What 0.1% is left when you kill 99.9% of germs with soap?

When we use a soap that says “kills 99.9% of germs” on its label to wash our hands, what happens to the 0.1% of germs in our hands? Does it get intentionally saved so it can live to tell the tale of how a mighty soap wipes out an entire population? What is really happening to that ultra-powerful one-in-1,000 germ?

Image by AJ Cann on Flickr

The battle between soap and germs

A germ is a microscopic organism that could potentially make us sick, be it bacteria or viruses. Germs can come in contact from daily objects such as our phones, banknotes, door handles, shaking hands with another person, and the list goes on.

How do they end up in our hands? There are sweat glands underneath our palms that produce sweat and oils that become an ideal medium for the germs to adhere nicely on our skin. Germs stick to our hands because of the oils on our skin.

So the main objective of soaps is to destroy the “home” of germs by removing the oil from our hand that will subsequently kill the germs. Alcohol and kerosene are solvents that can effectively destroy the oil, but they are quite toxic for frequent home use. Imagine if you smell like kerosene all day? This is why soaps in the hospitals and laboratories smell a bit funny because they contain alcohol and are generally stronger than soaps found in the grocery store.

A more ideal solution is to use soap, but this brings another problem. Since commercial soap normally does not contain alcohol unlike soaps in the hospitals, rather than destroying oil particles (and their associated germs), soap molecules work by dissolving the oil with water which is then washed away. Therefore, its efficacy in killing germs is always been debatable.

Image by Irina Ba on Unsplash

Does soap really clean our hands from germs?

Most soap manufacturers advertise their product capable of killing 99.9% of germs, but does it truly happen? One thing we know is that claiming just short of 100% avoids them from being sued, but their claim does come from lab results.

So the next logical question would be, how reliable are their lab results? Generally, their lab tests and results are trustworthy, but there is a big pitfall that not many people know. Their test procedures typically consist of placing germs on a surface, wiping that surface with the soap, and then finally the remainder of the germs are analyzed.

The pitfall arises because their test conditions are different from our messy everyday life. Obviously, the inside of a laboratory is much cleaner than the outside world, so the results may not be true in every condition.

One study demonstrated that handwashing with soap and water removes the presence of bacteria to only 8%. This indicates that washing hands with soap still does not leave germs to 0.1% even when done in a laboratory, let alone a real-world application.

One important thing to note is that soap is not really killing the germs in our hands, but rather washing them away. In a scenario where a soap really removes 99.9% of germs, the remaining 0.1% may sound insignificant. But when we are talking about the number of germs, we should be considering millions of them which is why it could still be dangerous. However, as long as we maintain regular sanitation habits supported by a strong immune system, we should be fine!

So when a soap manufacturer claims that their products kill 99.9% of germs, they are technically correct but practically wrong. Although their lab results are not entirely misleading, it is indeed a powerful marketing message!


18 Responses to “What 0.1% is left when you kill 99.9% of germs with soap?”

  1. Kevin Kusnadi says:

    @Hua Ye
    Yes you are correct. Some people just wet their hand with soap and water while there are also people that do the hand movement while washing. It’s also important to keep in mind that the way soap manufacturer test their product will always be different than washing hand in real life.

  2. Katie Loi says:

    Interesting blog post! I really enjoyed reading it, and it’s good to finally know what the actual results are behind that marketing strategy. I think it’ll also be interesting to read a section about hand sanitisers too, but your post will probably become too long. Right now, the message is nice and powerful already, so good job!

  3. Kevin Kusnadi says:

    I think there’s a slight misconception here. When you mentioned about “our” microbiota, that refers to the bacteria in our gut, in which the more diverse the better. All the microorganisms in our hands are possibly harmful and we definitely don’t want that!

  4. Kevin Kusnadi says:

    Hot water has always been the standard procedure to wash hand at least in my lab. But a big consideration is that people always scald their hand and people don’t like that. Sometimes it’s better to wash in cold water if that can encourage people to wash their hands!

