Chemists Decree: Don't Pee in the Pool

Urine reacts with the chlorine in swimming pools, creating potentially dangerous chemical byproducts.

Matt Dunham/AP

Here’s the thing. One-piece bathing suits, when wet, are very annoying to take off. And when you’re swimming three hours a day, as I did for practice on my high school swim team, climbing out of the pool, taking it off, and putting it back on every time you have to use the bathroom starts to feel burdensome. So maybe you just… go…somewhere in between the one millionth and one millionth and first lap you’ve swum that day.

Urine is sterile, and chlorine is sterilizing, right? This is the justification we offered ourselves, to counter our shame. Plus, decorated Olympic swimmers Michael Phelps and Ryan Lochte do it.

Turns out that was a pretty bad idea, for more reasons than just the ick factor. A new study published in the American Chemical Society’s journal Environmental Science & Technology, looked at the chemistry of what happens when urine meets chlorine, and it isn’t pretty.

The researchers mixed uric acid, found in both urine and sweat, with chlorine. Within an hour, they found that both trichloramine and cyanogen chloride had formed. These two chemicals are frequently found in chlorinated swimming pools—Ernest Blatchley III, one of the study’s authors, says that in the nearly 10 years he and his team have been studying swimming pool chemistry, they have found those two chemicals in every sample they’ve taken from a pool.

“We know that there are associations between some of these chemicals and adverse human health outcomes, so we’re motivated to understand the chemistry behind their formation and decay,” says Blatchley, a professor at the School of Civil Engineering and Division of Environmental and Ecological Engineering at Purdue University.

Exposure to trichloramine has been linked to respiratory problems, and cyanogen chloride can adversely affect the lungs, central nervous system, and cardiovascular system, according to the CDC.

Both of these chemicals contain nitrogen, Blatchley says, and uric acid also contains nitrogen, in a form the study’s authors suspected would be reactive with chlorine. They were right.

Though there is uric acid in sweat, it’s a pretty small amount, and it’s really urine we should be worried about, Blatchley says. Which is comforting, since if you’re swimming hard, there’s not much you can do about sweating, but you can always, you know, not pee in the pool. Some people do and some people don’t, but on average, a person leaves about 30 to 80 milliliters of urine behind each time they visit the pool.

A major concern, according to Blatchley, is times when there are a lot of people in one pool, such as at a swimming competition. In the presence of chlorine, cyanogen chloride in particular not only forms quickly, as shown in this study, but decays quickly as well. This means that if a lot of people are peeing in the pool, there’s the potential for a lot of cyanogen chloride to form, depleting the chlorine in the pool. While the cyanogen chloride would normally decay quickly, less chlorine means it might stick around longer, and that could be a real problem.

All of this is to say that peeing in the pool is not harmless, despite Phelps’ and Lochte’s claims that it’s normal and everybody does it.

“Chlorine kills it, so it’s not bad,” Phelps told the Wall Street Journal in 2012.

That gets Blatchley’s dander up.

“There’s a lot of people in the swimming community who look up to these people and listen to what they have to say,” he says. “[Phelps and Lochte] are not chemists and shouldn’t be making statements that are that false.”