Yes, swimmers pee in the pool. In anonymous surveys, 19% of adults admit to having done it at least once, and competitive swimmers are among the most frequent offenders. Ahead of the 2012 London Olympics, Michael Phelps told reporters, “I think everybody pees in the pool. Chlorine kills it, so it’s not bad.” His teammate Ryan Lochte agreed, saying something about chlorine water just makes you “automatically go.”
But Phelps was wrong about the chlorine part. Urine doesn’t just get neutralized. It reacts with pool chemicals to create compounds that irritate your eyes, damage your lungs, and produce that sharp “pool smell” most people mistakenly associate with cleanliness.
How Much Urine Is Actually in the Pool
Researchers at the University of Alberta developed a clever way to measure this. They tracked acesulfame potassium, an artificial sweetener that passes through the body unchanged and shows up reliably in urine. By measuring its concentration in pool water, they estimated that a standard 420,000-liter community pool (roughly 110,000 gallons) contains up to 30 liters of urine. That’s about 75 bladders’ worth floating around in a single pool at any given time.
The 19% admission rate from surveys is almost certainly an undercount. People tend to underreport embarrassing behavior, even anonymously. The chemical data tells a more honest story than the surveys do.
What Happens When Urine Meets Chlorine
Chlorine is added to pools to kill bacteria and viruses. When it reacts with urea, the main nitrogen-containing compound in urine, the result isn’t a clean kill. Instead, the chlorine bonds to nitrogen atoms in a series of steps, eventually producing a group of chemicals called chloramines. The most problematic of these is trichloramine, a volatile gas that escapes from the water surface into the air above the pool.
This process also generates cyanogen chloride, a toxic compound that irritates the eyes, skin, and respiratory tract. In high concentrations (far above what pools produce), cyanogen chloride is classified as a chemical warfare agent. Pool levels aren’t remotely that high, but even small amounts contribute to the cocktail of irritants hanging in the air at indoor facilities.
Every chlorine molecule that reacts with urine is a chlorine molecule no longer available to kill germs. So urine doesn’t just add irritants to the water. It actively weakens the pool’s ability to protect you from infections.
The “Pool Smell” Isn’t Chlorine
That strong chemical smell you notice at indoor pools? It’s not chlorine. It’s chloramines, the byproducts of chlorine reacting with urine, sweat, skin cells, and other organic matter swimmers bring into the water. A well-maintained, clean pool with adequate chlorine has almost no smell at all. The stronger the “pool smell,” the dirtier the water.
The same goes for red, itchy eyes after swimming. The CDC confirms that chloramines, not chlorine itself, cause eye and skin irritation. When chlorine combines with pee, sweat, dirt, and body products, it creates these irritants in the water and releases them as gas into the surrounding air. The CDC’s advice is blunt: never pee in the water.
Breathing Pool Air Can Affect Your Lungs
Trichloramine gas sits right at the water’s surface, exactly where swimmers breathe. A study published by the National Institutes of Health found that people exposed to trichloramine in indoor pool environments experienced measurable decreases in lung function, even after a single session. Among pool workers with long-term exposure, 23% reported persistent coughing, 29% had chronic nose irritation, 24% reported throat irritation, and 37% experienced ongoing eye irritation.
The research also found evidence that prolonged exposure to trichloramine may contribute to asthma development. Indoor pools concentrate these gases more than outdoor ones, where ventilation disperses them. If you’ve ever felt slightly short of breath or noticed a scratchy throat after swimming indoors, trichloramine is the likely culprit. The more urine and sweat in the pool, the more of this gas gets produced.
Pool Operators Can’t Fix This Quickly
Once urea enters pool water, chlorine breaks it down extremely slowly, over days or even weeks. Pool operators can perform “shock treatments,” temporarily raising chlorine to very high levels to convert chloramines into nitrogen gas that escapes into the air. Most health departments limit combined chlorine (the chloramine form) to 0.4 parts per million or less, and exceeding that threshold triggers a shock treatment.
But shocking a pool is a reactive fix, not a preventive one. It takes time, costs money, and the pool is often unusable during the process. The far simpler solution is for swimmers to stop adding urine in the first place.
The Urine-Detecting Dye Is a Myth
If you grew up believing that a special chemical in pool water would turn bright purple (or red, or blue) the moment someone peed, you’re not alone. A 2015 survey found that 50% of people believed this dye existed. The National Swimming Pool Foundation has called it “the most common pool myth of all time.”
No such dye has ever been used in pools. The chemistry makes it essentially impossible. Urine contains the same types of organic compounds found in sweat and other body fluids, so creating a chemical that reacts specifically and only to urine, without lighting up every time someone with sunscreen jumps in, remains impractical. The myth persists because it’s useful: generations of parents have wielded it to terrify children into using the bathroom first.
What You Can Do
The fix is straightforward. Use the bathroom before you get in. Shower before swimming to rinse off sweat, skin oils, and cosmetics that also react with chlorine. If you’re swimming with young children, take regular bathroom breaks rather than relying on swim diapers, which catch solid waste but do nothing for urine.
Choosing an outdoor pool over an indoor one reduces your exposure to airborne chloramines significantly. If you swim indoors regularly and notice persistent coughing, eye irritation, or breathing difficulty, poor air quality from chloramine buildup is a real possibility, especially at facilities with inadequate ventilation or heavy swimmer traffic.