I've seen innumerable antiseptic, mouthwash, handwash advertisements that claim to be able to eliminate as much as 99.9% of all germs over a surface...but why not the remaining 0.1% (i.e- why can't they eliminate all germs)?


What they mean by "germs":

Being a student of Biology, I can tell that the term "germs" is crudely defined. I would prefer to use "pathogens" (less ambiguous), and I suppose the guys that market these products have (roughly) the same idea in mind. Pathogens normally include bacteria (monerans), protozoans, fungi and viruses... so I guess these are the "germs" they're talking about.

What I'm looking for in an answer:

Why is it that these (commercial) products can't eliminate 100% of all pathogens? Is this due to the inability of antiseptics to act on a particular (class of) organisms? If so, what's the problem there? Or is it because, the guys who market stuff like this assume an arbitrary amount (0.1%) of the pathogens present on a surface (say, the human hand) is located in microscopic niches that are inaccessible to the antiseptic solution? (If it isn't possible to provide a blanket statement in this regard, use of Listerine as an example will suffice)

In other words:

Is an antiseptic's inability to eliminate 100% of all germs due to its "chemistry", or is it due to physical factors?


Do antiseptics/mouthwashes/handwashes even kill 99.9% of all germs in the first place? Or is it (as I strongly suspect) an example of marketing fraud?

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    $\begingroup$ I think they leave that 0.1% just so that they can pull their hands away in case of any infections. But your question is too broad, in the sense that you are asking too many qiestions at the same time. You need to narrow it down. $\endgroup$ Commented Jun 23, 2017 at 13:28
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    $\begingroup$ It's amazing that, for the layman (blame media on that), 99.9, 99.95, 99.992 or 99.9999435 are all the same thing. "99.9" is not a number anymore, it's just an idiomatic expression, meaning "almost all". $\endgroup$
    – user24284
    Commented Jun 23, 2017 at 13:30
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    $\begingroup$ In a first year lab I used to teach, the students would determine the zone of inhibition for different antiseptics in an agar diffusion test. Listerine didn't kill anything. $\endgroup$
    – canadianer
    Commented Jun 23, 2017 at 16:06
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    $\begingroup$ Microbes vary greatly in their ability to survive a particular method of sanitizing. For instance, giardia cysts (a protozoan) are hard to kill with halogens because they have a hard shell. Clostridium botulinum spores in food can survive cooking at 100 C and still cause botulism afterward. $\endgroup$
    – user4617
    Commented Jun 23, 2017 at 16:34
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    $\begingroup$ I also found 5 seconds rule which says exactly what 3 seconds rule says. And both are correct! Amazing thing, this science! $\endgroup$ Commented Jun 23, 2017 at 18:10

4 Answers 4


This is actually an interesting question! Let me answer both the parts separately, taking the example of Listerine® mouthwash.

Is an antiseptic's inability to eliminate 100% of all germs due to its "chemistry", or is it due to physical factors?

In most of the cases, this is due to the physical factors. Clearly, your mouth is not a flat surface. It has many depressions and elevations. And these irregularities are the perfect hotels for all kinds of pathogens. Also, it is difficult for most chemicals to reach those spots and stay there for long enough so that they can act on the pathogens. Thus, most of these chemicals would be unable to kill 100% pathogens because of their inability to reach all of them. Also, in rare cases, it might also be due to chemical factors(!) Yes, I'm talking about antibiotic resistance. And in that case, it would be practically impossible to kill 100% pathogens, no matter whether they're hidden or exposed.

Do antiseptics/mouthwashes/handwashes even kill 99.9% of all germs in the first place? Or is it (as I strongly suspect) an example of marketing fraud?

For that, lets first see what Listerine® contains. As given on its website, its main ingredients are (I'm simply copying what they write on their page, followed by checking whether its true):

  • Eucalyptol – with antibacterial properties, this eucalyptus-derived essential oil works as an anti-fungal agent within the mouth.

  • Methol – this natural oil as germ-killing abilities to help halt the growth of bacteria.

