I am confused about the needed ratio of bleach to water required to disinfect COVID-19 from surfaces.

The guide on CDC states that: https://www.cdc.gov/coronavirus/2019-ncov/community/home/cleaning-disinfection.html

Diluted household bleach solutions can be used if appropriate for the surface. Follow manufacturer’s instructions for application and proper ventilation. Check to ensure the product is not past its expiration date. Never mix household bleach with ammonia or any other cleanser. Unexpired household bleach will be effective against coronaviruses when properly diluted.

Prepare a bleach solution by mixing:

5 tablespoons (1/3rd cup) bleach per gallon of water or

4 teaspoons bleach per quart of water

This is a 2% ratio. My problem is that it is not clear what "bleach" means. Chemically I guess it's Sodium hypochlorite. What's not clear is that what is the strength of the solution I can buy in shops.

Wikipedia states that bleach in stores can be anything between 3-25%.

Does CDC recommend the final strength of Sodium hypochlorite : water to be 2% or "bleach in stores" : water to be 2% (which can be anything between 0.06% - 0.5%

Disregarding this confusing CDC direction: What is the scientific recommendation for the required sodium hypochlorite : water ratio needed for disinfecting COVID-19 from surfaces?

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    $\begingroup$ Just a note: bleach can discolor surfaces... $\endgroup$ – Jan Mar 12 at 13:03
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    $\begingroup$ SE Biology is concerned with the mechanisms of biological processes, and questions are generally answered by people with biological rather than medical or epidemiological expertise. Although I understand your concern about the coronavirus outbreak, your question is not about the biology of the disease but about personal or public hygiene and therefore is off-topic. I advise you to consult more appropriate reputable sources of information, some of which are listed here. $\endgroup$ – David Mar 12 at 13:47

Perhaps I can help answer your microbiology question which is really about SARS-CoV-2 which is an enveloped mRNA virus. There is a lot of misunderstanding about SARS-CoV-2 and the medical disease it causes, COVID-19. The confusion is understandable as this is a novel coronavirus with a high rate of transmission & people around the globe are still in a learning curve about it. It's important to understand how to decimate SARS-CoV-2 whenever possible. I'm an RDN & years ago I went to grad school on an Allied Health Traineeship thanks to citizens of the U.S.A. Although now retired, as well as having been a clinician, I'm a former prof. I believe in giving back to society which has supported me.

Many pieces in the press unfortunately use terms of SARS-CoV-2 and then COVID-19 interchangeably, which they should not. This adds to public confusion. Keep in mind that disinfection in this instance pertains to decimation of the virus SARS-CoV-2 per se.

Cleaning should precede disinfection. Disinfection is biocidal elimination on fomite surfaces of microorganisms, in this instance SARS-CoV-2. After applying disinfecting dilution be sure to allow it adequate action time and then allow surface to air dry.

When CDC says to use “5 tablespoons (aka 1/3 cup) bleach per gallon of water” or “4 teaspoons bleach per quart of water” that is in effect 1000 ppm. CDC guidance assumes household bleach concentration to be 5-6.25%, even though most NaOCl manufacturers int the U.S.A. went to 8.25% back around 2012-2013. Check any bottle you have (or box it came in) for the NaOCl concentration for that batch of bleach. When first manufactured concentration is higher, but over time actual available free chlorine level drops and adequate potency passed by 1 year out from manufacturing date. If stored at higher room temperature, cut that down to 6 months which is more realistic. Adequate 1000 ppm of NaOCl for enough time (minimum 1 minute) can kill SARS-CoV-2. If the non-porous fomite surface happens to be a countertop where inadvertently later food might be placed on, then after first cleaning, then disinfecting, be sure to rinse with potable H2O, then use a sanitizing solution dilution and allow to air dry.

