A new study has just been released on public swimming pools, and it’s made a doozy of a finding: they are filled with a lot more than just water and chlorine, and it could put your health at risk. The American Chemical Society published a paper that determines how much urine is in swimming pools based on measuring the levels of artificial sweeteners, which are not broken down by the body or by chlorine.

They found that a typical commercial-size swimming pool holding 220,000 gallons will have about 20 gallons of urine, and a small residential 20 by 40 foot pool that is five feet deep will have about two gallons of human waste. It’s not that much, but even trace amounts can be hazardous to your health.

Scientists also found other chemicals formed from when the chlorine combines with sweat and body oils. They based their research on analyzing water collected from pools and hot tubs in two cities in Canada. The pools were located in hotels and recreational facilities.

“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.”