Kills 99.9% of germs

You have likely seen this marketing phrase on disinfectant before, but what does it mean? Does it mean that of all the germs that exist, the product can kill 99.9% of the varieties? 

Not quite. Let’s indulge a little and dive into this. First of all, what’s a germ anyway? 

The dictionary defines a germ to be: a microorganism, especially one that is harmful to humans. 

How many kinds of these are there? It turns out that we only know of a tiny fraction of all the microorganisms. It is unclear how many there are in total, but estimates are millions to 10s of millions of species.

We pay closer attention to the ones that hurt. Surprisingly few of the total microorganisms are harmful to humans. In fact, there are approximately only 1,400 pathogenic species*. 

The pathogens are mostly bacteria and viruses, followed by dozens of fungi (which are mostly opportunists), some protozoa and a few multicellular pathogens like worms.

Livestock and humans have it worse than most species due to our numbers and spread, but all creatures seem to have some pathogens. In fact, amphibians have been struggling so hard against fungi that it has been causing a global reduction in their biodiversity! 

500 species of amphibian have been reduced by the Chytrid fungus with 90 of those presumed extinct. That’s out of roughly 10,000 species. It’s called the Amphibian Crisis and I’m surprised I had no idea it was happening. One of the victims is the cute golden frog in the picture at the end of this mini (the Panamanian golden frog).

Not just the frogs are ill, even bacteria and fungi can catch viruses… but now I’m getting a little off topic. 

We have established that there are roughly 1,400 species of pathogen. Great, so does that mean 99.9% of these are destroyed by disinfectant? Again, no. Of these, disinfectant you buy in the store is typically tested on a handful. When applied to one of those species, it will kill at least 99.9% of them. Meaning that for every 1000 on your kitchen surface, one will live to tell the tale**. 

Given that disinfectant is only tested on a few pathogens, why did we all start using it to try and avoid COVID, a new kid on the block? Well, the active ingredient in disinfectant is usually either quats (quaternary ammonium compounds), hydrogen peroxide, bleach or alcohol. These work in different ways to neutralise pathogens but they can all destroy the lipid outer layer of the covid virus and render it non-infectious. 

So, if the claim the disinfectant was making was about how many varieties of germ it could kill, what would the statistic be?

At a rough underestimate, Bleach and H2O2 will kill over 90% of all pathogens while Alcohol and Quats will kill at least 80%.

Happy scrubbing :)

* Microbiology by numbers, Nature

** Under perfect conditions, this is an underestimate - perfect conditions are clean surfaces, enough contact time, optimal temperature and so on.