The red, bloodshot eyes that people get after being in a swimming pool aren’t caused by chlorine, as thought — but by what happens when people urinate in the water.

People weeing in the pool means the urine reacts with chlorine to create a chemical compound that hurts the eyes, according to the US’s Healthy Swimming Program. And those chemicals can also create poisonous gases that can damage lungs, hearts and nervous systems.

“That ‘chlorine’ smell at the pool isn’t actually chlorine,” said Chris Wiant, chair of the US Water Quality and Health Council. “What you smell are chemicals that form when chlorine mixes with pee, sweat and dirt from swimmers’ bodies.”

Experts have pointed out that despite the story told to children that a dye in the water will show if they’ve urinated in the pool — as almost half of Americans believe — it’s actually very difficult to tell when it has happened. In fact, having red eyes are the biggest indicator, according to the National Swimming Pool Foundation.

Weeing into chlorinated swimming pools actually depletes the effect of the chlorine, meaning that more might need to be poured in. And even if it is, chlorine doesn’t get rid of the most common illnesses — many bacteria can continue to live on for days in the pool, once they’re in.

Experts say that bathers and swimmers must be extra careful to ensure that they’re properly washed before they get in, and relieve themselves elsewhere.