  5. Kevin Kusnadi says:

    Try washing your hand with hot water! Hot water is always the standard procedure for washing hand in my laboratory!

  6. Kevin Kusnadi says:

    So oil in our hand (Fatty Acid) are made out of a long chain of carbon. What alcohol does is to cleave the linkage between the carbon molecule, thereby destroying the physical state of the oil. Germs need oil as a media to survive, if the oil is destroyed, that will destroy the germs as well.

    When soap and water dissolve oil, it means that the soap molecules only bind to the carbon chain and therefore carry them together away when washed with water.

    Hope this answers your question!

  7. Kevin Kusnadi says:

    I’m glad that you enjoyed reading my blog. I must say that the number 99.9% is very effective in marketing and advertisement, but not so much in fighting germs.

  8. Kevin Kusnadi says:

    @Lau Li Ken
    Thanks for your suggestion! I’ll fix my blog later!

    The alcohol content is very low in hospital soap, so it shouldn’t cause any significant downside. You can definitely opt for alcoholic-soap for daily use at home especially if you don’t mind the smell. Some people would prefer to have a nice floral smell on their hand instead.

  9. Kevin Kusnadi says:

    I think it doesn’t really matter. Sometimes 99.99% is not always better than 99.9% because that is just entirely a marketing campaign. Hand sanitizers are useful because they are convenient to carry everywhere though, as many restaurants sometimes don’t have toilet to wash our hand

  10. Hua Ye says:

    Very interesting blog. I think the percentage of bacteria left also depends on how they wash their hands, whether they wash carefully using the surgery standard or just simply wash. It is tricky for those manufacturer to release such a message to the public.I remembered one class telling me that there are still some bacteria left after washing your hand carefully with soap. But they are normal bacteria living on human body and it doesn’t matter.

  11. Kaih Mitchell says:

    I’ve always been skeptical of those “kills 99.9% of germs” claims, but it’s interesting that they don’t even kill the germs, just wash them away. I recently heard people debating whether it was necessary to wash hands in hot water or if cold was fine. On one hand (pardon the pun), hot water can’t kill germs without scalding your hands, but then it is better at cleaning off the oils that germs live in. I wonder if it is that much more effective that we can justify the energy used to heat the water or if soap and cold water is just as good.

  12. chiengr says:

    Great read. I’ve always wondered this as well and just thought the 0.1% was our normal microbiota (but I guess they are not technically germs). Do you think soaps wash a few our own bacteria away too?

  13. Gustina Yasminisari says:

    This is really informative. Usually, I didn’t bother to check how a product being tested at lab. Now I feel that I should pay attention to this kind of thing, so I won’t take it blindly.

  14. Devanshi says:

    This is so interesting, I’ve always wondered about this when I’ve heard it on advertisements. I agree that soap is better than alcohol based hand sanitizers- because some bacteria and spores will not be killed by alcohol, so you need the physical action of washing them away from our hands.

  15. Matt says:

    Hi Kevin,

    Great article, I never knew that soap didn’t actually kill germs!

    Could I clarify something that I didn’t quite undertsand: when you say that alcohol and kerosene ‘destroy’ the oil on our hands, what does that actually mean?

    I understand how one could dissolve the oil with soap and water, but does alcohol do something chemically different?

  16. Zhao Yuan says:

    I always thought that soap actually do as they say. Thank you for spreading this knowledge. This is eye-opening. I think they should change their advertisements to “Removal of 90% germs”, rather than killing them. After all, it’s no antibiotic. Gotta keep our expectation at bay.

  17. Lau Li Ken says:

    This was an enlightening read. Fair point though: marketing a soap to kill off 92% of germs doesn’t sound quite as fancy.

    Assuming you don’t mind your hands smelling funky, are there any downsides to overusing hospital hand sanitisers that you know of?

    Also, you might want to add some hyperlinks to where you got the image from, and the image owner’s home page.

  18. Hanwen Hou says:

    Nice topic, do you think handwash liquids would be any better than soaps, do they contain more “germ-killing” components than soaps? Next time I will remember to get an alcohol wipe before eating fried chicken.