See: "COVID-19 – Disinfecting with Bleach" from Michigan State U Center for Research on Ingredient Safety which will explain how to read date code of manufacturing on a household bleach (NaOCl) bottle and how much bleach to add to cold water in order to reach what is effectively a 1000 ppm dilution if bleach is still ‘fresh enough’ aka adequate concentration in bottle as purchased. The bleach concentration bottle code information can be read as follows (quoted or paraphrased from article): Example: code E619337. ●First two characters E6 identify the company facility that manufactured the bleach.  ●Second two numbers 19 tell the year the company manufactured the bleach. ●Last three numbers 337 tell the day of the year the company manufactured the bleach. So, code E619337 tells us this bottle of bleach was manufactured at facility E6 in 2019 on the 337 day of the year (using a Julian calendar), which is December 3. This bottle of bleach technically expires one year from December 3, 2019, so it needs to be used or disposed of by December 2, 2020.  Similarly a product code A420027 tells us the product was manufactured at facility A4 in 2020 on the 27 day of the year, which is January 27. The product expires one year from January 27, 2020, so it needs to be used or disposed of by January 26, 2021.  Adjust the Julian calendar keeping in mind the year as 2020 is a leap year.  From experience using test strips with bleach concentrate dilutions, often that expiration timeline is closer to 6 months (commercial settings open bottle only 1 month). canr.msu.edu/news/covid-19-disinfecting-with-bleach

Public Health Ontario has an online "Chlorine Dilution Calculator" to help anyone determine how much NaOCl "household" bleach of a given concentration level to add to how much safe tap water to achieve a certain ppm of diluted chlorine sol'n. One can calculate sanitizing or even disinfecting diluted chlorine sol'n levels using the online calculator. Although the site is in Canada, they kindly include both English and Metric units. https://www.publichealthontario.ca/en/health-topics/environmental-occupational-health/water-quality/chlorine-dilution-calculator

Advisement of 1000 ppm for a NaOCl dilution based on publication: Kampf, G. et al. Persistence of coronaviruses on inanimate surfaces and their inactivation with biocidal agents. Journal of Hospital Infection, Volume 104, Issue 3, 246 - 251 (March 2020). https://doi.org/10.1016/j.jhin.2020.01.022 Fomite transmission (contact w/ inanimate or nonpathogenic object surface transmission exclusive of food components) reduction via biocidal agents such as 0.1%-0.5% NaOCl aka 1000-5000 ppm. “...Human coronaviruses on inanimate surfaces can be effectively inactivated by surface disinfection procedures with 62-71% ethanol, 0.5% hydrogen peroxide or 0.1% sodium hypochlorite within 1 minute.” from pre-print https://doi.org/10.1016/j.jhin.2020.01.022

Edit: The question is really about disinfecting for SARS-CoV-2, the mRNA virus which causes COVID-19 in humans. The edit is to: 1) Clarify the actual question, which deals w/ microbiology relating to SARS-CoV-2, not a medical disease condition COVID-19; 2) Acknowledge that the press has incorrectly used the two different terms interchangeably creating public confusion & explain why many professionals are responding in forums around the world trying to clear up that confusion by providing accurate information; 3) Include current full rather than shortened links to details of NaOCl disinfection of SARS-CoV-2 with enough detail that if links are later unavailable readers will still know what to do.

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  • $\begingroup$ Wow, thanks for this comment! I have changed the accepted answer to yours! $\endgroup$ – hyperknot Mar 30 at 1:14
  • $\begingroup$ I think Stackexchange doesn't like bit.ly links, it's better to write them out full or use the markdown editor. Hopefully someone will come over and fix the links. $\endgroup$ – hyperknot Mar 30 at 1:16
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    $\begingroup$ If anyone is having difficulty w/ bit.ly links, for COVID-19 – Disinfecting With Bleach just go to: canr.msu.edu/news/covid-19-disinfecting-with-bleach ; then for Public Health Ontario online "Chlorine Dilution Calculator" just go to: publichealthontario.ca/en/ServicesAndTools/Tools/Pages/… ; hope that helps. $\endgroup$ – Stephanie Mar 30 at 1:27
  • $\begingroup$ Welcome to Biology.SE! First, while this looks like a good answer it is to a question that is off-topic for this site (because it is a personal medical question — see help center) and it would be better to leave such questions unanswered. Second, for future posts please include the complete reference information since links can (and do) break. ——— You may also wish to take the tour and then consult the help pages for additional advice on How to Answer effectively on this site. Thank you! 😊 $\endgroup$ – tyersome Mar 30 at 22:43
  • $\begingroup$ In addition, comments are ephemeral on this site, so updates should be made by making an edit of your original post. $\endgroup$ – tyersome Mar 30 at 22:47

I have also been frustrated by use of percentages in protocols when making dilutions of stock bleach. It's helpful to look at the product labels, as they often have dilution factors for different uses as well as concentrations in parts per million.

