About one in five Americans admit they've peed in the pool. (We’re pretty sure the other four in five are lying) Even the world's most famous pool-user, Michael Phelps, says "everybody" does it.

Now scientists have finally figured out a way to quantify just how much urine is in our pools, and the results in Environmental Science & Technology Letters might make you think twice about taking a dip.

A research team testing 31 pools and hot tubs in Canada found evidence of urine in every single one of them. On average, there were 8 gallons of urine in a 110,000-gallon pool and 18.5 gallons of urine in a 220,000-gallon pool.

The results were even grosser for hot tubs. One hotel hot tub was found to have three times the urine level of the worst swimming pool.

Researchers figured this all out by measuring the levels of acesulfame potassium, or Ace-K, in pools and hot tubs. Ace-K is an artificial sweetener found in soup, diet soda, candy, yogurt, and much more. It doesn't occur in nature and 95% of it passes straight through the body unchanged, making it a good indicator of urine levels in water.

The authors of the study published in the journal Environmental Science and Letters say urine in swimming pools is a “public health concern.”

Although they note that urine itself is sterile, nitrogenous organics in urine can react with disinfectants like chlorine in swimming pools to form volatile and irritating compounds. Exposure to these volatile compounds in indoor swimming facilities can lead to eye and respiratory irritation and has even been linked to asthma, the researchers say.