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

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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.

Davor Geber/Shutterstock

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.

Gloved hands spraying and wiping a surface.
Diseinfectants can contain a range of ingredients. Maridav/Shutterstock

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.

A tub of cleaning supplies.
You might have noticed none of the cleaning products in your laundry cupboard kill 100% of germs. Africa Studio/Shutterstock

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.

A woman cleaning a kitchen counter with a pink cloth.
Disinfectants won’t necessarily work in your kitchen exactly like they work in a lab. Ground Picture/Shutterstock

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.

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  • Banana vs Goji Berries – Which is Healthier?

    10almonds is reader-supported. We may, at no cost to you, receive a portion of sales if you purchase a product through a link in this article.

    Our Verdict

    When comparing banana to goji berries, we picked the goji berries.

    Why?

    Both are great! But…

    In terms of macros, goji berries have much more fiber, carbs, and protein, thus making it the most nutrient-dense option, as we might expect from a dried fruit being compared to a non-dried fruit—since the non-dried fruit has water weight that the dried fruit doesn’t, its percentages of other things will be proportionally lower, because the percentages must still add up to 100%, and if 75% is water (as is the case for bananas, compared to goji berries’ 7.5% water), then that only leaves 25% to work with, while goji berries have 92.5% to work with. In short, an easy and expected win for goji berries.

    In the category of vitamins, bananas have more of vitamin B6, while goji berries have more of vitamins A, B1, B3, B5, B9, C, E, and K. A clear win for goji berries.

    When it comes to minerals, bananas are not higher in any minerals, while goji berries have more calcium, copper, iron, magnesium, manganese, phosphorus, potassium, selenium, and zinc. Another easy win for goji berries.

    As for polyphenols, you may well imagine that the brightly-colored bitter-tasting berries have more, and you’d be right; you can read more about the exciting phytochemical properties of goji berries in the links below.

    Meanwhile, adding up the sections show a clear overall win for goji berries, but by all means enjoy either or both; diversity is good!

    Want to learn more?

    You might like:

    Enjoy!

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  • Welcoming the Unwelcome – by Pema Chödrön

    10almonds is reader-supported. We may, at no cost to you, receive a portion of sales if you purchase a product through a link in this article.

    There’s a lot in life that we don’t get to choose. Some things we have zero control over, like the weather. Others, we can only influence, like our health. Still yet others might give us an illusion of control, only to snatch it away, like a financial reversal or a bereavement.

    How, then, to suffer those “slings and arrows of outrageous fortune” and come through the other side with an even mind and a whole heart?

    Author Pema Chödrön has a guidebook for us.

    Quick note: this book does not require the reader to have any particular religious faith to enjoy its benefits, but the author is a nun. As such, the way she describes things is generally within the frame of her religion. So that’s a thing to be aware of in case it might bother you. That said…

    The largest part of her approach is one that psychology might describe as rational emotive behavioral therapy.

    As such, we are encouraged to indeed “meet with triumph and disaster, and treat those two imposters just the same”, and more importantly, she lays out the tools for us to do so.

    Does this mean not caring? No! Quite the opposite. It is expected, and even encouraged, that we might care very much. But: this book looks at how to care and remain compassionate, to others and to ourselves.

    For Chödrön, welcoming the unwelcome is about de-toothing hardship by accepting it as a part of the complex tapestry of life, rather than something to be endured.

    Bottom line: this book can greatly increase the reader’s ability to “go placidly amid the noise and haste” and bring peace to an often hectic world—starting with our own.

    Click here to check out Welcoming the Unwelcome, and learn what’s practically a superpower in this sometimes crazy world.

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  • Entertaining Harissa Traybake

    10almonds is reader-supported. We may, at no cost to you, receive a portion of sales if you purchase a product through a link in this article.

    No, it’s not entertaining in the sense that it will tell you jokes or perhaps dance for you, but rather: it can be easily prepared in advance, kept in the fridge for up to 3 days, and reheated when needed as part of a spread when entertaining, leaving you more time to spend with your houseguests

    Aside from its convenience, it is of course nutritious and delicious:

    You will need

    • 14 oz cherry tomatoes
    • 2 cans chickpeas, drained and rinsed (or 2 cups cooked chickpeas, drained and rinsed)
    • 2 eggplants, cut into ¾” cubes
    • 1 red onion, roughly chopped
    • 1 bulb garlic
    • 2 tbsp extra virgin olive oil
    • 1 tbsp harissa paste
    • 1 tbsp ras el-hanout
    • 1 tsp MSG or 2 tsp low-sodium salt

    Method

    (we suggest you read everything at least once before doing anything)

    1) Preheat the oven to 400℉ / 200℃

    2) Mix the onion, eggplant, and garlic (whole cloves; just peel them and put them in) with the olive oil in a mixing bowl, ensuring everything is coated evenly.

