Hi, I’m Imogene Cancellare, I’m a big Cat scientist and  I’m a new host with Sci Show Chances are, if you’ve ever had  a fight with your roommate, i t was over dishes.

We’ve all argued about when to  throw out that ratty old sponge. Or, how long you can really get away  with leaving your dirty bowl in the sink.

Everyone cleans dishes differently. But is there a right way to do your dishes? A type of dish-maxxing, if you will?

Scientists have weighed in on this. Since at least the 1940s, we’ve been researching what washing techniques lead to the cleanest,  least-likely-to-get-you-sick, dishes. And, more recently, the most eco-friendly way too.

So, here is the right way to do your dishes, according to science. [intro music] Before we get too far into  the dish washing details, we need to establish if you’re  team human or team machine. If you ask my uncle MArcus, he’ll kiss  his bicep and say he is the dishwasher. And he’s not alone.

About a quarter of American households are not equipped with a dishwasher. And around 15% of those who  have one say they never use it. Whether it’s a matter of money, space, or habit, some folks just don’t use a dishwasher. … But their dishes are probably dirtier.

This is based on studies comparing  hand washing versus dishwashers. Those researchers didn’t investigate  modern household dishwashing, but they still counted how  many bacteria grew on dishes that were washed by hand versus those  that were run through a dishwasher. And they found that dishwashers  were better at removing microbes. a study from the 80s found that handwashed dishes had 4x more microbial life growing on them than those that went through the dishwasher.

But that was after washing clean dishes. And the microbe count was so low at the end that it wasn’t really a meaningful difference. In this case, the higher microbial  load on hand washed dishes was probably from reusing  the same towel to dry them.

But the researchers also  mentioned that dishwashers run for longer and are able to reach temperatures that human hands can’t deal with. That might kill off more bacteria. So machines get the hygiene high ground.

But ideally that would be validated  in more realistic contexts. And if you think you’re  sacrificing a bit of cleanliness for the sake of the environment, since dishwashers are a huge  waste of energy… they’re not. A study in 2011 comparing  handwashing and machine dishwashing across six European countries estimated that dishwashers used half the energy, and produced half the greenhouse gas  emissions, that handwashers used.

That’s because it takes energy to  clean, move, and heat your water. The more water we use, the more energy we use, which means more carbon dioxide and methane getting released into our environment. And many people who do their dishes by hand typically use a constant stream of water.

So depending on what equipment is available to you and on your particular methods, dishwashers are probably more resource efficient. if you’re the exceptional hand washer  that doesn’t keep the tap running, we’ll get to you later in the video. But there are still a lot  of ways to use a dishwasher. Like, you might debate whether your dishes would have come out cleaner  had you pre-rinsed them.

If you pre-rinse your dishes, just stop. Stop it right now. We just established that dishwashers  are probably more ecofriendly, especially if you don’t use a bucket-load  of extra water to wash your dishes before sticking your plate in the machine.

Many dishwashers already have a pre-rinse cycle. So you’re pre-rinsing the  pre-rinse! It’s out of control!

And I hate to break it to you, but you’re not as efficient at  pre-rinsing as the machines are. One study found that machines typically use around 1 gallon of water on pre-rinsing. But people might use as much as  25 gallons to do the same thing.

I know you’re already writing  a detailed essay to me about your peanut-butter covered knife that the dishwasher simply can’t handle. And for that situation, I will concede that you’re right, based on lived experience and no formal research. A little sponge action makes a big difference.

But you probably don’t need to  pre-rinse every dish like that. Nobody eats that much peanut butter, right?! And if you find that your  dishes need some elbow grease to remove their leftover grease, the tools you use make a difference.

But before we get into that  debate, it’s time for a quick ad. This SciShow video is supported by Brilliant, the online personal tutor for math, science, and engineering students from  grade 5 through undergrad. Brilliant’s view is that AI is  supposed to be making us smarter.