  • Thymol – this powerful oil is derived from the ajowan herb, and helps decrease the risk of gum disease.

  • Methyl Salicylate – for minty freshness from morning til night, the flavouring agent in this essential oil is, well… essential!

We just need to know whether these ingredients really work or not (why? We'll talk about this later on) and I'll skip methyl salicylate for this (they don't even claim that it is antimicrobial). Beginning with eucalyptol, it has been shown that oil of Eucalyptus globulus (of which eucalyptol is a component) has antimicrobial properties against Escherichia coli and Staphylococcus aureus (see Bachir et al, 2012). It has also been shown to possess antifungal properties (Safaei-Ghomi et al, 2010). Coming to methol, it has also been shown to have antibacterial properties against various Staphylococcus and Lactobacillus species (Freires et al, 2015). Finally, there have been lots of studies about the antimicrobial properties of thymol. You can check out the Wikipedia page for information.

So, where is the percentage? The point is, the exact percentage depends on a lot of factors. When an enterprise, such as Listerine®, claims that their product has been shown to be 99.9% (or any number) efficient against bacteria, they need to cite the particular study through which they claim this number. But they can not, in any case, be definite that their product will be 99.9% effective everytime. How effective a product is also depends on the conditions under which it is tested. Mostly, these tests are performed on a petri dish in a laboratory, something very different from your mouth. Thus, although they can claim that their product is scientifically proven to be 99.9% effective, they cannot claim it to be 99.9% effective when you use it. Again, they cannot claim 100% effectiveness because this makes them liable (saying our product has been shown to be 100% effective requires them to show that even a single microbe did not survive on the petri dish they used for experiment). Also, this gives them a way to escape in case anybody complaints about their product not being effective (since they caught infection even after using their product). In such situation, they can easily say that their product is not 100% efficient!

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    $\begingroup$ I really like this answer! As much as I want to "accept" this, I guess it would be wise to give the question a day for more answers to (hopefully) trickle in. Thanks again! ^_^ $\endgroup$ Commented Jun 23, 2017 at 14:30
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    $\begingroup$ I'm more familiar with the science having to do with water purification, e.g., using iodine tablets when hiking, but I think some of the science is the same. If there's dirt in your water, then there will be bacteria inside the dirt particles that can't be reached by the sanitizing agent. Similarly, if there's a speck of food caught between your teeth, listerine may be unable to reach the bacteria inside the speck of food. Pasteurization is much more effective at killing almost all organisms, because the temperature can't be sheltered against. $\endgroup$
    – user4617
    Commented Jun 23, 2017 at 16:28
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    $\begingroup$ @bencrowell agreed, but you can't pasteurize your mouth :P talking about soaps, most of them just help you banish those microbes by flushing them with water (only antibacterial soaps do seem to "kill" them, albeit this is debated too)... $\endgroup$ Commented Jun 23, 2017 at 16:58
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    $\begingroup$ For Listerine, you are missing the main ingredient -- alcohol. Depending on formulation, Listerine is about 22% to 26.9% alcohol. $\endgroup$
    – chadbag
    Commented Jun 23, 2017 at 18:52
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    $\begingroup$ Answers to this question point to this paper, which says "alcohols [are] significantly [less effective] at concentrations below 50% and is optimal in the 60 to 90% range". It also points out that alcohol is fungicidal, but not sporadical. As the question asks about "Antiseptic" and not specifically organic antibacterial/antibiotic agents, this seems very pertinent. I had another point I wanted some clarification on, but I can't remember it after brushing my teeth. $\endgroup$
    – Patrick M
    Commented Jun 23, 2017 at 19:15

What does "99.9%" mean, anyway?

@another'Homosapien' already provided a nice answer, which explains the biological aspects of the question. However, following his/her advice, I'd like to talk about the mathematical aspect of the question. I'm a biology teacher only, but I always liked math and I have a deep respect for statistics and its branches.

We tend to generalize when thinking about big numbers, small numbers or approximations. That's normal and actually expected. We say that a given galaxy has 400 billion stars, or that the human body has 1014 cells. Those numbers are not exact, and no one expects them to be.

