Swimmers have long known to be wary of pee in the pool. But new research indicates people leave behind poo as well. In a study of public swimming pools in Atlanta, the Centers for Disease Control and Prevention found traces of fecal matter in 58 percent of them.

In its most recent Morbidity and Mortality Weekly Report, the CDC reveals it found E. coli, a fecal indicator, in 93 of 161 pool filters last year. And although the findings could indicate swimmers are leaving floaters in the water, the CDC believes the germs are more likely coming from dingle berries left on swimmers’ backsides when they failed to properly wash before entering the water.

“The average person has .14 grams of feces on their rear end,” Michele Hlavsa, who led the CDC study, told NBC. “If that rinses off into the water, the amount from one person might not be that much. But as more and more swimmers introduce it that much, it does become an issue.”

Yuck. Imagine 1,000 kids at a water park. If they each have 10 grams of crap on their butts, that amounts to 10 kilograms—24 pounds—of poop in the water.

Although chlorine and other chemicals help kill microbes, the CDC explains that don’t kill germs instantly. Therefore, it’s important swimmers remember not to swallow pool water, and not take a dump in the pool. They can also help prevent the transmission of fecal matter by showering with soap before swimming and staying out of the water if they have diarrhea. A single incident of anal leakage can introduce up to 108 Cryptosporidium oocysts into the water, enough of the microbe to cause infection to someone who gets a mouthful of water.

The CDC found the highest levels of contamination in municipal pools rather than club-based pools. The increased rate of fecal indicators in city pools was likely the result of diapered babies and small children who are not 100-perceent potty trained.

“It’s time to stop treating the swimming pool as a toilet,” Hlavsa said. “Nowhere else except for the pool is it acceptable to poop in public or pee in public. In other places if we did this we’d be arrested.”