Life's Little Mysteries

How much pee is in swimming pools?

Before you take a dip, know this: There is definitely pee in the swimming pool. But, it's probably not that much.

About 1 in 5 people have admitted to peeing in a swimming pool at least once, according to a 2012 survey. And if you're asking Olympic swimmers, well, that rate is much higher: nearly 100 percent, said Carly Geehr, a former member of the U.S. national swim team.

The American Chemical Society (ACS) estimates that there are somewhere between 30 milliliters and 80 ml (1 to 3 ounces) of pee per person in a pool. And one study from 1997 estimated an amount in the middle: an average of 70 ml (2.4 ounces) of pee per person.

So even if you take the upper estimate of those findings, 80 ml of pee per person, you'd still need more than 12 people in a pool to get a liter (0.3 gallons) of pee. An Olympic-size pool, on the other hand, has about 2.5 million liters (660,430 gallons) of water. 

Related: How much of the ocean is whale pee (and worse?)

But these numbers are only estimates. Exactly how much pee is floating around in a given pool is a little harder to pinpoint.

The problem is that scientists still don't have a great way to measure urine levels in pools. (This means that, yes, the urban legend about a chemical in pools that will turn your pee purple, highlighting your deed for all to see, is just that: an urban legend.)

Pee is made of many chemicals, including water, salts, proteins and waste products. In a 2013 study, scientists estimated that urine contains at least 3,000 different chemicals.

And these compounds aren't necessarily unique to urine.

Urine contains a lot random organic compounds that look like a lot of other random organic compounds, said William Carroll, an adjunct professor of chemistry at Indiana University.

And when these compounds come into contact with a disinfectant — for example, chlorine — the disinfectant "tears them apart," Carroll told Live Science. That means that the only things left in pool water from the urine are the shards of the original molecules, and there's no way of knowing if these fragments came from urine or any other organic material, he said.

A sweet solution?

In one study, published in March 2017, researchers from Canada described a possible way to measure the pee in pool water, a method that involves sidestepping those chemical reactions. The researchers targeted a compound in urine that doesn't react with other chemicals in pee, nor with chlorinated pool water: an artificial sweetener called acesulfame potassium.

In the study, published in the journal Environmental Science & Technology Letters, researchers measured the concentration of acesulfame potassium in 22 swimming pools and eight hot tubs, along with concentrations in city tap water, in two Canadian cities,.

The scientists found that the concentrations ranged from 30 nanograms per liter up to 7,110 ng/l, a variation that could be explained by factors such as how the pool water was filtered and how many people were swimming (and potentially peeing) in the pool. On average, the concentration of this artificial sweetener in pee is 4,000 nanograms per milliliter of water, the study said.

Using acesulfame potassium concentrations from the 22 pools and eight hot tubs a guide, the researchers then collected 15 samples from two swimming pools over a three-week period. They estimated that a 220,000-gallon pool (830,000 l, or one-third the size of an Olympic pool) contained about 20 gallons (75 l) of pee, and a 110,000-gallon (420,000 l) pool contained about 8 gallons (30 l) of pee.

And yes, about 20 gallons of pee sounds gross — especially if you picture it as 20 milk jugs lined up in a row. But in a 220,000-gallon pool, that's only 0.01 percent of the total liquid in the pool — in other words, a drop in the bucket.

Need more proof that there's pee in the pool? Next time you swim, take a whiff of the water. That classic pool smell isn't actually the scent of chlorine, but instead a compound called trichloramine. It forms when a chemical called urea, which is found in pee and sweat, reacts with chlorine in the water, the ACS says.

Originally published on Live Science.

Sara G. Miller
Staff Writer
Sara is a staff writer for Live Science, covering health. She grew up outside of Philadelphia and studied biology at Hamilton College in upstate New York. When she's not writing, she can be found at the library, checking out a big stack of books.