For many, a trip to the local swimming pool also results in red, itchy eyes. So what’s the reason behind eye irritation? Brace yourself — the answer is grosser than you might think. According to The Washington Post’s Ariana Eunjung Cha, the answer isn’t chlorine. It’s urine.

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This nasty little bombshell was tucked in the Centers for Disease Control’s annual healthy swimming report. Though also shares less horrifying information like the number of pools in the United States (309,000 public pools, 10.4 million residential ones), the CDC also reports that a pool’s telltale “chlorine” smell — and red, stinging eyes after swimming — are caused by “pee, poop, sweat, and dirt from swimmers’ bodies.”

In fact, chlorine is still a culprit. Cha writes that when urine and chlorine mix, the combination turns chlorine into an ammonia derivative called chloramine, which has a distinctive smell, a reputation for causing respiratory problems, and a telltale effect on eyes.

But pee isn’t the worst thing that’s probably lurking in your pool. Actually, according to CDC healthy swimming spokesperson and epidemiologist Michele Hlavsa, it’s poop. She tells LiveScience’s Rachael Rettner that people with diarrhea can spread a filter and chorine-resistant Cryptosporidium, a parasite that is the leading cause of waterborne disease in the United States.

Luckily, there are several easy ways to prevent things from tainting your poolside experience forevermore. Always shower before entering, stay out of the pool if you’re not feeling well, and test water frequently if you own your own pool.