(Intro)

Michael: Ahh, summer, when the days are filled with sun, more sun, and sunburns.  Two years ago, we made a compilation of sun-themed episodes, but since then, we've produced so many more summery videos, so slather on the sunscreen and kick back in your favorite deck chair, because it's time for our second summer compilation.  This time though, we're diving into the science related to one of our favorite summer activities: swimming.  Speaking of diving into things, if you're a water lover, you've probably had the unfortunate experience of jumping into a pool or lake and getting water up your nose, which of course hurts.  A lot.  That's weird, because our nasal passages are wet all the time, and some people even clean them out with water using a neti-pot, so why does getting water up your nose hurt so much?  I'll pass it over to Hank to explain.

Hank: So have you ever done a cannonball into a swimming pool or a pond and accidentally,like, shot water all up into your brain through you nose? Neauuh, not a fan! And yet neti pots, those teapot-looking things that people use to squirt water into their noses when they're feeling stuffy, they're like, soothing and relatively painless. Turns out, the difference has to do with the conditions inside your nasal cavities.

See,the inside of your nose, as you may have notices, is full of nerve endings. And not to get too up-close-and-personal here, but it's also pretty warm up there, and kind of salty, with a salt concentration of about 0.9%. When you snort up fresh, cold swimming pool water, it's a shock to your system because. it's just so different form the natural environment inside your nose. cold temperature can make you fell like you're having a brain freeze, probably because it's making the blood vessels inside you nose constrict. And since the fresh water doesn't match up with your internal salt concentration, there's an uncomfortable feeling as they try to equalize, and the water flows into your cells.

Menawhile, your nasal membranes respond to the irritation by secreting a bunch of mucus, which is why your nose immediately starts running. Neti pots, which flush out your sinuses to remove allergens and thin out mucus, get around this by using warm, salty water that matches your internal environment.

But even though neti pots and accidental cannonball snorts feel totally different, they can both put you at risk for deadly infections. Though it's very rare, a snootful of pond water or an improperly cleaned neti pot just might expose you to brean-eating amoebas called Naegleria fowleri. Again, this is very rare, there are only three or four cases in the U.S, each year, but it's a water-dwelling parasite that if you snorted up your nose,can get into your brain and start destroying tissue.

I'm not saying you've got to give up cannonballs ... but maybe just hold your nose, do the thing that everybody does. You'll get around the pain and the risk of brain-eating infection at the same time!

Michael: If you can avoid getting water up your nose, spending hours upon hours in a cool swimming pool is pretty much the best thing on a sweltering summer day.  If you do that, though, you'll probably notice yourself getting a bit prune-y after a while.  Why does our skin do that?  Well, let me explain.  

Whether you’re swimming or washing the dishes or just taking nice, long, well-deserved bath --- if you’re immersed in water for longer than 10 minutes, chances are your fingers and toes will emerge looking like raisins. So what’s up with the wrinkled digits?

For years, scientists thought the phenomenon was the result of a type of osmosis, caused by water passing into the dry outer layer of skin. The influx of water, the thinking went, would expand the skin’s surface area, but not the tissue below it, so the skin would bunch up and wrinkle.

But in 1935, a pair of doctors noticed that this effect didn’t happen in their patients with nerve damage. One patient, for example, was a boy who had lost the feeling in three of his fingers. The researchers found that, when his hand got wet, the fingers that he could feel wrinkled as normal, but the ones that were numb remained smooth.

It turned out that pruney digits weren’t caused just by the passive flow of water through the skin -- it was an active response of the nervous system to prolonged moisture. The nervous system causes the wrinkling by constricting blood vessels below the skin, which causes the upper layers of skin to pucker.

Since the phenomenon is caused by an involuntary nerve response, some biologists have thought that it must have some evolutionary function. But what possible purpose could it serve? One recent theory suggests that wrinkly skin may have given our ancestors a better grip while working in wet conditions -- like gathering food from a stream or damp vegetation. And it may also have given us better footing while walking across slippery landscapes in the rain.

