In the midst of an ongoing pandemic— or any flu season, for that matter— it’s always a good idea to have  some hand sanitizer on you.

And antimicrobial handrubs are great! They’re convenient, portable, and really effective.

Pretty much every bottle of hand sanitizer features a statement saying it kills “99.99% of all germs.” I wish everything in my life  had that kind of success rate! But what’s up with the remaining 0.01%? You might assume that’s just  there for legal purposes, in case the stuff misses a germ here or there.

Or that it’s purely marketing, and nobody knows exactly how effective sanitizer is. Well, as it turns out, that tiny fraction of a percentage is there for a scientific reason. And there isn’t enough hand sanitizer in the world to eliminate it. [Intro music] While there are a lot of  hand sanitizer ingredients that are effective at killing germs, the most common one is alcohol.

Alcohol-based hand sanitizers work by destroying the external defenses of viruses and bacteria, usually a lipid membrane or a protein shell. It’s called denaturing, and  it’s a form of destruction specific to the microscopic  components making up cells. In viruses, this means breaking down the protective membrane called a viral envelope, and exposing the DNA inside  to be denatured as well.

In bacteria, the alcohol  destroys the cell membrane and can denature specific proteins to mess with their function. Either way, this spells death for the microbe— but only at the right concentration of alcohol. Hand sanitizer needs to be at least 60% alcoho l to maximize its impact on the membranes or proteins.

But studies have shown that the mechanism also depends on water, so nearly “pure” alcohol above 90% concentration isn’t as effective. But even at the right concentrations, there are some cases where alcohol isn’t enough to kill all germs. There are some types of viruses and bacteria that can survive even a generous splash of hand sanitizer.

These germs evade our alcohol-based attacks in a few ways. The first is by evolving resistance to it. Like the “superbugs” you hear about that are developing  resistance to antibiotics, some bacteria have evolved resistance to alcohol.

Enterococcus faecium is one of them. From the name, you can probably tell where this guy hangs out. It’s a gut bacterium that’s commonly spread in healthcare settings.

It’s already resistant to  many types of antibiotics, and now it’s developing  resistance to alcohol, too. Researchers have studied  the effectiveness of alcohol at killing E. faecium strains over the years, and found that our favorite  hand sanitizer ingredient is less effective against newer strains, particularly the ones that evolved after 2010. The genes involved in E. faecium’s resistance code for some proteins that compose the cell wall and others that help the  bacteria take up carbohydrates, which support the cell wall.

Unfortunately for us, the bacterial cell wall is one of the first things an  alcohol-based hand sanitizer will target. So the mutations E. faecium has evolved block the destructive mechanism of alcohol pretty effectively. Researchers suspect that widespread use of lower-concentration alcohol hand sanitizers might have contributed to this resistance.

So using stronger alcohol-based hand sanitizers might work better. Thanks to Skillshare for supporting this SciShow video! Skillshare is the largest  online learning community for creatives with thousands of classes led by industry experts across film, illustration,  design, freelance, productivity, and more.

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With Skillshare, you can join classes and  communities right up your alley, with support from fellow-creatives, to accomplish real growth. And the first 500 people to  click the link in the description will get a one month free trial of Skillshare. But evolved resistance isn’t the only way germs can evade hand sanitizers.

Some have shields that are just too tough for alcohol to begin with. Including the one protecting Norovirus. Norovirus is a non-enveloped virus, which means its function doesn’t depend on a membrane wrapped around it.

Instead, it has a protein shell called a capsid that’s extra difficult to break down. Enveloped viruses enter a cell by fusing their outer membrane to the cell’s membrane, then releasing their contents into the cell. While this mechanism is convenient and sneaky, it makes the virus really reliant on its membrane, which is pretty flimsy compared to a capsid.

These viruses spread less aggressively since they can’t survive as long outside of a host body. Norovirus, on the other hand, has high defense and high attack. It penetrates cell membranes when it infects them, ripping open the membrane  to get the viral RNA inside.

To do this, it needs a tough-as-nails capsid that won’t break apart before  it gets to a host cell. Now, this all might make Norovirus sound like an invincible viral supervillain. But while it’s super tough, it does have weaknesses: bleach and heat.

Those methods are obviously too harsh to use on ourselves, but surfaces and clothing can be disinfected with bleach or by washing and drying at high temperatures. As for your hands, it’s best to wash with soap and water to eliminate Norovirus. Because your hand sanitizer is about as effective on this virus as asking it to go away nicely.

While not all germs have  strong defenses like that, there is one last way that they can survive when they come in contact with hand sanitizer: they take a nap. Let me explain. Some types of bacteria have the defense mechanism of forming spores to deal with environmental stressors.

One of these is Clostridium difficile. Bacterial spores are basically a “dormant” form of the bacterium. Bacteria like C. diff form these spores and reduce all activity that could be disrupted by high temperatures, pH changes, and— you guessed it—alcohol.

So creating spores is basically a way for the bacteria to “go to sleep” until things get better. The cells shut down a lot of metabolic processes and wrap up their critical components in a tough shell. Without complex processes  keeping the cell running, antimicrobials only have  one target: the cell wall.

And as we’ve seen before, with a hard enough wall, alcohol doesn’t have a chance. The best way to deal with  C. diff spores on your body is … once again … handwashing. You basically can’t destroy them, so removing them is the only option.

Ultimately, hand sanitizers do work really well. Killing 99.99% of germs isn’t a stat to sneeze at. And the CDC highly recommends the use of alcohol-based hand sanitizers for healthcare workers, since they’re more effective than soap at actually killing germs that are susceptible to alcohol.

But as we’ve seen, not all germs are susceptible. Some bacteria and viruses have amazing defenses, evolve their resistance, or simply turn off and don’t care. Hand sanitizers don’t kill most of all germs.

They kill all of most germs. Until we develop something better, the surest way to stay safe  from viruses and bacteria won’t fit in a purse or backpack. It’s the restroom sink. [ OUTRO MUSIC ]