Why Disinfectants Typically Target Only 99.9% of Germs According to Their Labels

[Un article de The Conversation écrit par Hassan Vally – Professeur associé, épidémiologie, Deakin University]

In a world where science is doing so many incredible things, how is it possible that there is no 100% effective disinfectant?

To understand it, it is necessary to assimilate some elements of microbiology and mathematics. Explanations.

What is a disinfectant?

A disinfectant is a substance used to kill or inactivate bacteria, viruses and other microbes that are present on surfaces and objects around us. And there are many of them: there are literally millions of them in our domestic environment. Although most microbes are not harmful (and some are even beneficial), a small proportion of them can make us sick.

To disinfect our environment, it is possible to use physical interventions such as heat treatment or ultraviolet (UV) light.

However, when we think of disinfectants, we usually think of chemicals intended to kill germs.

These chemical disinfectants often contain so-called “active” ingredients such as alcohols, chlorine compounds and hydrogen peroxide (“oxygenated water”). Their role is to target essential components of microbes, in order to eliminate them.

The Mathematics of Microbial Elimination

In recent years, in the context of the spread of Covid-19 cases, we have become familiar with the concept of exponential growth, which translates into an increasingly rapid rate of increase.

Such growth can cause an explosion in the size of a population in a very short time. Thus, if the members of a colony of 100 bacteria double every hour, in 24 hours, this bacterial population will exceed 1.5 billion individuals.

Conversely, the destruction or inactivation of microbes follows a pattern of logarithmic decay, which in essence can be considered the opposite of exponential growth. In this case, the amount of microbes decreases over time, but the rate of destruction slows down as their number becomes lower.

For example, if a given disinfectant kills 90% of bacteria every minute, after one minute only 10% of the original bacteria will remain. After the next minute, only 10% of the remaining 10% will remain (i.e. 1% of the initial quantity), and so on.

Due to this logarithmic decay pattern, it is not possible to scientifically state that 100% of a microbial population can be eliminated. Indeed, we can only say 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.

This is also the case with other products, such as hand sanitizers and disinfectant wipes, which also often claim to eliminate 99.9% of germs.

Implications in real conditions

As is often the case in science, things get complicated when you leave the laboratory for the real world. Indeed, when it comes to evaluating the effectiveness of a disinfectant on a surface, various other factors come into play.

One of these factors is the size of the initial microbial population. The more contaminated a surface is, the more effective the disinfectant must be in eliminating microbes.

So, if a surface is contaminated with only 100 microbes and you eliminate 99.9% of them with a disinfectant, you can consider with a high level of confidence that all the microbes will have been eliminated (this is what (this is called sterilization).

In contrast, if a surface is contaminated with hundreds of millions or billions of microbes, even reducing the microbial load by 99.9%, it is likely that millions of microbes will still remain after disinfection.

To improve disposal efficiency, time is a key factor. It is possible to destroy a greater proportion of the microbial population living on a heavily contaminated surface by exposing it to the disinfectant for a longer period of time.

This is why you will find a statement on the labels of most household disinfectants recommending that you apply the product and then wait a certain amount of time before wiping. So always be sure to read the instructions for use carefully.

Outside of the laboratory, other factors, such as temperature, humidity and surface type, also influence the effectiveness of disinfectants.

Finally, in real conditions, “wild” microbes may prove to be more or less sensitive to disinfection than the strains used for laboratory tests.

Disinfectants and infection control

The fact that disinfectants cannot be considered 100% effective from a scientific point of view does not diminish their importance in infection control.

Their reasoned use plays an important role in our daily lives. Indeed, by reducing our exposure to pathogens, in other words the microbes that cause disease, these products reduce the risk of us getting sick.

However, to reduce the risk of infection, their use must always be supplemented by other control methods, such as hand washing.