“Kills 99.99% of germs” is a common statement found on hand sanitizing gels, as well as other products like soap, cleaning wipes, and aerosol sprays, but what does it mean? Why doesn’t it kill 100 percent? Is hand sanitizer okay to use instead of hand washing with soap and water?

First lets look at the word “germ.” While some products use other words like bacteria and virus, germ is a commonly used term that doesn’t really describe what exactly is being killed. Most dictionaries define a germ as: a microorganism, especially when disease-producing. By this definition, a germ could be harmful or neutral to humans without specifically telling us what type of organism it is (bacteria, virus, fungus, mold). It is important to know what types of organisms cause us to be sick, how they are spread person-to-person, and how each of them is prevented. For instance, some “germs” are only spread through certain bodily fluids, where others may be spread through the air or contaminated surfaces where hand sanitizer use isn’t appropriate.

The advertised 99.99 percent can also be confusing. Does that mean if I have 100 germ cells on my hands that it will kill 99 of them and leave one still alive? Or does it mean it kills 99 percent of the types of germs that exist in the world? There really is no definitive answer to that question. The main point is that there is nothing that will kill 100 percent of harmful microorganisms. There are germs like Noro virus, responsible for 58 percent of foodborne illnesses in the US, that are not killed or reduced by the use of hand sanitizer.

The 99 percent kill rate has come under quite a bit of scrutiny, and should not be relied on as always being true. When hand sanitizers are tested it’s done in laboratory conditions, not in people’s homes, at schools, or in situations humans are commonly in. Even if human subjects are used in the lab, their hands are perfectly clean and dry, ideal conditions for proper sanitizer effectiveness, but those conditions are unrealistic. It turns out that dirt (visible stuff like mud and grime as well as invisible stuff like dust and grease) has a serious negative affect on hand sanitizer and can reduce that 99 percent pretty quickly. Water/moisture and sweat can also reduce effectiveness. In the real world, people do not make sure their hands are clean and dry before using hand sanitizer; they use hand sanitizer in most instances because their hands are dirty. In fact, a study by professors at the University of Ottawa found that the top three brands of hand sanitizer reduced the amount of germs on 8th grade students hands by only 46-60 percent.

So, can you skip the soap and water and just use hand sanitizing gels? Absolutely not! There is no substitute for washing hands for a minimum of 20 seconds with soap and warm water. Hand sanitizer may help reduce the number of microbes on your hands, but only in ideal conditions when used as directed on the package. Michigan State University Extension recommends only using sanitizer after regular hand washing, or if there are no safe hand washing accommodations available.