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Instead, Koji will ask questions and guide  students to find the answer themselves. To get started with Brilliant’s tutor for free, click the link below or scan the QR code. You can upgrade to Premium to unlock all courses.

And right now, SciShow viewers can save 20% off an annual subscription at brilliant.org/scishow In a survey of 10 European countries, sponges were used on average  2.5 times more than brushes. But the Norwegians who hand-wash  their dishes really prefer brushes. And this might sound like a personal  preference that doesn’t make much difference, but the Norwegians might be onto something here.

In 2020, researchers compared microbial growth, including specific food pathogens like salmonella, on sponges and brushes. These pathogens can give you  the horrific double whammy of vomiting and diarrhea. So ideally we’d have very few of  those guys on our cleaning tools.

But Salmonella stuck around for a whole week on two of the three sponge  types the researchers looked at. And overall, they saw far more  bacteria on sponges than brushes. Like, the brushes ended up  with a level of contamination that an expert might consider slightly concerning, while the sponges got to the point  where slime can grow, which …ew.

Unfortunately, the major  reason sponges are so nasty is the same reason we like them for cleaning. They absorb things. They’re great at holding  soapy water while you scrub … but then they can keep holding onto  water for more than 24 hours afterward, depending on the kind of sponge and if you try to ring the water out or not.

Brushes, in comparison, tend to dry  out after four and a half hours. Faster drying brushes can help kill  microbes while those wet sponges become ideal habitats for  bacteria to survive and thrive. Which is why the researchers  advocate hard for the brush life.

But it can be hard to make a change like giving up the trusty sponge  you’ve used your whole life. And I mean the kind of sponge  you like to clean with. If you’ve been using the same  physical sponge your whole life, you definitely need to give that up.

So for folks in that boat, they  recommended cleaning sponges by soaking them in chlorine or boiling water. And as heated as the sponge vs. brush debate gets, it’s nothing compared with  this next item on our list. Maybe you remember the great TikTok suds debate.

You know, the global outrage at some British people who wash  their dishes with soapy water and then leave them to dry  without rinsing the soap off! I’m picking on Britain because I’m from the US, and we tend to wash off the suds here. But other countries, like  Brazil, seem to be on our side.

In a cross-cultural survey comparing  British households, Brazilian households, and migrants between the two, Brazilians said they rinsed their dishes. Multiple other respondents  reported not rinsing their dishes after giving them a good scrub. Sometimes, they used a  dishtowel to wipe them clean.

Other times, they just…let ‘em drip. This is baffling to me on face value. But it gets even whackier because  the British have studied this and found that a post rinse  leads to fewer microbes!

It was published in the  Cambridge University Press! You know how the whole point  of soap is to trap particles and wash them away? If you don’t do the “wash them away” part, then you might have removed many particles, but you’re still left with some that  re-grow bacteria after the wash.

This is in line with another study that found that using too much soap made dishes dirtier. Like, things got cleaner up to a certain point, and then the researchers think it got so foamy  that the particles weren’t being washed away. Because of all this, experts recommend that you  rinse your dishes in water after soaping them up.

So, combining all of this data,  there’s a right way to do your dishes. Earlier, we said that  dishwashers are more ecofriendly. And that’s true, most of the time.

But you can use fewer resources  with the two-tub-technique. Instead of running dishes  under a constant flow of water, experts recommend cleaning your dishes  in one tub filled with warm, soapy water, and then rinsing off the suds in another  tub filled with clean, cold water. And to save on how many times  you fill two tubs with water, wait until you have a critical mass of  dishes and then do them all at once.

So your college roommate  who left dishes out for days might have been onto something. A study that taught 53 Europeans  how to use this technique found that they used 60%  less water, 70% less energy, and 30% less detergent on average! That’s compared to previous  studies not using this technique.

So that’s the best way to wash  your dishes, according to science. But if it takes some time before  you fully implement these changes, you're not alone. Many of us hear the siren song of the sponge.

You’re wrong though. [ outro music ]