How soap can kill viruses: 20 seconds, lather and warm water

Since the outbreak of coronavirus in the UK, the government has been encouraging people to wash their hands frequently with soap in order to stop spreading the virus. The government's promotion successfully made people do this for a full 20 seconds by asking them to mentally sing "Happy Birthday" twice while washing our hands. (Those who don't like that particular song can now find suggestions for many other 20-second "hand-washing songs" online.) While it is easy to understand the reason for washing hands long enough, we may also like to know how we can optimise our hand washing efficiency using soap. An article with interviews from two professors gives us the answer: bubbles and foam, and warm water.¹

Before we go into what the two professors had said, we can have a brief understanding of the structure of an encapsulated virus (such as coronavirus) and how this kind of virus attaches to our hands. An encapsulated virus consists of three key components: genetic material made up of ribonucleic acid (RNA), proteins, and lipids (basically called fat). The virus replicates in the host and makes lots of these components which then self-assemble to form complete viruses. The proteins and lipids form the virus envelope (including a lipid bilayer and membrane proteins), giving protection and attachment ability to the virus when it is outside the host cell. When an airborne virus comes in contact with the skin, the envelope of the virus interacts with proteins and fatty acids in the dead cells on the skin's surface. However, the bondings that hold the three key components in the virus, and the bonding between the virus and the skin's dead cells, are not strong.*

Soap molecules have a special property that disrupts those weak bondings. Each soap molecule has two ends: a hydrophillic ("water-loving") head, and a hydrophobic ("water fearing") tail that is attracted to grease. When soap is applied, the hydrophobic tail of the soap molecule is drawn to the fatty outer layer of the virus, competing with the lipids in the membrane and thus prying it open. Once the virus splits open, its contents is spilled out into the soapy water and "dies", or, more correctly, becomes inactive.**

The combination of water and scrubbing with hands using soap creates more soap bubbles. You may wonder if these have an actual function. The foam or bubbles created can disrupt the chemical bonds that allow a virus to stick to the skin's surface. After 20 seconds of thorough scrubbing to get into every crack and crevice of your hands and fingers, all the viruses that have been damaged, or inactivated, by soap molecules are washed away when you rinse your hands. Therefore, according to Professor William Schaffner of the preventive medicine and infectious disease department at Vanderbilt University of Medicine in Nashville, the "bubbles and foam literally pick germs up and wash them down the drain." It is "an indication that the soap is ... trying to encapsulate the dirt and the bacteria and the viruses in them," said Dr. Bill Wuest, an associate professor at Emory University who studies disinfectants.¹

Dr. Wuest said that warm water can make everything bubbly more easily than can cold water: "Cold water will work, but you have to make sure you work really vigorously to get a lather and get everything soapy and bubbly." We may need to sing the "Happy Birthday" song three times instead of two. "Warm water with soap gets a much better lather, more bubbles."¹

Therefore next time you wash your hands with soap, please remember that besides 20 seconds, warm water and a lot of bubbles are also essential to destroy the viruses and wash them away from the skin effectively.

*The lipid bilayer membrane is assembled by noncovalent bonds, which are weak bonds that hold molecules. The proteins and lipid molecules in the virus membrane are held together by Van der Waals forces, which also hold together the hydrophobic tails of soap molecules, and hydrogen bonding, which also binds the hydrophilic heads of soap molecules with water. These weak bondings help to stabilize the lipid bilayer structure of the virus.

Washing off the virus with water alone, therefore, is not enough, as water cannot eliminate the interaction between the virus and the dead cells on the skin's surface. Coronavirus covered with a membrane of lipid bilayer, the "greasy" virus, thus cannot be simply separated from the dead cells of the skin by water. Soap and ethanol are the best tools to do that.

**Almost all sanitizer, which typically contains 60%-80% ethanol, "kills" viruses in a similar fashion as soap. Both dissolve the greasy coating of the enveloped virus.

Reference

1. "Why soap, sanitizer and warm water work against COVID-19 and other viruses" CNN Health, 24th March, 2020. https://edition.cnn.com/2020/03/24/health/soap-warm-water-hand-sanitizer-coronavirus-wellness-scn/index.html