Do hand sanitizers really kill 99.99% of germs? How can they prove that’s true?


Do hand sanitizers really kill 99.99% of germs? How can they prove that’s true?

In: Biology

Sample the surface of a hand before and after using sanitizer and culture the bacteria. Estimate the density of bacteria on the hand based on the culture. Compare the two. Do this many times to arrive at the average rate of sanitation, as well as the variance, though they don’t report that. It’s a simple experiment.

I’ve heard that the claim is a prevarication. That the product can kill 99.x% of the type of germs against which it was tested, not that it kills 99.x% of the germs on the surface on which it is being used.

Its more to help with plausible deniability. Lets suppose a sanitizer could kill 100% of every pathogen it was tested on. You would be tempted to write 100%, but you cant account for all the unknown pathogens you HAVENT tested.

No company manufacturing a sterilizing agent **can guarantee with absolute certainty** that **every pathogen on the face of the earth** is killed by their product. They cant possibly know that to be true, who knows what strains are out there we have yet to discover. There could be a bacteria that EATS ethyl alcohol for breakfast.

So to cover their own asses in any potential law suit they write 99.99%.

If you could prove your son got deathly ill from a surface you cleaned using their product and could also prove the pathogen responsible wasn’t being killed by their product which claimed 100%. Boom. Easy lawsuit.

EDIT: As others have pointed out this also applies to the micro scale. You cant prove 100% you’ve killed every pathogen on a surface or could kill every type of pathogen on earth.

ELI5 answer: they use the product on a surface covered in bacteria, fewer than 1 in 10,000 cells remain. Therefore they can say it’s 99.99% effective. Simple as that.

Longer answer:

First it’s important to note that “kills 99.99% of bacteria” doesn’t mean that it kills all of 99.99% of known bacterial species, or anything like that, it’s literally just number of cells present on a surface. So it doesn’t say anything specific about the type of bacteria that it is good against – it’s not that they know of one particular species that doesn’t die but the others all do. These hand sanitizers are broad in their action and don’t have much in the way of specific targets against specific things like an antibiotic does. Their active ingredients are various types of alcohol… which just generally kills stuff by denaturing proteins.

The 99.99% is just due to the methodology of testing these products. They’re saying that after the treatment, fewer than 1 cell in 10,000 remain – that’s pretty good!

If you wanted 100% effectiveness and all bacteria dead, you could try sticking your hand in bleach, or a flamethrower… but neither of those are going to do your hand much good. Hence the alcohol-based santizer is a good compromise between effectiveness and not damaging you.

Hand sanitizers will kill huge amounts of bacteria on your hands, but will not change the environment that they grow in. Once the effect ends (that is, the alcohol evaporates), surviving bacteria and new ones from the environment just go right back at it.

Washing hands with soap is really the best way to go because it actually changes the environment of your hands (its microbiome) for a period of time, making it inhospitable for the bacteria to live in the first place. As a result, they take a lot longer to come back.

I’m seeing a lot of incorrect information being put on here. Soap does not kill bacteria. Even antibacterial soap is not considered more effective than regular soap. It helps by washing them away and washing for 30 seconds is the standard in medicine, obviously more when scrubbing in for surgery. Hand sanitizers are considered the standard because they are considered as effective as a complete handwashing without the need to stand at a sink and then dry off. Yes it kills the vast majority of germs, but is ineffective against spores such as clostridium difficile which are ultra resistant.
Here’s a link to Harvard Med that provides a full breakdown.

Edit: please click the original link of the cease and desist letter. The problem is with the wordings used such as “kills more than 99.99%” and claims that it can “reduce student absenteeism by 51%”. Note that purell, lysol, etc all use the exact same “kills 99.9%” on their bottles and have not recieved complaints.

I also forgot something that may be of interest. We were told by sanitizer reps at one of my hospitals that we should wash our hands after every 3rd use of hand sanitizer. If you’re in healthcare or you just love using hand sanitizer, you’ll notice your hands will become almost tacky with a film over your hands after multiple uses.

Most sanitizers and disinfectants are created to kill a list of different bacterias that are quite common. However there are some that they can’t kill. A good example of this would be c-diff. Typically you need 200ppm solution to sanitize for any number of “germs” on most sanitizers list. With c-diff you need 5000ppm. So typically sanitizers will kill most “germs” on, and even off its list, it will however not kill some of the nastier ones. The reason for this can be found in how a “germ” dies. Not all germs die the same way, so an alcohol bases sanitizer which basically defeats “germs” by drying them out doesn’t so well against a germ the creates spores at death that can survive alcohol.

Tldr: some germs die differently and therefore you can’t kill them all the same way. Regularly and properly washing your hands has a better likelihood of keeping you “germ” free than just squirting sanitizer in your hands.

Edit: I was a sanitary supply rep for years and this was always the big question. Every year when some big germ craze would happen everyone would flip out. Reality was when they went heavy on hand sanitizer more people got sick than when they would push employees, students, or patrons to wash their hands properly.

They don’t have to test each product.

There are different models of sanitation, and certain methods have defined/researched *log reductions* by the CDC and WHO.

Isopropyl and Ethyl Alcohol is considered a 3 log reduction. If you product has enough alcohol, it qualifies.

Hand washing with proper technique ( e.g. EN 1499 and EN 1500) has a greater log reduction and alcohol based sanitizers should never be used as a replacement for washing hands.

Chemist/Microbiologist here. I make disinfectant cleaners for a living. The 99.9% is a specific claim, that has a specific legal connotation and a scientific basis as dictated by a test method. Bacterial kill is measured in “percent log reduction” and each 9 correlates to the degree of “%LR”. So if you start with a known amount of bacteria on a surface, apply your product and count the bacteria left via extraction, serial dilution and agar plating, you can correlate the %LR to a claim (claims can range from bacterial protection to sanitization to bacterial kill, etc).
99% is 2 logs, 99.9% is 3 logs, 99.99% is 4 logs and so on. To release a product into the consumer market, 99.9% reduction is the standard. That’s why bottles say “Kills 99.9%” – if the product yielded only a 99%LR, it would say “protects against 99% of bacteria.” Each word of the bottle is chosen very carefully based on the test method results.
You can pick and choose which microorganisms you test against but typically they are dictated by the test method. Staph, klebsiella, e.coli, e.hirae and salmonella are the most common. If you can make the claim with those, you are very likely to be effective against most other organisms. Of course there are exceptions which require different test methods but those products typically are used in hospital settings, not released to the general consumer.
You’d be surprised what you can find out by reading the fine print on the back of the bottle. Most people misuse their cleaners at home, don’t realize it and don’t get the protection they think they’re getting.