Why do disinfectants only kill 99.9% of germs? Here’s the science

Have you ever wondered why most disinfectants claim to kill 99.9% or 99.99% of germs but never promise to remove all of them? Perhaps this thought occurred to you halfway through cleaning the kitchen or bathroom.

Surely in a world where science can do all sorts of amazing things, someone would invent a disinfectant that is 100% effective?

Answering this puzzle requires understanding a bit of microbiology and a bit of math.

What is a disinfectant?

A disinfectant is a substance used to kill or inactivate bacteria, viruses, and other microorganisms on inanimate objects.

There are literally millions of microorganisms on surfaces and objects in our environment. Although most microbes are not harmful (and some are even good for us), a miniature proportion of them can make us diseased.

Although disinfection may include physical interventions such as heat treatment or using UV featherlight. Typically, when we think of disinfectants, we think of the apply of chemicals to kill microorganisms on surfaces or objects.

Chemical disinfectants often contain active ingredients such as alcohols, chlorine compounds and hydrogen peroxide, which can attack indispensable components of various microorganisms in order to kill them.

The mathematics of microbial elimination

We have all become familiar with the concept over the last few years exponential growth in the context of the spread of Covid cases.

This is where the numbers are growing at an accelerating rate, which could very quickly lead to an explosion the size of something. For example, if a colony of 100 bacteria doubles every hour, within 24 hours the bacterial population will exceed 1.5 billion.

Conversely, the killing or inactivation of microorganisms occurs: logarithmic decay patternwhich is essentially the opposite of exponential growth. In this case, although the number of microbes decreases over time, the rate of death becomes slower as the number of microbes decreases.

For example, if a particular disinfectant kills 90% of bacteria every minute, after one minute only 10% of the original bacteria will remain. After another minute, 10% of the remaining 10% (or 1% of the original amount) will remain and so on.

Due to this logarithmic decay pattern, it can never be claimed that 100% of any microbial population can be killed. All you can scientifically say is that you are able to reduce the microbial load by a fraction of the initial population. That’s why most disinfectants sold for home apply claim to kill 99.9% of germs.

Other products, such as hand sanitizers and disinfecting wipes, which also often claim to kill 99.9% of germs, follow the same principle.

Real-world implications

As with many fields of science, things get a little more complicated in the real world than in the lab. When assessing a disinfectant’s likelihood of removing microbes from a surface, a number of other factors must be considered.

One of these factors is the size of the initial microbial population you are trying to get rid of. This means that the more contaminated the surface, the more powerful the disinfectant must be to eliminate microorganisms.

For example, if you started with just 100 microbes on a surface or object and removed 99.9% of them with a disinfectant, you can be very confident that you have successfully removed all the microbes from that surface or object (called sterilization).

On the other hand, if you have a enormous initial microbial population of hundreds of millions or billions of microbes contaminating a surface, even a 99.9% reduction in microbial load can still mean that potentially millions of microbes could remain on the surface.

Time is a key factor determining the effectiveness of microbial killing. So exposing a heavily contaminated surface to a disinfectant for an extended period of time is one way to kill more of the microbial population.

Therefore, if you look closely at the labels of many popular household disinfectants, they will often suggest that to disinfect, you apply the product and then wait a certain period of time before wiping it off. Therefore, always check the label of the product you apply.

Other factors such as temperature, humidity and surface type also have an impact how well the disinfectant works outside the laboratory.

Similarly, real-world microbes may be more or less sensitive to disinfection than those used for testing in the laboratory.

Disinfectants are one of the elements of infection control

The judicious apply of disinfectants plays an critical role in our daily lives by reducing exposure to pathogens (disease-causing microbes). They can therefore reduce ours chances of getting sick.

The fact that disinfectants cannot be scientifically demonstrated to be 100% effective in no way diminishes their importance in infection control. However, their apply should always be complemented by other infection control practices such as washing handsto reduce the risk of infection.