Question of the Week

What happens if you use 99.9% effective sanitizer twice?

Sun, 30th Jan 2011

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Emilio Romero Jouvin asked:

My daughter Camila raised a very interesting question while watching a commercial for a sanitizing or disinfecting gel that claimed to get rid of 99.9% of germs: other than being curious as how do they know with such accuracy the performance of the product, she asked what would happen if one uses the gel twice... will it get rid of 100% of the germs (sounds logical ha ha).


Thank you and best regards as always. Great show, keep up the good work.


Greetings from Ecuador




We put this question to Jane Greatorex from the Health Protection Agency in Cambridge:

Jane -  Well, it’s not a straightforward answer.  The 99.9% claim that manufacturers put on their products is true based upon the tests that they carry out, but these are carried out under laboratory conditions in test tubes and on plates.  It’s a little bit more complicated than that in real life and it really depends on what we’re talking about here. 

(c) Lars Klintwall Malmqvist @wikipedia" alt="Person washing his hands" />If we’re talking about a cough droplet, a cough droplet might contain about 200 million 'flu germs in the winter season and about a million or so bacteria; and if you use that hand gel, then you will indeed get the numbers of viruses and bacteria down probably to levels below that which is infectious.

However, you will not remove all of them and that's because of two things.  One, your hands are very good at retaining bacteria and viruses, and two, the numbers of bacteria and viruses that you start with are such that even 0.1% is still a big number of bacteria and viruses.  If you kill even that 0.1% with another hand wash, you'll still be left with some remaining.

Now if, for instance, we were talking about faecal contamination – not a nice subject, but then we’re talking about much larger numbers of viruses and bacteria, and the big complication there is, you need fewer of those to cause an infection. 

So, the straight answer is, you won't remove all the bacteria and viruses, and that's in some situations, it’s really not very good news.

Diana -   Two scrubbings will remove more germs, but 99.9% of a lot is still going to be a lot.  It all depends on what the infectious doses of a specific pathogen.  If you only need one bacterium to infect you and sometimes even the most thorough washing isn’t enough.


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Its quite logical to think it that way. For example, when you brush your teeth once it is not as effective as you brush your teeth twice. As for sanitizers, I think it has something to do with how much of it you apply to lets say your hand.

But one thing that I can think of is that sanitizers cannot totally remove all bacteria. Its like a person being able to remove dirt from a cupboard, by cleaning using a cloth. But it has reached its limits. The cloth can't clean stains stuck on the cupboard. So even if you change the cloth. The results will still be the same. 99.9% clean not 100%

Well, thats the best I can think of. Best! The Scientist, Tue, 25th Jan 2011

Well, obviouisly, you'll kill 99.999% of the germs. And, if you wash your hands three times, you'll kill 99.99999% of the little blighters.  Geezer, Tue, 25th Jan 2011

The question should be why you are not killing 0.1% of the "germs".

Is that 0.1% just the ones hiding under your nails, cuticles, and other places on your hands?

Or, are they fully or partly resistant to the alcohol.

Bacteria tend to be more susceptible to alcohol than viruses, although the notes I was reading indicated that the common cold transmission is at least reduced by these hand cleansers.

Some molds and yeasts, of course, will produce various types of alcohol, so one might expect some resistance.  The same would be true of a few types of bacteria.

The risk of overuse of antimicrobials is that we could be inadvertently breeding the next generation of "super-bugs". CliffordK, Wed, 26th Jan 2011

Come to think of it, we might ask how they know it kills 99.9% Did they actually count them?

(Bet it really does a lot better than that, but their Legal Department insists that if the Marketing guys have to put a number on the bottle, it better have a generous safety margin.) Geezer, Wed, 26th Jan 2011

The exact phrase used in the advertisement is that it " kills 99% of all known germs ... dead ".
The percentage of germs that are unknown is, unsurprisingly, unknown : now that's what I call a large error margin.

The the known 1% which survive the disinfectant and an unknown % of unknown germs may include extremophile* bacteria which flourish in bleach. Repeated applications of bleach wouldn't reduce their number, quite the reverse, ( cf antibiotic resistance).

RD, Wed, 26th Jan 2011

They should.

Actually, what they'll probably do with bacteria is to plate them on a substrate like agar, and count the colonies that grow.  So, if pre-treatment you get 1000 colonies.  With treatment you get 1 colony, and the control gives 0.  You would conclude that you killed 99.9%

Hopefully they can actually verify in a "real-world" situation with having a person use the disinfectant, then scrub their hands with sterile water, and plate it on the agar. 

If "Germs" include viruses.  The viruses wouldn't necessarily grow on Agar.  So they would have to develop a confirmation that would test for all types of "germs", not just the ones they can easily grow on agar.

Also make sure it is an actual "Kill" and not just growth inhibition.

