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If you’ve ever been a competitive swimmer – or, just spent a lot of time at the pool – you might have peed in the water once or twice.

Or maybe a lot. According to interviews, Olympic swimmers pee in the pool all the time.

Even though it sounds disgusting, a lot of people say urine is sterile, plus chlorine is a disinfectant. So what’s a little pee between friends… right? Well, you might actually wanna take your business to the bathroom.

For one, it turns out that pee isn’t all that sterile. But there’s another problem, too: Mixing urine with the chlorine in your pool can make chemicals that might cause respiratory and nervous system problems. Urine is mostly water, but it contains a lot of junk your body doesn’t want anymore, including uric acid and urea, both nitrogen-containing molecules.

Uric acid is produced when your body breaks down molecules called purines, like the ones in some of your DNA bases. And urea is made from the breakdown of proteins. When these waste molecules mix with chlorine in the pool that’s meant to destroy bacteria and viruses, they react to form disinfection by-products, or DBPs for short.

Specifically, urea reacts to create a type of chemical called chloramines, swapping out its hydrogen atoms for chlorine atoms. Trichloramine, especially, is pretty reactive, and can corrode the metal in and around pools. And you might know it by its smell – that classic chemical “pool smell” is caused by chloramine gases, not chlorine.

A lot of people, like lifeguards, have reported they get red eyes, a runny nose, or a scratchy voice after being around the pool too much, which could potentially have something to do with irritation from lots of trichloramine. Some researchers think chloramines could cause respiratory problems in swimmers, too, since they probably breathe in a bunch more than your average pool-goer. But we’ll need to do more research to really understand the health effects.

We’ve known about the connection between urea and trichloramine for a while now, but we’ve pretty recently found a link between uric acid and a molecule called cyanogen chloride. Cyanogen chloride gas doesn’t have a familiar smell, but it’s real bad news and can cause respiratory, cardiovascular, and central nervous system problems. It’s part of a group of chemicals called cyanides, which all have a carbon atom bonded to a nitrogen atom.

They’re toxins, and nasty ones at that. These chemicals mess with how your cells use oxygen, so your cells struggle to produce energy, and if the concentrations are high enough, all kinds of things can go wrong. In one study, published in the journal Environmental Science & Technology in 2014, researchers created synthetic urine and combined it with various concentrations of chlorine.

And within an hour, the uric acid created some cyanogen chloride. The amount varied based on the chlorine concentration they used, but it was around 2 to 8 milligrams per liter. Now, there aren’t many official guidelines about what concentration of liquid cyanogen chloride is dangerous, but some sources recommend avoiding exposure to more than 0.6 milligrams per cubic meter of the gas form.

So getting 2 to 8 milligrams per liter of cyanogen chloride sounds like a huge deal. But it’s pretty unlikely you’ll ever find that much in your swimming pool, because this experiment used higher concentrations of chlorine than you’d find outside of a lab. So, you probably don’t need to panic if you go to a pool party every once in a while.

DBP levels might be an issue at large swim meets, where hundreds of swimmers are probably peeing in the water. Especially if a lot of people are peeing in the same spot – say, at the foot of the diving board – that area will have higher concentrations of DBPs like trichloramine or cyanogen chloride. And researchers are trying to figure out if long-term exposure to DBPs are related to the unusually high amounts of asthma reported among competitive swimmers, which has been documented in multiple surveys.

For now, it looks like DBPs probably aren’t a life-or-death issue in the pool, although they’re not harmless either. So getting out of the pool, drying off, and going inside to use the bathroom is kind of a pain, but your lungs – and probably your friends – will thank you. And thank you for watching this episode of SciShow!

If you want to impress your friends with science at the pool that you’re definitely not gonna pee in, you can get your very own SciShow beach towel, available at dftba.com/scishow.