In today’s #SkepticalTuesdays post, I tackle the age-old question on just how to set up your toilet before you flush.

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Seat up or down? If you ask the Mythbusters, it doesn’t really matter. In fact, when you bring up the question on reddit’s r/skeptic community, that’s what most of us will probably tell you, too. And in a sense we’re right. But as always, there’s more to it.

What we’re talking about, of course, isn’t politeness or the battle of the sexes. I don’t really care whether it’s chivalrous to put the seat down or not (though, as a “medievalist by day,” I can actually tell you that no, that has nothing to do with chivalry). What we’re talking about is aerosolized poop.

Back in ’99, The Straight Dope did a good little explainer on the issue, referencing the research of Dr. Charles Gerba. The long and short of it is that fecal bacteria are everywhere. In your bathroom, sure, but also in your kitchen, in your laundry room — pretty much everywhere.

In more recent years, the Mythbusters did a show on the topic — as well as a nice mini-myth on toothbrush placement — that seemed to show that pretty much no matter what you did, there’d be tiny amount of fecal bacteria on your toothbrush. Even if you kept it in the kitchen under a glass dome.

But one thing really stuck in my craw about the Mythbusters’ experiment, and it was that their toilet was one of those weird institutional toilets that doesn’t have a lid. In my house, we always solved the “seat up or down” debates by closing the seat and the lid, on the “a compromise is a solution in which nobody wins” model. But also, I thought, maybe this would keep the amount of aerosolized fecal matter down to a minimum as well.

On the other hand, some people suggested that closing the lid would actually make things worse, because it would somehow disperse more — but remember that aerosolization isn’t the same thing as pressurization. The inside of your toilet bowl isn’t suddenly going to experience a spike in pressure unless you have a high-pressure water injection system (and maybe, like a hospital, you do). But in mine, the water falls out of a small tank and spirals down the hole along with whatever’s in there, so my inclination has always been that you wouldn’t be creating a bacterial spray-nozzle around the edges.

And wouldn’t you know, someone did a study on this exact thing.

Published in the Journal of Hospital Infection in January of 2012, the study, entitled “Potential for aerosolization of Clostridium difficile after flushing toilets: the role of toilet lids in reducing environmental contamination risk,” examined the question of C. diff spread with open and closed toilet lids over time. Since it’s behind a paywall, I’ll do a little description:

The researchers selected a standard lidded toilet at the Leeds Teaching Hospitals Trust and disinfected the place with bleach and then neutralized the bleach with sodium thiosulphate (plus flushing the sanitized bowl four times with the lid down after that, just to make sure of a controlled environment). They did this between runs as well.

Then they took C. diff-free feces from five volunteers and made a uniform fecal mixture (sounds like a fun time), as well as culturing C. diff to add to it, so that for each flush the exact same feces and the exact same concentrations of C. diff were present. This was their C. diff infection diarrhea simulant.

They recorded levels of aerosolized C. diff from air samples at the seat height as well as 10cm above the handle height (25cm above the seat). They also placed C. diff-selective agar plates around the bathroom in six places — on the right and left sides of the seat, on top of the tank, and in three places around the floor — which were taken for testing after 90 minutes following each flush. They also ran controls, including leaving agar plates out for 24 hours beforehand without any flushing (which found no C. diff when they hadn’t added any).

Then they tested with the lid up and down, multiple times.

Now, for the aerosolization aspect, C. diff was found with both the lid up and down, suggesting that some aerosolized particles were making their way past the lid. However, with the lid down the amount in the air was greatly reduced by comparison with the lid up. But the really interesting thing is that with the lid down, all of the agar plates came back with no settled C. diff on them, while the amounts with the lid up were much higher. So what does this mean for our question?

One, it means your lid isn’t airtight, and you’re not going to be keeping all the fecal bacteria in there. Two, it means that closing the lid does seem to reduce the effect, especially reducing the contamination of horizontal surfaces after the fact.

So here’s my TL;DR for you — If you want less fecal bacteria around your home, close the lid. It’ll probably help. If you want no fecal bacteria around your home, invest in technology to upload your brain to a computer, because you’re living inside a walking, talking disease factory, and at least some degree of contamination is essentially inevitable.

Have a great week, all.

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Richard Ford Burley is a doctoral candidate in English at Boston College, where he’s writing about remix culture and the processes that generate texts in the Middle Ages and on the internet. In his spare time he writes about science and skepticism here at This Week In Tomorrow.