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

Have you ever wondered why most disinfectants say they kill 99.9% or 99.99% of germs, but never promise to kill them all? Maybe this thought crossed your mind in the middle of cleaning your kitchen or bathroom.

Surely, in a world where science is capable of doing all sorts of amazing things, someone would have invented a 100% effective disinfectant?

The answer to this riddle requires understanding a little microbiology and a little math.

What is a disinfectant?

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

There are literally millions of microbes on surfaces and objects in our home environment. Although most germs are not harmful (and some are even good for us), a small proportion can make us sick.

Although disinfection can include physical interventions such as heat treatment or the use of UV light, when we think of disinfectants we generally refer to the use of chemicals to kill microbes on surfaces or objects .

Chemical disinfectants often contain active ingredients such as alcohols, chlorine compounds and hydrogen peroxide that can target the vital components of different microbes to kill them.

The Mathematics of Microbial Elimination

Over the past few years, we have all become familiar with the concept of exponential growth in the context of the spread of COVID cases.

This is where the numbers increase at an ever-increasing rate, which can lead to a very rapid explosion in the size of an object. For example, if a colony of 100 bacteria doubles every hour, the bacterial population would exceed 1.5 billion in 24 hours.

Conversely, the destruction or inactivation of microbes follows a pattern of logarithmic decay, essentially the opposite of exponential growth. Here, as the number of microbes decreases over time, the mortality rate slows 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 the next minute, 10% of the remaining 10% (or 1% of the initial amount) will remain, and so on.

Because of this logarithmic decay pattern, it is never possible to claim that you can kill 100% of a microbial population. We cannot scientifically affirm that we are capable of reducing the microbial load of a proportion of the initial population. This is why most disinfectants sold for home use say they kill 99.9% of germs.

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

Real-world implications

As with much science, things get a little more complicated in the real world than in the lab. There are a number of other factors to consider when evaluating a disinfectant’s ability to eliminate microbes from a surface.

One of these factors is the size of the initial microbial population you are trying to get rid of. In other words, the more contaminated a surface is, the stronger the disinfectant must act to eliminate microbes.

If, for example, you were to start with just 100 microbes on a surface or object and eliminate 99.9% of them using a disinfectant, you could be confident that you have effectively eliminated all of the microbes on that surface or object. surface or object (called sterilization).

In contrast, if you have a large initial microbial population of hundreds of millions or billions of microbes contaminating a surface, even a reduction in microbial load of 99.9% can still mean that there are potentially millions of microbes remaining. on the surface.

Time is a key factor in determining how effectively microbes are killed. Thus, exposing a highly contaminated surface to a disinfectant for a longer period of time is one way to ensure that more of the microbial population is destroyed.

This is why, if you look closely at the labels of many common household disinfectants, they often suggest that to disinfect, you should apply the product and then wait a specified amount of time before wiping it off. So always consult the label of the product you are using.

Other factors such as temperature, humidity and surface type also influence the effectiveness of a disinfectant outside of the laboratory.

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

Disinfectants are part of infection control

Judicious use of disinfectants plays an important role in our daily lives by reducing our exposure to pathogens (disease-causing microbes). They can therefore reduce our risk of getting sick.

The fact that disinfectants cannot be proven to be 100% effective from a scientific point of view does not detract from their importance in infection control. But their use should always be supplemented with other infection control practices, such as hand washing, to reduce the risk of infection.

This article is republished from The Conversation under a Creative Commons license. Read the original article.