I have a bottle of Clorox brand concentrated bleach sitting next to me, and the label says this solution is 8.25% sodium hypochlorite. The label also says that 8.25% sodium hypochlorite yields 7.85% available chlorine, and that 4 oz. of this product in 1 gallon of water yields 2,400 ppm chlorine, implying that the stock concentration is 76,800 ppm if 1 gallon (128 oz.) is the total volume or 79,200 ppm if the 1 gallon is added to 4 oz. (that is, 132 oz. total). For simplicity, let's say that the stock solution is 78,500 ppm so that 1% available chlorine is exactly 10,000 ppm chlorine.

The CDC website cites a study that states 200 ppm chlorine is sufficient to inactivate 25 different viruses. Sadly, I cannot find the text of that study. Here's the citation:

Klein M, DeForest A. The inactivation of viruses by germicides. Chem. Specialists Manuf. Assoc. Proc. 1963;49:116-8.

So, using the low end of your given values, a 3% stock solution of bleach will have ~2.85% available chlorine, following the yield ratio above. Diluting 5 tbsp. (2.5 oz.) of this stock in 1 gallon of water (total volume 130.5 oz.), as recommended, will yield ~0.05% available chlorine, or 500 ppm, which is sufficient for viral inactivation.

Edit - a bulletin from Clorox (PDF, cached HTML) states that 2400 ppm available chlorine is recommended for disinfecting hard, nonporous surfaces. The bulletin also lists enveloped and non-enveloped viruses against which this product is effective (at the "99.9%" level), including Human Coronavirus (Strain 229E, ATCC VR-740)

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The World Health Organization states[*]:

1:100 dilution of 5% sodium hypochlorite is the usual recommendation. Use 1 part bleach to 99 parts cold tap water (1:100 dilution) for disinfection of surfaces.

They say most household bleach solutions contain 5% sodium hypochlorite (50 000 ppm available chlorine), which does seem to be the typical concentration in the U.S. The recommended dilution yields 0.05% sodium hypochlorite (or 500 ppm available chlorine).

That same source page has a few helpful points to guide usage. Most importantly they point out: "Do not use bleach together with other household [cleaning agents], because this reduces its effectiveness and can cause dangerous chemical reactions. For example, a toxic gas is produced when bleach is mixed with acidic detergents, ... and this gas can cause death or injury. If necessary, use detergents first, and rinse thoroughly with water before using bleach for disinfection." The latter is important because they also say "Organic materials inactivate bleach; clean surfaces so that they are clear of organic materials before disinfection with bleach."

[*] World Health Organization. "Infection Prevention and Control of Epidemic- and Pandemic-Prone Acute Respiratory Infections in Health Care" (2014), "Annex G - Use of disinfectants: alcohol and bleach".

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  • $\begingroup$ That page seems to only link for influenza viruses, nothing COVID-19 specific. $\endgroup$ – hyperknot Mar 13 at 12:44
  • $\begingroup$ @hyperknot, that page is part of a large document not about influenza, but about "Pandemic-Prone Acute Respiratory Infections" (exactly what we're facing). Of course it doesn't mention SARS-CoV-2 (the virus that causes COVID-19) since it was written in 2014. It does mention the SARS-CoV virus, the cause of the 2003 SARS epidemic (and a close relative of the virus causing COVID-19). In any case, pretty much all types of virus particles are all vulnerable to harsh chemicals like bleach. $\endgroup$ – mgkrebbs Mar 14 at 0:24

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