Plenty of swimmers blame chlorine in pools for symptoms like red, stinging eyes and nasal irritation, but the reality is a bit more complicated and a lot more disgusting, according to the Centers for Disease Control and Prevention. The CDC says the red eyes are caused by "chlorine binding with sweat, urine, and other waste from swimmers," creating irritants that can spread from the water to the air as the amount of waste in the pool increases, KFOR reports. This makes eye irritation especially bad at many indoor pools, according to the CDC, which says pool operators should make sure there's plenty of fresh air circulating when there are plenty of swimmers.
Dr. Michael Beach, a man the CDC describes as its "foremost expert on waterborne illnesses," tells Women's Health that the CDC thinks swimming is a "fantastic activity" and the agency wants to keep it that way by keeping people healthy. That means advising them not to pee in the pool—and to have a good shower before getting in, especially if they happen to be battling diarrhea. "We have a new parasitic germ that has emerged that's immune to chlorine," continues Beach, explaining that it's spread through people—and not just babies—swimming when they have diarrhea. "We've got to keep it out of the pool in the first place." And if you think you're quasi-protected thanks to the dye that supposedly changes the color of the water when someone pees in it, well, it's a "complete myth," Beach adds. Read more on the harm that peeing in the pool can cause—and how many adults admit to doing it. (Read more swimming pool stories.)