About one in five Americans say they’ve peed in the pool. 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 probably won’t please swimmers.

A research team testing 31 pools and hot tubs in Canada found evidence of urine in every single one of them, Gizmodo reports. 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.

According to the Guardian, 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, NPR reports. 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. While urine itself isn’t harmful, it can react with chlorine to create “disinfectant byproducts,” which can be.

Still, researchers say they aren’t trying to turn people off of swimming; they’d just like to remind everyone to please not pee in the pool. (That classic summertime pool smell? It’s pee.)

This article originally appeared on Newser: Scientists Confirm Worst Fears About Pee in Pools

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