By Zack Dumont

I don’t even need to provide an example. We’ve all seen it before. I’m referring to the lofty claim that some new and/or improved soap “kills 99.9% of bacteria”. In the spirit of Scientific Literacy week, I’d like to share what goes through my mind when I see claims like this. My intention is not to be smug, but inspire critical thought in others when they’re evaluating scientific claims. Perhaps if you’re already on board with these views of mine then revisiting may not serve you well… but maybe, just maybe, you could find these questions useful next time you hear someone else at risk of being deceived. Here goes my breakdown of this ubiquitous claim.

Regarding “Kills”:

  • Where has this killing been shown to occur? On my skin? On someone else’s skin? On another living creature’s skin? In a lab somewhere (eg, on a Petri dish)? The proximity to the proposed site of action is critical. Just because something can be killed in a lab doesn’t mean it can be killed on my skin. I’m not expecting the company to test the soap on me, but a lab bench experiment is pretty far removed from measuring real world effects. There are many scientific claims based on what is seen in a laboratory and then never pan out when put into the field.

Regarding “99.9%”:

  • If you have even just a rough idea of how much bacteria can be found on any surface, let alone something warm and organic (such as the skin on our hands), then you’ll know that even 0.1% left over is a lot! And of what’s left over… shouldn’t I be concerned that a super-powerful soap couldn’t kill it, and now it’s left unchecked with all that food and nourishment around?

Regarding “bacteria”:

  • Going back to the first stream of questions – did the soap kill 99.9% of a single species of bacteria in a lab somewhere? Or does it kill 99.9% of the different types of bacteria (of which there are thousands of species and strains!)?
  • Do I want to kill the bacteria? Are all bacteria bad? Aren’t some bacteria good? Don’t we buy products with probiotics in them?

Governments and regulators with some impetus to be business-friendly (self-imposed or not), will have a hard time protecting consumers against these seemingly benign marketing claims; especially considering how many are out there and the resources that would be required to police it. Further, claims like these are difficult to forbid as they will have some element of truth to them – the soap probably did kill most of some bacteria somewhere. With these factors combined, it’s left to consumers to cut through the marketing and hype. At this, some people are better trained and more experienced than others. Time-permitting, and with skill for teaching adults (which is quite different from teaching children), you can share your insight with others. But in reality, time is tight and most others’ patience for learning new things is even more scarce. So, if nothing else, asking out-loud questions like the ones above are a quick and easy way to minimally create some cognitive discrepancy in those willing to listen. Plant some seeds of doubt, then with tolerance, watch to see if something takes root. If yes, nourish it. If not, keep planting those seeds!

For more on this topic, check out Sabrina Stierwalt’s article in Scientific American<1>.


<1> Stierwalt S. Does Soap Really Kill 99.9 Percent of Germs? Scientific American Jly 11, 2016. Avialable from: