It is a truth universally acknowledged that a pool in good use by humans will contain pee.

As Ars has reported before, those disturbing warm spots create harmful chemicals. Urine and chlorinated water react to form trichloramine and cyanogen chloride. At certain doses, these can cause respiratory and eye irritation. At high doses, cyanogen chloride can kill—it’s considered a chemical warfare agent. The levels found in pools, however, are nearly negligible. Long-term exposure by professional swimmers and pool workers may be linked to asthma, but otherwise it’s unlikely to cause many problems. And as Ars has also reported, you’d need a hellish scenario of two-parts water, one-part chlorine, and the wee of three million people in a pool to get to a lethal situation.

Still, it’s hard not to be curious about how much pee we let slip while taking a dip—and now we have the answer.

By analyzing pool water for a sweetener commonly found in human urine, Canadian researchers estimated that there were about 75 liters of pee in a large public pool—which contains about 833,000 liters or is a third of an Olympic pool. That’s about half a bathtub’s worth of pee. In a pool half the size of the first, the researchers reported an estimated 30 liters of pee. Their study was published Wednesday in Environmental Science & Technology Letters.

“We want to use this study to promote public education on appropriate swimming hygiene practices,” lead author Lindsay Blackstock of the University of Alberta, Edmonton, told The Guardian. “We should all be considerate of others and make sure to exit the pool to use the restroom when nature calls.”

To calculate the yellow to blue water ratio in pools, the researchers tracked levels of the artificial sweetener, acesulfame potassium (ACE). This is found in many processed foods, from soda to baked goods, yogurt, and soups. It zips right through our bodies, and researchers knew its average concentration in urine.

By sampling 31 pools and hot tubs for ACE, the researchers could estimate how much swimmers were contributing to water levels. They took 250 samples over the course of three weeks. Concentrations ranged from 30 to 7,110 nanograms per liter of water. That’s up to a 570-fold increase of ACE levels found in tap water.

Some of the hot tubs were found to be most toilet-like. One hotel Jacuzzi had three times the ACE concentration found in the most pee-filled pool.

And once you’ve peed in a pool, the urine is likely to stick around. As Ernest Blatchley III, an environmental engineer at Purdue University, told NPR: "It's not uncommon for water in a pool to go unchanged for years." Pool owners tend to simply add more water, which is cheaper than draining and refilling.