Health officials say that holding in your urine when you really have to go can be harmful. But every public pool has signs that prohibit peeing in the pool.

Yet a lot of Olympic swimmers admit to doing it anyway and if you are visiting a public water park and it's not 20 percent urine, count yourself lucky. 

In season 5 of Seinfeld, George and Jerry had this very discussion:

George Costanza: It's not good to hold it in. I read that in a medical journal.
Jerry: Did the medical journal mention anything about standing in a pool of somebody else's urine?

According to a new report, Jerry's skepticism is warranted. Writing in Environmental Science&Technology, the authors say that, when mixed, urine and chlorine can form substances that can cause potential health problems beyond bathing in urine. 

Adding chlorine to pool water is the most common way to kill disease-causing microbes and prevent swimmers from getting sick but they found that chlorine mixes with sweat and urine and makes other substances. Two of these compounds, including trichloramine (NCl3) and cyanogen chloride (CNCl), are ubiquitous in swimming pools. The first one is associated with lung problems, and the second one can also affect the lungs, as well as the heart and central nervous system.

Peeing in the pool. Yes, it's a bad idea. Credit: 
DOI: 10.1021/es405402r

Scientists have not yet identified all of the specific ingredients in sweat and urine that could cause these potentially harmful compounds to form. So Li's team looked at how chlorine interacts with uric acid, a component of sweat and urine. 

They mixed uric acid and chlorine, and within an hour, both NCl3 and CNCl formed. Though some uric acid comes from sweat, the scientists calculated that more than 90 percent of the compound in pools comes from urine.

They conclude that swimmers can improve pool conditions by simply urinating where they're supposed to — in the bathrooms.

Citation: Lushi Lian, Yue E, Jing Li, and Ernest R. Blatchley, III , 'Volatile Disinfection Byproducts Resulting from Chlorination of Uric Acid: Implications for Swimming Pools', Environ. Sci. Technol., 2014, 48 (6), pp 3210–3217 February 25, 2014 DOI: 10.1021/es405402r