https://www.npr.org/sections/health-shots/2017/03/01/517785902/just-how-much-pee-is-in-that-pool said:
You know that sharp odor of chlorine from the swimming pool you can recall from earliest childhood? It turns out it's not just chlorine, but a potent brew of chemicals that form when chlorine meets sweat, body oils, and urine.
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The scientists calculated that one 220,000-gallon, commercial-size swimming pool contained almost 20 gallons of urine. In a residential pool (20-by-40-foot, five-feet deep), that would translate to about two gallons of pee.
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Apart from being gross, that's also a potential health hazard. Chlorine reacts with urine to form a host of potentially toxic compounds called disinfection byproducts. These can include anything from the chloramines that give well-used pools the aforementioned odor, to cyanogen chloride, which is classified as a chemical warfare agent. There are also nitrosamines, which can cause cancer. There's not enough evidence to say whether the nitrosamine levels in pools increase cancer risk, Blatchley says, but one study in Spain did find more bladder cancers in some long-term swimmers.
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Once someone does pee in the pool, the only way to truly get rid of it is to replace the water. "It's not uncommon for water in a pool to go unchanged for years," Blatchley says, since many pool owners or operators just add water as needed rather than completely replacing it, which is more expensive.
And the longer water sits in a pool, the worse it gets, his research has shown. Over time, people add more chlorine to the water, which is converted to a form called chloride that builds up and encourages the formation of yet more disinfection byproducts.
