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

Have you ever wondered why many disinfectants claim to kill 99.9% or 99.99% of germs, but never promise to eliminate them all? Perhaps the thought crossed your mind in the middle of cleaning your kitchen or bathroom.

Surely, in a world where science can do all kinds of amazing things, could anyone have invented a 100% effective disinfectant?

The answer to this conundrum requires an understanding of microbiology and a little math.

What is a disinfectant?

A disinfectant is a substance used to kill or inactivate bacteria, viruses, and other organisms on non-living things.

There are millions of microbes on the surface of the water and things in our home environment. Although most microbes are harmless (and some are good for us), a small percentage can make us sick.

Although disinfection may involve physical interventions such as heat treatment or the use of UV light, most often when we think of germs we are talking about the use of chemicals to kill germs. which are on the surface of water or in objects.

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

Numbers to eliminate germs

Over the past few years we have all become familiar with the idea of ​​a dramatic increase in the number of cases of COVID.

This is where the numbers grow at an increasing rate, which can lead to an explosion of size very quickly. For example, if a colony of 100 bacteria doubles every hour, in a 24-hour period the number of bacteria would exceed 1.5 billion.

On the other hand, the killing or inactivation of organisms follows a logarithmic decay pattern, which is actually the opposite of exponential growth. Here, as the number of parasites decreases over time, the death rate slows down as the number of parasites decreases.

For example, if a certain 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 original amount) will remain, and so on.

Because of this logarithmic decay method, it is impossible to say that you can kill 100% of any germs. You can only say scientifically that you are able to reduce the burden of germs by a fraction of the original population. This is why most disinfectants sold for home use are shown to kill 99.9% of germs.

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

Real world consequences

Like most sciences, things get more complicated in the real world than they are in the lab. There are several other factors that should be considered when evaluating the effectiveness of a disinfectant.

One of these factors is the size of the initial population of viruses you are trying to eliminate. That is, the dirtier the surface, the harder the disinfectant needs to work to kill the germs.

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

On the other hand, if you have a large population of hundreds of millions or billions of microbes contaminating the surface, even reducing the microbial load by 99.9% may mean that there are millions of microbes left on the surface of the water.

Time is an important factor in how germs are killed. So exposing a heavily infected area to a disinfectant for a longer period of time is one way to ensure that you kill the majority of germs.

This is why if you look carefully at the labels of many common household disinfectants, they will often suggest that to kill germs you should use the product and wait a certain time before wiping. So always check the label of the product you are using.

Other factors such as temperature, humidity and surface type also affect how well a disinfectant works outside the lab.

Similarly, real-world pathogens may be less or less sensitive to antimicrobials than those used for laboratory testing.

Antibiotics are one aspect of infection control

The rational use of germs plays an important role in our daily life to reduce our exposure to germs (pathogens that cause disease). Therefore they can reduce the chances of getting sick.

The fact that antibiotics cannot be shown to be 100% effective from a scientific point of view in no way detracts from their importance in infection control. But their use should always be complemented by other infection control methods, such as hand washing, to reduce the risk of infection.