    3) Add in 1 tbsp of the harissa paste, 1 tbsp of the ras-el hanout, and half of the MSG/salt, and again mix thoroughly to coat evenly.

    4) Bake in the oven, in a walled tray, for about 30 minutes, giving things a stir/jiggle halfway through to ensure they cook evenly.

    5) Add the cherry tomatoes to the tray, and return to the oven for another 10 minutes.

    6) Mix the chickpeas with the other 1 tbsp of the harissa paste, the other 1 tbsp of the ras-el hanout, and the other half of the MSG/salt, and add to the tray, returning it to the oven for a final 10 minutes.

    7) Serve hot, or set aside for later, refrigerating once cool enough to do so. When you do serve, we recommend serving with a yogurt, cucumber, and mint dip, and perhaps flatbreads (you can use our Healthy Homemade Flatbreads recipe):

    Enjoy!

    Want to learn more?

    For those interested in some of the science of what we have going on today:

    Take care!

    Share This Post

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  • Is still water better for you than sparkling water?

    10almonds is reader-supported. We may, at no cost to you, receive a portion of sales if you purchase a product through a link in this article.

    Still or sparkling? It’s a question you’ll commonly hear in a café or restaurant and you probably have a preference. But is there any difference for your health?

    If you love the fizz, here’s why you don’t have to pass on the sparkling water.

    Brent Hofacker/Shutterstock

    What makes my water sparkle?

    This article specifically focuses on comparing still filtered water to carbonated filtered water (called “sparkling water” or “unflavoured seltzer”). Soda water, mineral water, tonic water and flavoured water are similar, but not the same product.

    The bubbles in sparkling water are created by adding carbon dioxide to filtered water. It reacts to produce carbonic acid, which makes sparkling water more acidic (a pH of about 3.5) than still (closer to neutral, with a pH around 6.5-8.5).

    Which drink is healthiest?

    Water is the best way to hydrate our bodies. Research shows when it comes to hydration, still and sparkling water are equally effective.

    Some people believe water is healthier when it comes from a sealed bottle. But in Australia, tap water is monitored very carefully. Unlike bottled water, it also has the added benefit of fluoride, which can help protect young children against tooth decay and cavities.

    Sparkling or still water is always better than artificially sweetened flavoured drinks or juices.

    Isn’t soda water bad for my teeth and bones?

    There’s no evidence sparkling water damages your bones. While drinking a lot of soft drinks is linked to increased fractures, this is largely due to their association with higher rates of obesity.

    Sparkling water is more acidic than still water, and acidity can soften the teeth’s enamel. Usually this is not something to be too worried about, unless it is mixed with sugar or citrus, which has much higher levels of acidity and can harm teeth.

    However, if you grind your teeth often, the softening could enhance the damage it causes. If you’re undertaking a home whitening process, sparkling water might discolour your teeth.

    In most other cases, it would take a lot of sparkling water to pass by the teeth, for a long period of time, to cause any noticeable damage.

    How does drinking water affect digestion?

    There is a misconception drinking water (of any kind) with a meal is bad for digestion.

    While theoretically water could dilute stomach acid (which breaks down food), the practice of drinking it doesn’t appear to have any negative effect. Your digestive system simply adapts to the consistency of the meal.

    Some people do find that carbonated beverages cause some stomach upset. This is due to the build-up of gases, which can cause bloating, cramping and discomfort. For people with an overactive bladder, the acidity might also aggravate the urinary system.

    Interestingly, the fizzy “buzz” you feel in your mouth from sparkling water fades the more you drink it.

    Is cold water harder to digest?

    You’ve chosen still or sparkling water. What about its temperature?

    There are surprisingly few studies about the effect of drinking cold water compared to room temperature. There is some evidence colder water (at two degrees Celsius) might inhibit gastric contractions and slow down digestion. Ice water may constrict blood vessels and cause cramping.

    However other research suggests drinking cold water might temporarily boost metabolism, as the body needs to expend energy to warm it up to body temperature. This effect is minimal and unlikely to lead to significant weight loss.

    Which water wins?

    The bottom line is water is essential, hydrates us and has countless other health benefits. Water, with carbonated bubbles or without, will always be the healthiest drink to choose.

    And if you’re concerned about any impact to teeth enamel, one trick is to follow sparkling water with a glass of still. This helps rinse the teeth and return your mouth’s acidity back to normal.