However, the problem with "99.9%" is different. That's not an approximation or an extrapolation for something that would be very hard or impossible to count, like the precise number of stars in M83 galaxy. That "99.9%" has become just an idiomatic expression.

99.76, 98.58, 99.98 and 99.92 are not the same number, They are close, but not the same. And, given the context, the difference can have a huge significance. Follow me:

Imagine that a given birth control pill A has a theoretical effectiveness of 99.96%, while a given birth control pill B has a theoretical effectiveness of 99.98%. They are both very high, so we will simply say "99.9%". However, the number of couples that get pregnant using the pill A is the double of the number of couples that get pregnant using the pill B (0.04% versus 0.02%).

I've heard several times that hormonal birth control methods have an effectiveness of 99.9%. That's just a generalisation. Let's see some real numbers (this data is from Hatcher, R. (2004). Contraceptive technology. New York: Ardent Media, Inc.) for some methods:

  • Progestin pill: 99.7% (it's not 99.9%)
  • Cooper T IUD: 99.4% (it's not 99.9%)
  • Depo-provera: 99.95% (it's not 99.9%)
  • Evra patch: 99.7% (it's not 99.9%)
  • Mirena: 99.9% (well...)

The examples abound. For instance, you can hear that DNA polimerase is accurate 99.9% of the time, or even 99.99%, if the speaker wants to transmit a higher precision. However, the real number is closer to 99.999999% (given an error rate of 10-8 per bp).

But nothing beats the TV ads for antiseptics and cleaning products. They all say "99.9%", and some of them say 99.99% (to make the message even more clear). However, these products don't kill "99.9%" of bacteria (be this either the number of cells or the number of species). The real number may be 98.46%, or 99.12%, or 99.98%... As @mmesser said in his/her comment, " that 99.9% has more to do with marketing and lawyers than biology".

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"Huston, we have a winner": this brand not only added another "9" to the sequence, but also claims to be the fastest gun in the west.

Conclusion: in non-scientific media and TV ads in particular, "99.9%" is not a number anymore, it's just an idiomatic expression, meaning "almost all".


Well as a Microbiologist by trade, we use this numbers based on log reductions on the forming units of the organism being tested. There is a limitation on the testing methods that cannot determine an absolute elimination of the pathogen. For example, I perform the test and perform serial dilutions of the recovered organism and plate it on agar to be counted. Higher concentrations are hard and nearly impossible to count so we must use dilutions to achieve this and then multiply for the dilution factor to react an "exact number". So when I perform a 100 times dilution and the agar plate comes clean (no organism) growing the recorded number is not 0 but <100 because we cannot be sure about how many organisms should be there. I know this is a bit confusing but it is more like a safeguard to not give false information.

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    $\begingroup$ +1 Came here to say this - although the others make some interesting points and speculations about liability etc, this is actually the correct answer. Legally, a chemical must result in a 3-log reduction in titre during the specified contact time to be described as a disinfectant. (Source: I run a high containment pathogen lab and we use similar protocols to validate disinfectants whenever we add a new pathogen, want to test a new chemical, or want to modify our protocol - reduction in contact time, validation at low temperatures, etc.) $\endgroup$
    – arboviral
    Commented Mar 7, 2018 at 13:55
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    $\begingroup$ You could improve the answer by adding a labelled diagram of a serial dilution or some links, though. $\endgroup$
    – arboviral
    Commented Mar 7, 2018 at 13:56

I agree with answer of @another 'Homo sapien' - to 99.9% :-) but there is another angle on it:

From millions of bacteria present, few might be genetically different enough to be able to deal with whatever chemical mechanism it supposed to kill them (have resistance to it), and survive. Evolution at work.

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    $\begingroup$ Unlike you, I am just a computer programmer who had a pleasure to work for bioinformatics in academia for a decade. So I prefer to read explanations by experts, instead of pretending to be one. I am just a lurker here. Making my answer longer would add fluff, and I like concise answers. $\endgroup$ Commented Jun 23, 2017 at 18:06

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