The stinging red eyes you get from swimming underwater are not caused by chlorine - or not chlorine by itself.
That's according to Dr Michael Beach, an associate director of the American Centre for Disease Control's (CDC) Healthy Water programme, who claims that the stinging is "chlorine binding to urine and sweat" already in the pool.
We double-checked it, and the CDC's website backs Beach up. It notes: "Irritants in the air at swimming pools are usually the combined chlorine by-products of disinfection. These by-products are the result of chlorine binding with sweat, urine, and other waste from swimmers."
As for dyes which change the colour of water when someone pees? We have more bad news. Dr Beach tells Women's Health that they are a "complete myth". "It’s about scaring people into not urinating in the pool." According to myth-busting site Snopes, he is right.
"No matter what your parents might have told you, there isn't any magical chemical that when added to a swimming pool will reveal the presence of urine in the water by producing a brightly-colored cloud," the site says.
Just when you thought it was safe to go back into the water...