How Does Soap Work to Kill Germs?

Once you have the virus there are no drugs that can kill it or help you get rid of it. But  your grandmother’s bar of soap kills the virus.

Soap works because the virus is a self-assembled nanoparticle in which the weakest link is the lipid (fatty) bilayer. Soap dissolves the fat membrane and the virus falls apart and dies – or rather, we should say it becomes inactive as viruses aren’t really alive.

The slightly longer story is that most viruses consist of three key building blocks: ribonucleic acid (RNA), proteins and lipids. A virus-infected cell makes lots of these building blocks, which then spontaneously self-assemble to form the virus. Critically, there are no strong covalent bonds holding these units together, which means you do not necessarily need harsh chemicals to split those units apart. 

When an infected cell dies, all these new viruses escape and go on to infect other cells. When you cough, or especially when you sneeze, tiny droplets from the airways can fly up to 10 yards. 

These tiny droplets end up on surfaces and often dry out quickly, but the viruses remain alive. Human skin is an ideal surface for a virus. It is “organic” and the proteins and fatty acids in the dead cells on the surface interact with the virus.

When you touch a surface with a virus particle on it, it will stick to your skin and hence get transferred on to your hands. If you then touch your face, especially your eyes, nostrils or mouth, you can get infected. And it turns out that most people touch their face once every two to five minutes.

Washing the virus off with water alone might work. But water is not good at competing with the strong, glue-like interactions between the skin and the virus. Water isn’t enough.

Soapy water is totally different. Soap contains fat-like substances known as amphiphiles, some of which are structurally very similar to the lipids in the virus membrane. The soap molecules “compete” with the lipids in the virus membrane. This is more or less how soap also removes normal dirt from the skin.

The soap not only loosens the “glue” between the virus and the skin but also the Velcro-like interactions that hold the proteins, lipids and RNA in the virus together. 

Alcohol based products, which pretty much includes all “disinfectant” products, contain a high-percentage alcohol solution (typically 60-80% ethanol) and kill viruses in a similar fashion. But soap is better because you only need a fairly small amount of soapy water, which, with rubbing, covers your entire hand easily. Whereas you need to literally soak the virus in ethanol for a brief moment, and wipes or rubbing a gel on the hands does not guarantee that you soak every corner of the skin on your hands effectively enough.

So, soap is the best, but do please use alcohol-based sanitizer when soap is not handy or practical.

How soap works

Soap doesn't kill germs on our hands, it removes them. 

Germs stick to the oils and grease on our hands (sounds yucky, but it's totally normal). Water alone won't remove much of the germs on our hands because water and oil don't like each other, so they won't mix. But soap likes both water and oil. That's because soap molecules are a type of surfactant, which means they have one end that's water loving, or hydrophilic, and one end that's oil loving, or hydrophobic. 

When you wash your hands with soap, the soap molecules act as a mediator between the water and oil molecules, and bind with both of them at the same time. Then when you rinse everything off, the soap carries away the germs with the water. 

For the most effective hand washing, you must use soap and you must be thorough. Work up a lather because the friction helps lift dirt and oils from your skin. How long you should scrub depends on how dirty your hands are, but most health authorities recommend at least 20 seconds, or as long as it takes to sing "Happy Birthday" twice. And don't forget to scrape underneath your fingernails. That area is prime real estate for germs. 

Once you've washed, be sure to air-dry or towel-dry. There's no agreed-upon best practice for drying, but wet hands are more likely to spread germs than dry ones, the CDC says. 

Is antibacterial soap even better? Nope. 

Antibacterial soaps have added ingredients that can penetrate bacterial cell membranes and kill the bacteria. Sounds impressive, but studies have shown that antibacterial soaps are no more effective than regular soaps at removing germs.  

In 2016, the FDA issued a rule that antibacterial soaps were no longer allowed to be marketed to the public. 

What about hand sanitizer? 

The CDC recommends cleaning hands with soap and water, but if that's not an option, then hand sanitizer is a good backup. Studies have found that hand sanitizers with alcohol concentrations of 60-95% are more effective at killing germs than no alcohol or low-alcohol sanitizers. 

Hand washing with soap is, by far, the most effective way to keep harmful germs at bay. 

Stay safe everyone!