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 don’t promise to remove them all? Perhaps you’re cleaning your kitchen or bathroom? This thought may have crossed your mind while you were cleaning it.

Indeed, in a world where science makes all sorts of amazing things possible, has someone invented a disinfectant that is 100% effective?

Solving this difficult problem requires knowledge of microbiology and mathematics.

What is a disinfectant?

Disinfectants are substances 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 home environment. Although most microorganisms are harmless (some are even good for us), a small number can make us sick.

Although disinfection involves physical interventions such as heat treatment and the use of ultraviolet light, when we think of disinfectants we are usually referring to the use of chemicals to kill microorganisms on surfaces and objects.

Chemical disinfectants often contain active ingredients such as alcohol, chlorine compounds, and hydrogen peroxide, which can target and kill key components of various microorganisms.

Mathematics of microbial removal

Over the past few years, we have all become familiar with the concept of exponential growth during the coronavirus pandemic.

Here, numbers are increasing at an accelerating rate, and something can explode in size very quickly. For example, if a colony of 100 bacteria doubles every hour, after 24 hours there will be more than 1.5 billion bacteria.

Conversely, microbial death or inactivation follows a logarithmic decay pattern, which is essentially the opposite of exponential growth. Here, the number of microorganisms decreases over time, but the lower the number of microorganisms, the slower the rate of death.

For example, if a particular disinfectant kills 90% of germs every minute, only 10% of the original germs remain after one minute. After the next minute, 10% of the remaining 10% (or 1% of the original amount) remains.

Because of this logarithmic decay pattern, it is impossible to claim that 100% of the microbial population can be killed. All science can say is that the microbial load can be reduced by a percentage of the initial population. This is why most disinfectants sold for home use are labeled to 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 impact

As with much science, things get a little more complicated in the real world than in the lab. There are many other factors to consider when evaluating how well a disinfectant can remove microorganisms from surfaces.

One of these factors is the size of the initial microbial population you are trying to remove. This means that the more contaminated the surface, the more powerful the disinfectant needs to work to remove microorganisms.

For example, if you start with only 100 microorganisms on a surface or object and use a disinfectant to remove 99.9% of them, you can be very confident that you have effectively removed all microorganisms from that surface or object. (This is called sterilization.)

In contrast, if you have a large initial microbial population of hundreds of millions or billions of microorganisms contaminating a surface, even if you reduce the microbial load by 99.9%, there are still potentially millions of microorganisms contaminating the surface. It may mean that the

Time is an important factor in determining how effectively microorganisms are killed. Therefore, exposing highly contaminated surfaces to disinfectants for long periods of time is one way to ensure that larger microbial populations are killed.

This is why if you look closely at the labels of common household disinfectants, they will tell you to apply the product, wait a certain amount of time, and then wipe it off to disinfect. Therefore, always check the label of the product you are using.

Other factors such as temperature, humidity, and surface type also affect how effective disinfectants are outside of the lab.

Similarly, real-world microorganisms may be more or less sensitive to disinfection than microorganisms used in laboratory tests.

Disinfectants are part of infection control measures

The judicious use of disinfectants plays an important role in our daily lives in reducing exposure to pathogens (disease-causing microorganisms). Therefore, they can reduce our chances of getting sick.

The fact that disinfectants cannot be proven to be 100% effective from a scientific perspective in no way diminishes their importance in infection control. However, their use should always be complemented by other infection prevention measures, 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.