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Scientists have found high levels of urine in Canadian public swimming pools Pedro Armestre/AFP

Olympic legend Michael Phelps says that "everybody" does it – but very few people admit it. Now scientists have discovered the answer to the question that you may not want to know: just how much pee is there in your swimming pool?

Researchers have analysed urine levels in 31 swimming pools and hot tubs in two Canadian cities. The results suggest that the 23-time Olympic champion's claim may not be that wide of the mark.

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One regular-sized community pool contained 75 litres of pee, while in a single child size pool there were 30 litres of the yellow stuff. The team found evidence of urine in every pool and hot tub they tested.

The experts employed a new test method to infer their results: they measured the level of acesulfame potassium, an artificial sweetener found in processed foods that is passed through the body without being broken down, and estimated the quantity of urine in the pool based on this.

"Our study provides additional evidence that people are indeed urinating in public pools and hot tubs." one of the researchers, graduate student Lindsay Blackstock told The Guardian.

"We want to use this study to promote public education on appropriate swimming hygiene practices. We should all be considerate of others and make sure to exit the pool to use the restroom when nature calls."

A 2012 paper by US scientists found 19% of adults admitting to having peed in the pool. The quantitative methods employed by the Canadians could not shine a light on the question of how many people discreetly releasing their waste under the water.

Blackstock said: "We did not monitor the number of pool users over the three week time period ... so there is no way we could estimate the number of individual urination events per day."

The team found 75 litres of urine in an 830,000 litre swimming pool, which is one-third the size of an Olympic pool. They found and 30 litres in a second pool, which was half the size of the first. On average hot tubs contained more urine than swimming pools.