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Maybe you've had this experience.  After a long day at the pool, your eyes start stinging a little.  They might even turn all red, a burning, itchy, bloodshot mess.  Now, you might think that's the price you pay for swimming without goggles and that your eyes are just sensitive to the chlorine in the water.  The real answer is a lot more disgusting.  Apologies in advance.

The stinging redness actually means the water you've been in for the past hour includes a fair amount of urine, sweat, or even poop.  Yup.  Your eyes are actually doing you a sort of favor and screaming out in their bloodshot pain that you've been in water that's less than sanitary.  

To be fair, chlorine is still involved.  The real offenders here are a class of chemicals called chloromines which form when chlorine reacts with nitrogen-rich ammonia or similar compounds in those bodily fluids.  Pee and sweat both, for example, contain urea, a molecule with two ammonia-like parts.  When urea comes in contact with chlorine, it swaps out its hydrogens for chlorine and voila, chloramine is born.  That chloramine is what actually irritates your eyes, not the cleaning stuff that was put in the pool originally.  

So how do you know whether you should risk jumping into the water?  Go with your nose.  It turns out that the chlorine odor you associate with pools isn't actually chlorine.  That's also chloromine, and not only is the smell a sign that there's a lot of bodily fluid in the pool, but because all that chlorine is bound to nitrogen, it means that there's probably not that much free chlorine left to do what it's meant to do--disinfect.  

That's because chlorine only works well, ripping apart bacteria and destroying viruses, when it's dissolved in water as hyprochlorous acid.   Once its part of a chloromine molecule, that killing power goes way down.  So if there's a strong smell to the pool, you might wanna reconsider your swim, and if you wanna help rather than hinder the red eye problem, take a quick shower before you swim.  In about a minute, you can wash away any sweat or dirt that you might otherwise bring into the pool that would make more chloromines, and I shouldn't have to say it, but pretty please, don't pee in the pool, and thank you for watching this episode of SciShow.

We will be uploading more summer themed videos all week, so make sure to subscribe.  If you wanna learn more weird facts like this and if you wanna impress your friends with science while you're swimming, check out these awesome new SciShow beach towels, available at DFTBA.com/scishow.  That's right, we're making beach towels.  Sorry to everyone in the Southern hemisphere, I know that it's winter there.

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