Canadian researchers have figured out a way to test just how much urine can be found in a swimming pool.
The results? In some cases, about 0.01% of pool water is urine.
It is a small amount but likely more than enough for most swimmers - and enough to be a public health concern.
In one instance, the researchers estimated that an 833,000-litre (220,000-gallon) pool could contain about 75 litres of urine (20 gallons) on average.
A smaller 416,000-litre (110,000-gallon) pool was estimated to contain 30 litres (eight gallons).
The University of Alberta researchers analysed more than 250 samples from 31 pools and hot tubs. The samples were collected in two undisclosed Canadian cities, from public and private pools, hotels, and hot tubs.
Dr Xing-Fang Li, with the University of Alberta's Department of Laboratory Medicine and Pathology, and her colleagues said recent instances - like the diving pools at the 2016 Rio Olympics turning green - highlight the importance of monitoring water quality.
The scientists used the amount of acesulfame potassium (ACE), a common artificial sweetener that passes that through the body and is excreted in urine, as a marker for their tests.
ACE was found in all the study samples and allowed them to roughly measure how much urine is actually in a pool.
Researcher Lindsay Blackstock says that "the only logical explanation for why it would end in swimming pools and hot tubs in such elevated levels is urine".
The team knew going into the study that though people do relieve themselves when swimming.
"Although considered a taboo, 19% of adults have admitted to having urinated in a swimming pool at least once," they note in the study.
When mixed with chlorine in a pool or hot tub, urine can contribute to the formation of so-called "disinfection by-products" in pool water that can be harmful to a swimmer's health.
Those compounds. specifically one called trichloramine, can potentially cause eye irritation, respiratory problems, and has been linked to occupational asthma for people who spend hours in pools, like pool workers and professional swimmers.
Ms Blackstock said cleaning chemicals like chlorine, which help stop the transmission of water-born pathogens, are not the problem - people are.
"If you're peeing in the pool, you're contributing to a potential health risk," she said.
Urine is not the only factor in the formation of those disinfection by-products - sweat, body lotions and hair care products contribute to the problem as well.
The Edmonton-based researchers suggested that a public health campaign focusing on pool hygiene, like taking a quick shower before jumping in the pool, might help reduce exposure to those disinfection by-products.
The research was published on Wednesday in the Environmental Science and Technology Letters journal.