Remember when swimming star Michael Phelps told The Wall Street Journal that peeing in the pool was completely normal and totally harmless? He’s wrong about the last part. According to findings published in the Environmental Science & Technology journal and reported by The Atlantic, pee can mix with the chlorine in pools to form chemical byproducts that are dangerous to swimmers’ health.

For the study, researchers mixed uric acid (an ingredient found in urine and sweat) with chlorine. Within just one hour, researchers found the mixture formed the chemical compounds trichloramine (NCl3), which the Centers for Disease Control and Prevention links to respiratory problems, and cyanogen chloride (CK), which the CDC says can adversely affect the lungs, central nervous system and cardiovascular system.

Pool chemists said they were not particularly surprised by the findings because over the past 10 years they have found both chemicals in nearly every water sample they’ve taken from a public pool.

In addition, Ernest Blatchley III, PhD, a professor at the Division of Environmental Ecological Engineering at Purdue University and one of the study’s authors, said there’s another huge concern. When hordes of people pee in a pool, CK not only forms quickly but also decays fast—which can deplete the chlorine in the water and result in yet more CK and pool problems.

So, next time you think about skipping the bathroom for some sweet relief, ignore Phelps and remember this age-old courtesy: Please, don’t pee in the pool.

Chlorine can also wreak havoc on African-American hair. Click here for tips on how to protect your crowning glory while taking a dip in the deep end.