Chlorine has long had a bad rap for irritating the eyes of swimmers, especially in crowded public pools.

As it turns out, however, it's not the chemical itself turning your eyes red after a swim — it's everything else in the water that chlorine goes in to kill. 

Specifically, human urine.

The U.S. Centers for Disease Control (CDC) recently teamed up with both the Water Quality and Health Council and the National Swimming Pool Foundation to warn the public about health risks associated with summer.

"Smell that 'chlorine'?" reads a fact sheet released by the coalition last month. "It's not what you think. What you smell are actually chemicals that form when chlorine mixes with pee, poop, sweat, and dirt from swimmers' bodies.… These chemicals — not chlorine — can cause your eyes to get red and sting, make your nose run, and make you cough."

CDC peeing in the pool fact sheet

Swimming with someone who has diarrhea is more than just gross. According to the CDC, it can be dangerous. (U.S. Centers for Disease Control)

A companion report published by the National Swimming Pool Foundation, an American non-profit dedicated to aquatic health and safety, elaborates on what it is about urine that leads to red eyes.

"Chlorine and other disinfectants are added to a swimming pool to destroy germs," said Michele Hlavsa, chief of CDC's Healthy Swimming Program, in a release. "Peeing in a pool depletes chlorine and actually produces an irritant that makes people's eyes turn red."

To eliminate the irritants caused by nitrogen-containing compounds found in urine, more chlorine may need to be added to a pool, she said.

Red eyes are one of several colour-related topics tackled by the nationwide campaign.

Another relates to what the swimming pool foundation calls "the most common pool myth of all time" — one that nearly half of all Americans surveyed by researchers believed was true.

"Parents have long used the story of a chemical that changes colour in the presence of pee to keep their children from peeing in the pool," reads the report.

Foundation CEO Thomas M. Lachocki made the truth clear, saying "there isn't a dye that turns red. It's the eyes that turn red. Swimmers' eyes are the real colour indicator that someone might have peed in a pool."

As gross as that sounds, it's one of the mildest potential ailments that can be caused by human waste in a public pool.

Swimming at an indoor pool is particularly risky, according to the CDC, as the irritants mentioned above can move into the air surrounding a pool and trigger coughing, wheezing, or even asthma attacks.

And then there's the issue of infectious diseases.

In an interview about the campaign with Women's Health, the CDC Healthy Water program's associate director noted that there's been an increase in outbreaks of recreational water illnesses over the past decade.

Cryptosporidium virus

Cryptosporidium is a leading cause of waterborne disease, according to the U.S. Centers For Disease Control. (U.S. Centers for Disease Control)

The reason for this? People who swim while they have diarrhea and unleash even very tiny amounts of germs like Cryptosporidium (or "crypto" for short), norovirus, and E. coli. into the water. 

"Diarrhea and swimming don't mix!" reads the CDC's website. "Swimmers who are sick with diarrhea — or who have been sick in the last two weeks — risk contaminating pool water with germs. Certain germs that cause diarrhea can live from minutes to days in pools, even if the pool is well-maintained. Once the pool has been contaminated, all it takes is for someone to swallow a small amount of pool water to become infected."

Approximately 58 per cent of Canadians admitted to peeing in the pool at least once in a recent survey of 9,500 people conducted by Travelocity.

If you're one of them, Hlavsa has some advice:

"The solution isn't rocket science; it's common courtesy. Swimmers should use the pool to swim, the restroom to pee and the showers to wash up before getting in the pool. It's that simple."