The American Chemical Society has released an astonishing new report that makes a rather unsettling claim about public pools. It’s been tough to tell exactly what is in the pools, but researchers have found that they can tell how much pee is in the pool by measuring the artificial sweeteners in people’s urine, as some sweeteners aren’t broken down by the body or in pool water.

Scientists estimated that a 220,000 gallon commercial-size swimming pool has about 20 gallons of urine. A small residential pool, which is about five feet deep and is 20 by 40 feet, would have about two gallons of pee. In reality, that’s not that much, but even a small amount can be a health hazard.

And it’s not just pee. Scientists found a mixture of a lot of chemicals that result when chlorine mixes with sweat and body oil. Researchers based their findings on collecting water form pools and hot tubs at hotels and facilities in two cities in Canada.

“Recent studies have shown that nitrogenous compounds (e.g., urea) in urine and sweat react with chlorine to form disinfection byproducts (DBPs), including trichloramine, that can cause eye irritation and respiratory problems,” the statement reads. “Xing-Fang Li, Lindsay K. Jmaiff Blackstock and colleagues say this evidence has highlighted the need for improved understanding of pool chemistry to raise awareness and educate the public on the importance of swimming hygiene practices. To estimate how much urine — and potentially DBPs — might be in a given pool, Li’s team needed to identify what compound might consistently be present in urine. So the researchers turned to the artificial sweetener, acesulfame potassium (ACE), which is marketed as Sunett and Sweet One. The sweetener, which is often used in processed foods like sodas, baked goods and even in other sweeteners, is widely consumed, chemically stable and passes right through the digestive tract and into consumers’ urine.”