The Eternal Lie of the Pools That Turn Blue If You Pee in Them

Everyone knows about urine-indicator dye, despite the fact that it has 100 percent never existed

If you pee in the pool, it’ll go blue! There’s a special dye in there, designed to detect urine, and it’ll billow around you in a big, embarrassing, pissy cloud, and everyone will know you’ve done it, and you’ll be hounded out of town as a known pool-piddler. 

Everybody knows that, right?

Except it isn’t true. It doesn’t even stand up to any scrutiny as an idea — what chemical specifically detects wee-wee? Would something that would react that dramatically to a dribble of micturition be safe to swim in? And if it happened, and someone was suddenly surrounded by a cerulean aura of widdly-woo, what would the protocol be for lifeguards, pool staff and fellow swimmers? If a pool had to be evacuated every time someone leaked in it, nobody would have ever completed a length. Plus, what would happen next — would they pour in an anti-piss substance that returned it to normal? Would they drain the pool and refill it with untainted water? (Answer: Highly unlikely). 

Despite being clearly nonsense, it still persists as an idea. “I’ve heard many people suggest that there’s a chemical that will react with urine to turn a certain color,” says environmental engineer Ernest R. “Chip” Blatchley III of Purdue University, who has spent a lot of his career studying the chemistry of swimming pools, and has become the media’s go-to figure when discussing people relieving themselves in the shallow end. “But as far as I can tell, it’s a myth. I’m not aware of any chemical that’s added to pools for this purpose.”

“I think everyone who works in the pool trade has been asked about it,” agrees Sue Pace of H2O Swimming Pools Ltd. “But unfortunately, it’s a complete myth.”

It isn’t just limited to word-of-mouth, either – it’s shown up on the big screen, most notably in Adam Sandler’s 2010 film Grown Ups, in which Kevin James’ character Eric “makes a sissy” in the pool at a theme park. (I reached out to the production company, Happy Madison, for some background on what inspired Sandler and co-screenwriter Fred Wolf to include the dye, but didn’t hear back.)

Swimming pools are obviously filled with urine. As Blatchley previously confirmed to NPR, “I think you can assume that if people are using your pool, they’re peeing in it.” Swimmer Michael Phelps, the most successful Olympian of all time, once admitted to Ryan Seacrest, “Everybody pees in the pool. It’s kind of a normal thing to do for swimmers. When we’re in the water for two hours, we don’t really get out to pee.” A study at the University of Alberta, in fact, concluded that an Olympic-sized swimming pool was likely to contain about 225 liters of urine. 

Which sounds like, to quote another of Sandler’s films, a shitload of piss. But, in a swimming pool, is it? The urine content of even the most urine-heavy of pools is, percentage-wise, minute. Those 225 liters of aforementioned urine are surrounded by enough water that the actual piss mass is less than one thousandth of one percent of the pool volume, meaning any pee-seeking dye would have its work cut out for it.

“Most pools are 20,000 gallons (91,000 liters) or more, so to make a few ounces of urine show up as a bright color would take some serious chemistry,” says bzsteele, a former pool supplies store employee, who recalls new pool owners asking about the dye. “There are cheap tests that could detect urine, but things like sweat, detergent and lotions would also be likely to spike them, so you’d be thrown off by all kinds of false positives. And once the reaction had happened, I’m not sure how you would undo it and get the pool back to stable.”

There’s also the fact that disinfection byproducts, or DBPs — created when the chlorine in pools reacts with the endless streams of pee released into them — are far more harmful than chlorine or urine would be on their own. Haloacetic acid, trihalomethane and chlorite can all be created by chlorine and organic matter (sweat as well as pee) reacting together, and can lead to respiratory issues, eye complaints, “lifeguard lung” and asthma. Adding more volatile chemicals, then, is unlikely to improve matters. And although pool disinfection techniques that require less chlorine (such as UV light, saltwater and hydroxyl-based systems) are increasingly being taken up by pool owners concerned about DBPs, a color-changing substance to stop people peeing in the pool is still nowhere in sight.

“Peeing in the pool isn’t great for the water chemistry,” says bzsteele. “However, average pools are huge, have proper sanitation and great filtration where every drop is filtered and circulated multiple times a day. In the long run, whatever [dye] you added would probably be more expensive and worse for you and the pool equipment than just having a random kid pee in it every now and then.”

The real power of the urine-indicator dye myth, of course, is that it might make a few more people get out and go to the bathroom rather than relieve themselves in the pool, which can only be a good thing — we can probably assume that the original rumor was started for this very purpose. Less wee means less DBPs, so despite being a lie, maybe the dye myth continues because everyone knows, deep down, they probably shouldn’t be peeing where they swim. 

It’s a miniature version of the larger lies humanity has been telling itself for millennia: People fervently believe in something without ever having seen any evidence, and modify their behavior out of fear of the consequences of transgressing. The story of urine-indicator dye, at the risk of sounding slightly dramatic, is essentially the story of God.