Swimmers who emerge after a few laps in the pool with red, stinging eyes will most likely attribute it to the chlorine found in the water.
Not necessarily, according to a study done by the Centres for Disease Control and Prevention (CDC) in the United States. The truth is less palatable - the redness is a sign of irritation caused by chemical compounds formed when chlorine reacts with urine.
"Peeing in a pool depletes chlorine and actually produces an irritant that makes people's eyes turn red," said Ms Michele Hlavsa, chief of the CDC's Healthy Swimming Programme.
The chemical is chloramine, and it forms when compounds containing nitrogen, found in human urine and sweat, combine with the chlorine in pools.
While the thought may put off people getting into pools, the irritation caused by chloramine is unlikely to have any long-term ill effects, said general practitioner Philip Koh, chairman of the medical board of Healthway Medical Group.
But to be on the safe side, and for the sake of comfort, people should wear goggles to protect their eyes when swimming.
Dr Lee Hung Ming, medical director of Lee Hung Ming Eye Centre at Gleneagles Hospital, said chloramine is not the only potential irritant in pools.
"Other factors that can cause redness of the eye are excessive amounts of chlorine or if the water is too acidic or alkaline," he said.
Chloramine can also irritate the skin and respiratory system, causing runny nose and coughing, according to the report.
The chlorine used in Singapore's swimming pools may differ in strength or concentration from that in the US, said Mr Jeffrey Teh, general manager of swimming pool equipment company CU Water Services.
"But at the end of the day, chlorine is still chlorine, and will cause the same kind of reaction (with urine) here as it does there," he said.
National Environment Agency guidelines state that all licensed swimming pools must be chlorinated. Operators must ensure that the chlorine level in the pool is maintained between 1mg per litre (mg/l) and 3mg/l.
At least once a month, pool water has to be tested by laboratories for chemical and bacteriological quality.
Chlorine is added to water to kill germs and to prevent potential outbreaks caused by bacteria or viruses such as Escherichia coli, or E. coli, and norovirus.
Those are mainly spread by people with diarrhoea - which is why those who are suffering from bowel diseases should not enter swimming pools.
Said general practitioner Leong Choon Kit: "Chlorine remains an important agent for sterility."
He added it has been well-established that chlorine, on its own, can irritate the eyes.
Swimming instructor Sam Tan, 38, said that in his experience, people are generally responsible and refrain from peeing in pools, although a rogue leak may occur occasionally.
"It's more common among young kids, as they sometimes cannot hold it in," said Mr Tan.
Dr Lee said that he hopes the report will remind pool users about the importance of being civic- minded.
"Peeing in pools is not only unhygienic, but can potentially cause irritation of the eyes to others."
This article was first published on June 27, 2015.
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