Does Soap Really Kill 99.9% of Germs?

Does soap really kill germs like it's supposed to?

Sabrina Stierwalt, PhD
3-minute read
Episode #196

Eighty million. That’s the number of germs exchanged in a kiss. Ten to two hundred million. That’s the number of germs that are found on an average cell phone.

What is a clean freak to do? How can we possibly combat all of those germs? This question comes from listener Geraldo in Brazil and I think it’s a great one. Does soap really kill 99.9% of germs?

How Does Soap Clean?

Remember that a germ is what we call any microscopic particle or organism that can make us sick, so this includes viruses and bacteria. Most of the gunk we want to wash off of our hands, whether it be dirt or germs, adheres to us thanks to the oils on our skin. Destroying the oil with a solvent like alcohol or kerosene will thus remove the associated germs.

However, although soaps used in hospitals are often strong, alcohol based versions, alcohol and kerosene are themselves toxic to varying degrees and thus not ideal for frequent in-home use. Imagine smelling like kerosene all day? Luckily, we have soap.

Rather than dissolve oil, soap molecules work in a more mechanical way to essentially convince the oil particles (and thus their associated germs) to join up with water molecules so that they can be more readily washed away. Each soap molecule typically looks like a long chain of carbon and hydrogen atoms with one end that readily joins water molecules (called the hydrophilic end). The other end of the soap molecule prefers to avoid water (called the hydrophobic end) and instead happily attaches to grease.

Thus, when you are washing your hands, the hydrophobic end grabs on to the germ-hosting oil particles, while the other, hydrophilic end coaxes the oil toward the water to eventually be flushed away.


Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.