ROCKFORD, Ill. (WTVO) — Yes, it’s a fact: many people are peeing in their swimming pools. Even U.S. Olympic Swimmer Michael Phelps confessed to it. But is it safe to swim in a pool once someone has urinated in it?

According to the Centers for Disease Control and Prevention (CDC), the answer is: no.

“Pee in the toilet, not in the pool!” the CDC said in 2021. “When pee and chlorine mix in the pool, there is less chlorine available to kill germs.”

Worse: “Pee mixed with chlorine creates chemicals that can make your eyes red and itchy,” the CDC added.

Chrlorine is a chemical that is added to pool water to kill germs, by creating a mild acid when mixed with water to kill off bacteria including salmonella and E.coli.

Pool chemicals can kill most germs within minutes, but others can survive in the pool for days.

According to Dr. Jamie Alan, an associate professor of pharmacology and toxicology at Michigan State University, “Chlorine needs the right pH to work properly,” but “urine alters the water’s pH.”

Chlorine and urine combine to make chloramines, and “this reaction sucks up free chlorine, making it less effective.”

“Urine, as well as sweat, contains urea, a nitrogen-containing compound that can combine with chlorine to produce other compounds, leaving less of the chlorine available to kill bacteria,” said Dr. Mark Conroy, associate professor of emergency medicine and medical director of the University Hospital emergency department at The Ohio State University Wexner Medical Center.

“Urinating in a pool is simply a bad habit,” he added. “While urine itself is generally considered sterile, the chlorine in the pool is there to protect us from other bacteria. Ultimately it’s best to simply get out of the pool, head to a restroom, and then return to avoid making swimming uncomfortable for themselves and other swimmers.”