Pool pee isn’t just gross—it’s also harming swimmers

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Swim against the Olympians Michael Phelps or Ryan Lochte and, chances are, you’re bathing in their urine. And they’re not alone. Nearly one in five American adults (pdf, p.5) admits to having urinated in public swimming pools. Phelps, for one, doesn’t think a little in-pool relief is such a big deal.

“I think everybody pees in the pool,” Phelps told The Telegraph in 2012. “It’s kind of a normal thing to do for swimmers. When we’re in the water for two hours, we don’t really get out to pee. Chlorine kills it so it’s not bad.”

But though chlorine might kill bacteria, it also reacts with chemicals in human urine to create harmful gases. Scientists have long-known that when urea—a compound present in urine—combines with chlorine, it gives off nitrogen trichloramine (NCl3), a poisonous gas that can cause acute lung injury. Scientists monitoring the presence of nitrogen trichloramine during a national swimming competition found that it doubled after the first day, increasing by three to four times throughout the rest of the four-day event.

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