Is It OK to Dilute the Last of Our Hand Soap? We Asked a Microbiologist

You’ve been washing your hands nonstop lately. You scrub under your nails, get between your fingers and count to 20. Good job, you. But this time, you go to the sink and find that the soap is so low, it’s not reaching the pump. You could walk to linen closet and get another bottle, but you probably do what most of us do: Unscrew the pump and add a splash of water. We never gave it much thought before COVID-19, but is doing this even, um, safe?

We asked Jason Tetro, microbiologist, host of the Super Awesome Science Show and author of The Germ Files, if we should be worried. Here are the facts.

Before you start having nightmares about invisible creepy-crawlies all over your hands, take a deep breath. Liquid hand soap is still effective after a splash of water—phew—with one caveat. “As long as there’s surfactant, you will be able to remove [germs], although you may need more as dilution increases,” explains Tetro. In other words, the surfactant, aka the soap, will work as long as there’s enough of it in the mix.

We know what you’re thinking: how do I know it’s enough? There’s no exact science about how much water is OK to add—we asked. “Honestly, I don’t believe anyone has done that experiment…It’s not supposed to be diluted, but everyone does it,” says Tetro. “The ratio of soap to water will determine the level of removing power that occurs. So, if you dilute a little, then it’s not a large problem.” 

Instead of keeping a Pyrex under the sink for measuring, don’t stress. You can make an educated guess based on how many suds bubble up while you wash, regardless of the type of hand soap (foam, bar, liquid) you’re using. “As long as you are able to coat your hands with the active ingredient, usually shown by lather, then there really is no better or effective type of soap,” Tetro reassures us.

You also might have noticed that Tetro said surfactant *removes* germs, not kills them. As it turns out, most hand soaps don’t. “Hand soaps are for the most part designed to remove microbes from the skin. They are not good at killing germs unless they have been specially formulated to do so.” The more you know.

Soaps that kill germs will say antibacterial, antimicrobial or antiseptic on the bottle, but don’t stress about finding one of these, especially during quarantine times. In addition to having more added chemicals, there isn’t enough evidence to show that antibacterial soap is better at preventing illness than regular, according to the FDA.

That may make soap seem somehow less protective, but hold that thought. There’s a reason the CDC has made such a ruckus about washing our hands properly. While you may have thought it’s the soap’s germ-killing chemicals that clean your hands, it’s actually the friction between your hands and the surfactant that breaks apart microscopic germs. That’s why hand sanitizer is only ideal in a pinch when you’re not near a sink. 

Just in case you don’t have these steps memorized by now, here are the CDC’s guidelines for keeping your hands fresh and clean:

1. Wet your hands with clean, running water. The temperature isn’t important. Turn off the tap and apply soap to your hands.
2. Lather your hands—the backs, between the fingers and under the nails—with soap.
3. Scrub for at least 20 seconds (count or sing the “Happy Birthday” song twice in your head). “The key to proper handwashing is to ensure that soap coats the whole hand and that there is friction for a few seconds on the entire area,” says Tetro. “That will help to remove the germs and the dirt and grime and help you to have that nice clean hand.”
4. Rinse your hands with clean, running water.
5. Dry off with a clean towel, or allow your hands to air-dry.