So when you pee in a pool, your pee mixes with chlorine, creating a by-product which can cause negative health effects.
If you've ever felt your eyes burn and go red when you're near a pool, there's a high chance that someone took a leak in it.
The result of peeing in a pool can also lead to respiratory problems such as asthma and can leave the pool smelling.
Researchers at Purdue University in the US already estimate that pools can contain as much as 30-80 millilitres of urine per person.
But it's not only pee that can react badly with the chemicals in the pool.
Organic matter from your body such as sweat, dirt and lotion can leave the pool unclean.
So do yourself and everyone a favour and use a bathroom... or hold it in.