When it comes to swimming dos and don'ts, taking advice from Olympic swimmers Michael Phelps and Ryan Lochte seems like an obvious place to start.

In an interview with The Wall Street Journal during the 2012 Summer Olympics, the pair admitted that peeing in the pool is something every swimmer does. “Chlorine kills it so it’s not bad,” Phelps said.

But new evidence published by the American Chemical Society has shown that even the U.S.’s most decorated swimmer may be in the wrong.

When chemists mixed the uric acid found in human urine with chlorine, they found that two harmful chemicals formed within an hour.

Worse Yet: Who’s Putting Poo in the Pool? »

Urine + Chlorine = Toxic

In research published in the journal Environmental Science & Technology, Jing Li, Ernest Blatchley, III, and colleagues identified the chemicals that can be created when urine and chlorine mix.

While chlorine is the most common way for pool owners to kill disease-causing microbes and prevent swimmers from getting sick, if mixed with urine, it can create trichloramine and cyanogen chloride. These two chemicals are ubiquitous in public pool water samples, the researchers said.

Both chemicals are associated with lung problems, but cyanogen chloride can also affect the heart and central nervous system.

In fact, it is classified as a schedule 3 controlled substance under the U.S. Chemical Weapons Convention due to its potential to be used in chemical warfare. Not only is it toxic, cyanogen chloride is also easy to deliver because it can penetrate most gas masks.

Some uric acid is excreted through sweat, but researches say 90 percent of the quantities found in pools comes from urine.

While the researchers aren’t suggesting that your local pool should be closed down by the Department of Homeland Security, they do suggest that people do their business where they're meant to—in the bathroom.

“Moreover, given that uric acid introduction to pools is attributable to urination, a voluntary action for most swimmers, these findings indicate important benefits to pool water and air chemistry that could result from improved hygiene habits on the part of swimmers,” the authors concluded.

Help Keep Your Local Pool Safe

To keep summer fun from becoming a public health emergency, the U.S. Centers for Disease Control and Prevention (CDC) recommend that you:

  • Shower before swimming.
  • Wash your hands after using the toilet or changing diapers.
  • Take children on bathroom breaks or check diapers often.
  • Change diapers in a bathroom and not poolside, and thoroughly clean the diaper changing area.
  • Refrain from swimming if you have diarrhea.
  • Avoid swallowing pool water or even getting it in your mouth.