on amount of free chlorine molecules – Graham Chiu Mar 27 at 10:24
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My short answer is, "I don't exactly know," but maybe the discussion will still be interesting and a step in the right direction?

Also, it has been a while since I looked into it and I don't remember the citations, so it might be worthwhile to follow up elsewhere.

My understanding is that lowering the pH of a bleach solution converts the chlorite ion to hypochlorite ion, which is a more effective disinfectant, all else being equal. Diluting the stock bleach also dilutes the added sodium hydroxide (added so as to convert the bleach to the more shelf-stable chlorite form), lowering the pH and converting more of the available to chlorite to hypochlorite.

But, while chlorite seems to be less effective, it doesn't seem to be wholly ineffective, so is there a point where a greater amount of a less effective substance equals or outweighs a lesser amount of a more effective substance? That's where I wasn't really coming up with a good answer. Research results seem to conflict pretty dramatically depending on how they were testing efficacy, and a lot of available research seems predicated on the assumption that you are going to follow the existing recommendations.

In general, dilute bleach seems sufficiently effective, and it has the advantages of increased economy (unless you're buying the smallest available amount and not using it up before it expires), diminished risk of damaging the item you are cleaning, and decreased amounts of chemical irritant in the air (so perhaps a reduced risk of developing COPD or something if you do a lot of disinfection).

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  • Welcome to Medical Sciences.SE This seems to be more of a discussion than an answer. Please take the tour and read the help center to get a better idea of how this site works. Also be aware that answers here are required to provide supporting references for any claims made. – Carey Gregory Jun 15 at 21:28

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