This article is more than 2 years old.

June 24, 2015 This article is more than 2 years old.

Planning to cool off this summer by hitting the pool? You might want to pack your goggles to prevent your eyes from turning bloodshot, from… No, not chlorine.

Contrary to common belief, it’s not chlorine—not chlorine by itself, anyway—that reddens your eyes. It’s the chemical compounds formed when chlorine reacts with human urine.

That’s according to the Healthy Swimming Program, a collaboration between the US Centers for Disease Control, the Water Quality and Health Council, and the National Swimming Pool Foundation (h/t Women’s Health).

The prevailing association of red-eye with chlorine hints at how common the ailment is—and, therefore, how much urine we’re splashing around in. It also has some high-profile apologists. Olympian swimmers Michael Phelps and Ryan Lochte have confessed to sneaking an underwater leak, as Quartz reported last year.

But while red-eye is gross, it’s just a minor downside of the compounds created when chlorine mixes with urine (and to a much lesser extent, sweat and dirt). Recent research reveals that chlorine’s reaction with two chemicals in urine—urea and uric acid—creates two poisonous gases that can hurt people’s lungs, hearts, and central nervous systems (see link above).

On top of all that, chlorine doesn’t kill some of the most insidious types of bacteria fast enough to prevent infections. For instance, a bacteria called Cryptosporidium—“Crypto” for short—can linger in chlorinated pools for days.

In spite of widespread chlorine use, outbreaks of recreational water illnesses (RWIs) have climbed in the past decade, says the CDC. During that time, more than 20,000 people have picked up diarrheal illnesses from water they swallowed in US swimming pools, water parks, and other disinfected swimming venues.

How might bacteria be getting into pools in the first place?

Brace yourself for ick. It seems that bathers aren’t just peeing in pools; they’re also leaking diarrhea. The CDC advises people to “never swim when you have diarrhea.” This one-minute YouTube clip—the winner of CDC’s Healthy Swimming program video contest—is a pretty hilarious example of its pool safety outreach:

Of course, there’s no real-life patrol of bathers’ intestinal woe. That makes the CDC’s other big piece of anti-RWI advice all the more immediate: that those braving the waters should keep their mouths firmly shut.