If you've ever gone to a public pool worrying that someone might've peed in the water, you have good reason to be anxious because someone almost certainly has. Lots and lots of people, in fact. The average commercial-sized pool has a volume of 20,000 gallons. And nearly 20 gallons of that is urine, according to researchers from the University of Alberta.

The average bladder holds about 500 millilitres (or 1 US pint) of fluid. That means roughly 150 people pee in the average pool. 

If you haven't thrown up yet, you're probably assuming that swimming in urine doesn't matter since chlorine disinfects the water. Not so, according to science. Chlorine is like a bouncer at a club. It's job is to weed out the bad patrons, but it rarely spots them before they run amok on the dance floor - or in this case, your immune system.

Although highly effective at killing bacteria and parasites, chlorine doesn't work instantaneously, so you can still get sick from filthy water — especially if someone's using it as a super-sized toilet. 

For more info, check out this video from Vox.