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

What is a disinfectant?

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

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

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

Conversely, the killing or inactivating of microbes follows a logarithmic decay pattern, which is essentially the opposite of exponential growth. Here, while the number of microbes decreases over time, the rate of death becomes slower as the number of microbes becomes smaller.

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 that remaining 10% (or 1% of the original amount) will remain, and so on.

Disinfectants are one part of infection control

The sensible use of disinfectants plays an important role in our daily lives in reducing our exposure to pathogens (microbes that cause illness). They can therefore reduce our chances of getting sick.

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

The answer to this conundrum requires understanding a bit of microbiology and a bit of mathematics.

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

Time is a key factor that determines how effectively microbes are killed. So exposing a highly contaminated surface to disinfectant for a longer period is one way to ensure you kill more of the microbial population.

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

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This is why, if you look closely at the labels of many common household disinfectants, they will often suggest that to disinfect you should apply the product then wait a specified time before wiping clean. So always consult the label on the product you’re using.

Surely, in a world where science is able to do all sorts of amazing things, someone would have invented a disinfectant that is 100% effective?

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