One in five of us will do the unthinkable this summer: take a leak in the pool.

The lazy act is more than gross, though. It results in toxic chemicals, albeit in very small amounts. “There's this perception that peeing in a pool is okay because there's chlorine, and that's just not true,” says Ernest Blatchley, a chemical engineer at Purdue University. In a pool, chlorine's job is to kill bacteria. It does not take care of bodily functions. In fact, it readily reacts with uric acid, the nitrogen-containing chemical that gives urine its name. The resulting compounds are cyanogen chloride (CNCl) and trichloramine (NCl3), which are potentially hazardous and present in every pool Blatchley has sampled for the past 10 years. His most recent study, published in the journal Environmental Science & Technology, shows that 93 percent of the uric acid used to make these compounds comes from human urine (sweat also contains uric acid).

And it does not take very much pee to drive levels of these chemicals above acceptable limits set by the U.S. Environmental Protection Agency: a previous study found that levels of trichloramine rose by a factor of four after swim meets. Other scientists have found frequent exposure to trichloramine and cyanogen chloride in pools—like swimmers and lifeguards experience—raises the likelihood of asthma and other respiratory problems, although these links require more study.

So if the ick factor isn't enough already, please, don't pee in the pool.