Michael Phelps does it. So does Ryan Lochte.
But please think twice before you, ahem, go in the water.
It may be convenient. But peeing in a pool also triggers chemical reactions that can be hazardous to your health.
Phelps, the most decorated Olympian in modern history, was asked in 2012 if he urinates in the water.
"I think everybody pees in the pool," Phelps told the Wall Street Journal. "It's kind of a normal thing to do for swimmers. We don't really get out to pee. We just go whenever we are on the wall.
"Chlorine kills it, so it's not bad," Phelps said.
But that's not just gross, it's wrong, according to the American Chemical Society.
A new video the group produced (see above) explains how urine in a pool reacts with chemicals that are used to sanitize the water. The reaction creates toxic gases, one of which, in much greater concentrations, is classified as a chemical weapon.
Any pool is going to attract microorganisms. E. coli, giardia and salmonella thrive in untreated water. Swimmers who don't shower before diving into the pool bring other bacteria and traces of fecal matter into the mix. That's why swimming pools are treated with chlorine, sodium hypochlorite and calcium hypochlorate.
"The chlorine will kill poop bacteria," said Pam Lawn, director of Environmental Health Services for the Montgomery County Health Department. "But it doesn't eliminate all the hazards."
When a swimmer pees in a pool, the uric acid in the urine combines with those molecules to create disinfection by-products (DBP). One of those by-products, called trichloramine, is what gives swimming pools their distinctive odor, said Susan Richardson, a chemistry professor who researches drinking water and swimming pool water at the University of South Carolina.
"When you're at an indoor pool, that's not chlorine you're smelling," Richardson said. "That's evidence that people have peed in the pool."
In small quantities, DBPs corrode stainless steel pool fixtures, cause red eyes, skin irritation, and can aggravate respiratory ailments.
In 2007 at a waterpark in Ohio, noxious DBPs caused symptoms in 665 people, according to the Centers for Disease Control and Prevention.
At very high concentrations, DBPs can cause coma and death. (Though there's no evidence that pool DBPs have ever caused swimmers or pool workers to suffer a catastrophic illness.) Cyanogen chloride, which is also produced when urea reacts with chlorine, is classified as a chemical weapon.
Preliminary studies have linked DBPs to higher incidence of asthma and bladder cancer among competitive swimmers.
"It's something we have to tell people," Lawn said. "Don't pee in the water."
Holding it until you can get to a toilet limits the formation of DBPs. But it doesn't totally eliminate the problem.
Sweat contains trace amounts of urea, which also reacts with chlorine to produce DPBs.
"That's just one of the reasons we encourage our patrons to shower before getting into the water," Lawn said. "We want them to rinse that off."
Researchers estimate that the problem is so widespread, each swimmer contributes an average of two ounces of urine to the water. Some swimmers don't even realize they're adding to the problem, as small amounts of urine can be released unintentionally, according to an ACS report.
Four years ago, 12-time Olympic medalist Lochte created a satirical video spoofing his self-admitted pool peeing for the website Funny or Die.
"The way I see it, the pool is the biggest and most expensive toilet and it's all mine," Lochte said with a straight face. "Like animals, they mark their territory. I pee in the pool. That's my territory."
Richardson appreciates Lochte's sense of humor, but given the hazards to health, strongly advises against peeing in pools.
Peeing in the ocean, however, is perfectly fine.
"There's no chlorine out there. The whales and fish pee all the time," Richardson said. "I say if you're out there, go for it."