Keep in mind...  99% of the "germs" in the environment are not harmful to humans, especially if you don't get them into a skin break.  CliffordK, Wed, 26th Jan 2011

When you clean everything with that product but then catch some nasty little germ, guess what.... Yep, it was the 0.1% that the product didn't kill. Its the manufacturer's 'get out' clause.

Even if you clean things twice, you'll still only be killing the same 99.9% of all known germs, the 0.1% it didn't kill first time around are the same 0.1% it doesn't kill second time around.

The problem with being overly hygienic with all these cleaners is that they are indiscriminate, they kill the harmful, harmless and even the beneficial. By sanitising everything, we leave our body's immune system with nothing to do. So when something does break through this barrier, the question is, will our defence system be in any fit state to deal with it? Don_1, Wed, 26th Jan 2011

Oh, this is fun! ....

Great, all great, but if we know that there is a strand of germs which always survive, then which are those super-ultra-invincible-puppies…? shouldn’t we be trying to study and get to know them?
And if we manufacture the next sanitizer to kill that strand… will it still be 99.9% effective?

Emilio Romero, Wed, 26th Jan 2011

We recieved this from Ian Smith.  I'm sure he'll be happy for me to post it here:

Dear Dr. Chris,

As for the origins of the "kills 99.9% of all germs" slogan, I am not sure, but I have always suspected it is the manufacturer believes the product kills 100% but can't say that in case it is sued for a germ not being killed.

As for whether washing a surface twice will kill all the germs, the only article I found swiftly with germ numbers/ densities ( noted 1500 germs per cm sq.. I suspect this varies dramatically depending upon the person and what they have been doing, and their cleanliness.

If we assume that all germs are equally resistant to the product, then the first wipe will kill 99.9% of the germs. That leaves 1 germ per thousand original germs. If we wash again, we are down to 99.9% of those remaining germs killed - or about 1 in a million. If we assume that a man's palm is c. 10cmx10cm, we have approximately 1500x100 germs - or 150,000 germs. However, we missed the fingers, and more importantly, we missed the ridges that form fingerprints. These ridges will be much harder to clean, and offer a larger surface area, so I think by the time you add the backs of the hands, the fingers, and allow for fingerprints, it is debateable as to whether you would kill all the germs in two washes - assuming they are all equally vulnerable. And if they are all equally vulnerable, why aren't 100% killed first time? ;-)

Kind Regards,

Ian Smith BRValsler, Wed, 26th Jan 2011

It's good to see kids thinking skeptically. First the claim of 99.9% germ killing is largely theoretical. Your biggest variable is the surface on which you want to clean. If the surface is PERFECTLY smooth even down to the microscopic scale (The scale of most "germs") and the antiseptic is PERFECTLY made and used (all three highly unlikely) then you'll kill 99.9% of the germs present. You'll be left with about 1 in 1000 germs left, still a lot of germs. If you clean again you'll not kill 99.9% of the germs left as these are the germs that happened to be in places where they are protected or are for other reasons immune. It's the second one, the immunity, that is a bit scary. Those bugs that survive will now breed and their dependents will also be immune, which means your antiseptic will be less effective. This is how evolution works. Take a group of organisms and introduce a stress, like a toxin, that kills most, but not all the individuals who experience the stress. Now wait a while for your population to rebuild and introduce the stress again. Again a lot of individuals will be killed but, all other things being equal, you'll have a lot more survivors than you did the first time. Repeat this cycle over and over and soon the stress will only kill a small number of individuals. mountaineirc1969, Thu, 27th Jan 2011

For proper hand washing technique....  you should follow the example of a surgeon.  It should take you a few minutes to complete the process.

Wikipedia has notes about hand washing.

Clostridium is actually spore-forming digestive system bacteria, usually not bad, except with colonizing the body after courses of antibiotics.

Again, this is the risk of lowering the antibacterial susceptibility of widespread environmental bacteria so that your treatment is less effective when you really need it.

CliffordK, Thu, 27th Jan 2011

Well... Mutations occur with every generation of a certain organism, one of the generations for a specific bacteria could be immune to the chemicals in the sanitizer gel.
Actually, because of this, the educated urge others not to overuse sanitizer, as it would promote the spreading and evolution of bacteria. Locke, Wed, 2nd Feb 2011

It's incredibly stupid to want to kill of the organisms we live with. Use soap, it will clear of the dirt to about 99%, but stop trying to kill the natural bacterias you have. We have evolved with those, and a effect of blind faith in 'hygiene' is that your kids will have a lower resistance to a lot of common diseases.

Asthma and similar allergic reactions is on the rise in all Western societies, not only due to new types of manufacturing and their wastes but, as a guess, also depending on all this hygiene and the absence of contact with nature, dirt etc, city living introduce. America is incredibly weird in their fear of bacteria, I remember seeing plastic disposal wraps on the toilet seats and..

Nobody dies of a little dirt, but the opposite? yor_on, Fri, 4th Feb 2011

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