    Christian Moro, Associate Professor of Science & Medicine, Bond University and Charlotte Phelps, Senior Teaching Fellow, Medical Program, Bond University

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

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  • 5 Minute Posture Improvement Routine!

    10almonds is reader-supported. We may, at no cost to you, receive a portion of sales if you purchase a product through a link in this article.

    McKay Lang walks us through it:

    Step by Step

    Breathing exercise:

    • Place your hands on your lower abdomen.
    • Take three deep breaths, focusing on body tension in the shoulders and neck… And release.

    Shoulder squeeze:

    • With your hands on your hips, inhale and squeeze your shoulders upwards.
    • Hold your breath for 3–4 seconds, then exhale.
    • Repeat two more times, holding the squeeze a little longer each time.

    Upper shoulder massage:

    • Massage your upper shoulder muscles to release tension stored there.

    Overhead arm stretch:

    • Raise your arms above your head, clasping each elbow with the opposite hand.
    • Inhale deeply, stretch upwards, then exhale and release.
    • Repeat, alternating elbows.

    Neck and head push:

    • Place your palms on the back of the head, and push your head into your hands (and vice versa, because of Newton’s Third Law of Motion).
    • Do the same sideways (one side and then the other), to engage the other neck muscles.

    Cool down:

    • Gently unclasp your hands, bring your head upright, and massage your muscles. And breathe.

    For variations and a visual demonstration of all, enjoy:

    Click Here If The Embedded Video Doesn’t Load Automatically!

    Want to learn more?

    You might also like to read:

    6 Ways To Look After Your Back

    Take care!

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  • When Did You Last Have a Cognitive Health Check-Up?

    10almonds is reader-supported. We may, at no cost to you, receive a portion of sales if you purchase a product through a link in this article.

    When Did You Last Have a Cognitive Health Check-Up?

    Regular health check-ups are an important part of a good health regime, especially as we get older. But after you’ve been prodded, probed, sampled and so forth… When did you last have a cognitive health check-up?

    Keeping on top of things

    In our recent Monday Research Review main feature about citicoline, we noted that it has beneficial effects for a lot of measures of cognitive health.

    And that brought us to realize: just how on top of this are we?

    Your writer here today could tell you what her sleep was like on any night in the past year, what her heart rate was like, her weight, and all that. Moods too! There’s an app for that. But cognitive health? My last IQ test was in 2001, and I forget when my last memory test was.

    It’s important to know how we’re doing, or else how to we know if there has been some decline? We’ve talked previously about the benefits of brain-training of various kinds to improve cognition, so in some parts we’ll draw on the same resources today, but this time the focus is on getting quick measurements that we can retest regularly (mark the calendar!)

    Some quick-fire tests

    These tests are all free, quick, and accessible. Some of them will try to upsell you on other (i.e. paid) services; we leave that to your own discretion, but the things we’ll be using today are free.

    Test your verbal memory

    This one’s a random word list generator. It defaults to 12 words, but you can change that if you like. Memorize the words, and then test yourself by seeing how many you can write down from memory. If it gets too easy, crank up the numbers.

    Click here to try it now

    Test your visual memory

    This one’s a series of images; the test is to click to say whether you’ve seen this exact image previously in the series or not.

    Click here to try it now

    Test your IQ

    This one’s intended to be general purpose intelligence; in reality, IQ tests have their flaws too, but it’s not a bad metric to keep track of. Just don’t get too hung up on the outcome, and remember, your only competition is yourself!

    Click here to try it now

    Test your attention / focus

    This writer opened this and this three other attention tests (to get you the best one) before getting distracted, noting the irony, and finally taking the test. Hopefully you can do better!

    Click here to try it now

    Test your creativity

    This one’s a random object generator. Give yourself a set period of time (per your preference, but make a note of the time you allow yourself, so that you can use the same time period when you retest yourself at a later date) in which to list as many different possible uses for the item.

    Click here to try it now

    Test your musical sense

    This one’s a pitch recognition test. So, with the caveat that it is partially testing your hearing as well as your cognition, it’s a good one to take and regularly retest in any case.

    Click here to try it now

    How often should you retest?

    There’s not really any “should” here, but to offer some advice:

    • If you take them too often, you might find you get bored of doing so and stop, essentially burning out.
    • If you don’t take them regularly, you may forget, lose this list of tests, etc.
    • Likely a good “sweet spot” is quarterly or six-monthly, but there’s nothing wrong with testing annually either.

    It’s all about the big picture, after all.

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