Have you ever wondered why most disinfectants indicate they kill 99.9% or 99.99% of germs, but never promise to wipe out all of them? Perhaps the thought has crossed your mind mid-way through cleaning your kitchen or bathroom.

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?

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

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 domestic environment. While most microbes 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, typically when we think of disinfectants we are referring 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 which can target vital components of different microbes to kill them.

The maths of microbial elimination

In the past few years we’ve all become familiar with the concept of exponential growth in the context of the spread of COVID cases.

This is where numbers grow at an ever-accelerating rate, which can lead to an explosion in the size of something very quickly. For example, if a colony of 100 bacteria doubles every hour, in 24 hours’ time the population of bacteria would be more than 1.5 billion.

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.

Because of this logarithmic decay pattern, it’s not possible to ever claim you can kill 100% of any microbial population. You can only ever scientifically say that you are able to reduce the microbial load by a proportion of the initial population. This is why most disinfectants sold for domestic use indicate they kill 99.9% of germs.

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

Real-world implications

As with a lot of science, things get a bit more complicated in the real world than they are in the laboratory. There are a number of other factors to consider when assessing how well a disinfectant is likely to remove microbes from a surface.

One of these factors is the size of the initial microbial population that you’re trying to get rid of. That is, the more contaminated a surface is, the harder the disinfectant needs to work to eliminate the microbes.

If for example you were to start off with only 100 microbes on a surface or object, and you removed 99.9% of these using a disinfectant, you could have a lot of confidence that you have effectively removed all the microbes from that surface or object (called sterilisation).

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

Time is 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.

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.

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

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

Disinfectants are one part 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.

The fact disinfectants can’t be shown to be 100% effective from a scientific perspective in no way detracts from their importance in infection control. But their use should always be complemented by other infection control practices, such as hand washing, to reduce the risk of infection.

Hassan Vally, Associate Professor, Epidemiology, Deakin University

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

Pratik Raul, University of Canberra; Jeroen van Boxtel, University of Canberra, and Jovana Acevska, University of Canberra

Autism is a neurodevelopmental difference associated with specific experiences and characteristics.

For decades, autism research has focused on behavioural, cognitive, social and communication difficulties. These studies highlighted how autistic people face issues with everyday tasks that allistic (meaning non-autistic) people do not. Some difficulties may include recognising emotions or social cues.

But some research, including our own study, has explored specific advantages in autism. Studies have shown that in some cognitive tasks, autistic people perform better than allistic people. Autistic people may have greater success in identifying a simple shape embedded within a more complex design, arranging blocks of different shapes and colours, or spotting an object within a cluttered visual environment (similar to Where’s Wally?). Such enhanced performance has been recorded in babies as young as nine months who show emerging signs of autism.

How and why do autistic individuals do so well on these tasks? The answer may be surprising: more “neural noise”.

What is neural noise?

Generally, when you think of noise, you probably think of auditory noise, the ups and downs in the amplitude of sound frequencies we hear.

A similar thing happens in the brain with random fluctuations in neural activity. This is called neural noise.

This noise is always present, and comes on top of any brain activity caused by things we see, hear, smell and touch. This means that in the brain, an identical stimulus that is presented multiple times won’t cause exactly the same activity. Sometimes the brain is more active, sometimes less. In fact, even the response to a single stimulus or event will fluctuate continuously.

Neural noise in autism

There are many sources of neural noise in the brain. These include how the neurons become excited and calm again, changes in attention and arousal levels, and biochemical processes at the cellular level, among others. An allistic brain has mechanisms to manage and use this noise. For instance, cells in the hippocampus (the brain’s memory system) can make use of neural noise to enhance memory encoding and recall.

Evidence for high neural noise in autism can be seen in electroencephalography (EEG) recordings, where increased levels of neural fluctuations were observed in autistic children. This means their neural activity is less predictable, showing a wider range of activity (higher ups and downs) in response to the same stimulus.

In simple terms, if we imagine the EEG responses like a sound wave, we would expect to see small ups and downs (amplitude) in allistic brains each time they encounter a stimulus. But autistic brains seem to show bigger ups and downs, demonstrating greater amplitude of neural noise.

Many studies have linked this noisy autistic brain with cognitive, social and behavioural difficulties.

But could noise be a bonus?

The diagnosis of autism has a long clinical history. A shift from the medical to a more social model has also seen advocacy for it to be reframed as a difference, rather than a disorder or deficit. This change has also entered autism research. Neuroaffirming research can examine the uniqueness and strengths of neurodivergence.

Psychology and perception researcher David Simmons and colleagues at the University of Glasgow were the first to suggest that while high neural noise is generally a disadvantage in autism, it can sometimes provide benefits due to a phenomenon called stochastic resonance. This is where optimal amounts of noise can enhance performance. In line with this theory, high neural noise in the autistic brain might enhance performance for some cognitive tasks.

Our 2023 research explores this idea. We recruited participants from the general population and investigated their performance on letter-detection tasks. At the same time, we measured their level of autistic traits.

We performed two letter-detection experiments (one in a lab and one online) where participants had to identify a letter when displayed among background visual static of various intensities.

By using the static, we added additional visual noise to the neural noise already present in our participants’ brains. We hypothesised the visual noise would push participants with low internal brain noise (or low autistic traits) to perform better (as suggested by previous research on stochastic resonance). The more interesting prediction was that noise would not help individuals who already had a lot of brain noise (that is, those with high autistic traits), because their own neural noise already ensured optimal performance.

Indeed, one of our experiments showed people with high neural noise (high autistic traits) did not benefit from additional noise. Moreover, they showed superior performance (greater accuracy) relative to people with low neural noise when the added visual static was low. This suggests their own neural noise already caused a natural stochastic resonance effect, resulting in better performance.

It is important to note we did not include clinically diagnosed autistic participants, but overall, we showed the theory of enhanced performance due to stochastic resonance in autism has merits.

Why this is important?

Autistic people face ignorance, prejudice and discrimination that can harm wellbeing. Poor mental and physical health, reduced social connections and increased “camouflaging” of autistic traits are some of the negative impacts that autistic people face.

So, research underlining and investigating the strengths inherent in autism can help reduce stigma, allow autistic people to be themselves and acknowledge autistic people do not require “fixing”.

The autistic brain is different. It comes with limitations, but it also has its strengths.

Pratik Raul, PhD candidiate, University of Canberra; Jeroen van Boxtel, Associate professor, University of Canberra, and Jovana Acevska, Honours Graduate Student, University of Canberra

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