If you’ve ever swam at a public pool, you’ve probably emerged from the water at some point with a stinging sensation in your eyes. We’ve grown accustomed to blaming this unpleasant sensation on there being too much chlorine in the pool but the Center for Disease Control claims new research shows that the true cause of getting red eyes while swimming is something much, much worse.

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It seems that the chlorine alone isn’t enough to cause eye irritation — instead, it takes a mix of chlorine and other human waste byproducts such as sweat… and urine.

“Irritants in the air at swimming pools are usually the combined chlorine by-products of disinfection,” writes the CDC. “These by-products are the result of chlorine binding with sweat, urine, and other waste from swimmers… if the air is saturated with irritants, new irritants produced in the water will stay in the pool water causing further irritation for swimmers, such as stinging or red eyes.”

This sort of problem is most likely to occur in indoor pools where there’s poor ventilation and where irritants are constantly recycled in the air. If you have an indoor pool and swimmers are getting red eyes from using it, the CDC says you should “open all of the doors and windows in the pool area or to use fans to boost airflow over the pool surface when many swimmers are using the pool.”

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