She loves to swim in the big pee bath (Image: AP)

Pools are giant toilets. Around one in five Americans admit to having peed in a swimming pools at least once. Every pool probably contains a shot glass full of urine per swimmer, according to a story by Chemical and Engineering News from last year.

The people around you might not know you peed. But thanks to the artificial sweeteners you eat, science knows. Science always knows.

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Sure, people think pee is sterile (it’s not), but its chemicals can react with the pool water to form so-called “disinfectant byproducts,” or DBPs. Some of these chemicals can even be toxic, but that C&EN story reports that we don’t know if swimmers are exposed to enough DBP in pools to cause health effects. Regardless, it would be nice to know how much pee was in the pool so more people don’t pee in it. According to a recent study in Environmental Science and Technology Letters, the answer might be in the artificial sweeteners you eat.

A team of researchers looked at one common sweetener in particular, called acesulfame-potassium, or Ace-K. People eat lots of Ace-K, because it shows up in diet sodas, candy, Jell-O, and plenty of other products, wrote Bill Chameides, a Duke University professor emeritus. Even better, almost all the Ace-K you eat, 95 percent of it, leaves the body in the urine, Xing-Fang Li, professor in the Department of Laboratory Medicine and Pathology at the University of Alberta told Gizmodo. Using it for the study “makes sense,” she said. “It’s representative of urine, and it’s not naturally occurring.”

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The simple analysis Li and her team performed involved taking pool and hot tub samples and passing them through a high performance liquid chromatography-mass spectrometer system, one of the most common instruments for determining what chemicals are in a given thing. The bad news: 100 percent of the tested pools contained at least some Ace-K. Through some estimation, and by testing 20 Canadians’ urine samples, the researchers found 110,000 gallon pools contained around 8 gallons (30 liters) of urine on average, and 220,000 gallon pools contained around 18.5 gallons (70 liters) on average, Lindsay Blackstock, a University of Alberta graduate student and the study’s first author told Gizmodo.

That doesn’t mean you shouldn’t swim in the pool, though. “The benefits of keeping active through swimming outweigh the potential risks we use as rationale for the study,” said Blackstock. So, what does this mean for you? Li doesn’t want to shame anybody, but “this is really public education,” she said. “Just don’t do it, don’t pee in the pool!”

As far as peeing in oceans, that’s still fine. “The products excreted by humans are very similar to the products excreted by fish,” said Blackstock. “You’re sort of adding to the mix there so I wouldn’t be worried about that.”

Correction: This post has been updated to reflect the fact that urine is not sterile.

[Environmental Science Technology Letters]