FILE: A boy dives in a basin of the Freibad Lister Bad public swimming pool in Hanover, central Germany. (JULIAN STRATENSCHULTE/AFP/Getty Images)

Like many people who swim in the summer, you might have assumed it's the chlorine that makes your eyes so red in pools.

That's not exactly true, the Centers for Disease Control says in its annual healthy swimming report. It turns out the cause is actually urine binding with chlorine to turn into something called a chloramine. In addition to being an eye irritatant, chloramine, a derivative of ammonia, has been linked to respiratory problems among some swimmers.

"When we go swimming and we complain that our eyes are red, it's because swimmers have peed in the water," Michele Hlavsa, chief of CDC's healthy swimming program, told "Today."

The CDC also said in its report that the chlorine smell of pools isn't actually what you think. "What you smell are actually chemicals that form when chlorine mixes with pee, poop, sweat and dirt from swimmer's bodies... These chemicals -- not chlorine -- can cause your eyes to get red and sting, make your nose run, and make you cough."

"Yuck!" the CDC concluded.

Read more:
Good news, chocolate lovers: The more you eat the lower your risk of heart disease
Common heartburn medications linked to greater risk of heart attack
Here's how a five-day diet may 'reboot' the body and reduce cancer risk