In a 2013 study, evolutionary biologists tested this theory by asking subjects with either wrinkly and non-wrinkly fingers to pick up a variety of wet and dry objects, like marbles. They found that the subjects with wrinkly digits picked up the wet objects 12 percent faster than their counterparts. But there was no difference when it came to picking up dry objects. The wrinkles apparently helped channel the water away, much like the treads on your car’s tires.

But then this raises the question: If wrinkly skin gives us a better grip, then why isn’t our skin wrinkly ALL of the time? Well, maybe because shriveled fingers and toes are less sensitive, which is no advantage at all.

Raisin-y fingers isn't the only thing pool water can do to your body.  If you're swimming around for a long time in a pool, you might find that your eyes get all red and kind of burn.  You might think that's just a reaction to the chlorine, but it's not.  Here's Hank with more.

Hank: 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 chloramines 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 chloramine, 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 chloramine 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 chloramines, and I shouldn't have to say it, but pretty please, don't pee in the pool.

Michael: Well, I bet you're not going to be able to forget that lovely bit of information about pools anytime soon, and it gets worse, because all that pee in the pool that's making your eyes red, it could be downright dangerous for you.  Here's Stefan with why.

Stefan: If you’ve ever been a competitive swimmer – or, just spent a lot of time at the pool – you might have peed in the water once or twice.

Or maybe a lot. According to interviews, Olympic swimmers pee in the pool all the time.

Even though it sounds disgusting, a lot of people say urine is sterile, plus chlorine is a disinfectant. So what’s a little pee between friends… right? Well, you might actually wanna take your business to the bathroom.

For one, it turns out that pee isn’t all that sterile. But there’s another problem, too: Mixing urine with the chlorine in your pool can make chemicals that might cause respiratory and nervous system problems. Urine is mostly water, but it contains a lot of junk your body doesn’t want anymore, including uric acid and urea, both nitrogen-containing molecules.

Uric acid is produced when your body breaks down molecules called purines, like the ones in some of your DNA bases. And urea is made from the breakdown of proteins. When these waste molecules mix with chlorine in the pool that’s meant to destroy bacteria and viruses, they react to form disinfection by-products, or DBPs for short.

Specifically, urea reacts to create a type of chemical called chloramines, swapping out its hydrogen atoms for chlorine atoms. Trichloramine, especially, is pretty reactive, and can corrode the metal in and around pools. And you might know it by its smell – that classic chemical “pool smell” is caused by chloramine gases, not chlorine.

A lot of people, like lifeguards, have reported they get red eyes, a runny nose, or a scratchy voice after being around the pool too much, which could potentially have something to do with irritation from lots of trichloramine. Some researchers think chloramines could cause respiratory problems in swimmers, too, since they probably breathe in a bunch more than your average pool-goer. But we’ll need to do more research to really understand the health effects.

We’ve known about the connection between urea and trichloramine for a while now, but we’ve pretty recently found a link between uric acid and a molecule called cyanogen chloride. Cyanogen chloride gas doesn’t have a familiar smell, but it’s real bad news and can cause respiratory, cardiovascular, and central nervous system problems. It’s part of a group of chemicals called cyanides, which all have a carbon atom bonded to a nitrogen atom.

They’re toxins, and nasty ones at that. These chemicals mess with how your cells use oxygen, so your cells struggle to produce energy, and if the concentrations are high enough, all kinds of things can go wrong. In one study, published in the journal Environmental Science & Technology in 2014, researchers created synthetic urine and combined it with various concentrations of chlorine.

And within an hour, the uric acid created some cyanogen chloride. The amount varied based on the chlorine concentration they used, but it was around 2 to 8 milligrams per liter. Now, there aren’t many official guidelines about what concentration of liquid cyanogen chloride is dangerous, but some sources recommend avoiding exposure to more than 0.6 milligrams per cubic meter of the gas form.

So getting 2 to 8 milligrams per liter of cyanogen chloride sounds like a huge deal. But it’s pretty unlikely you’ll ever find that much in your swimming pool, because this experiment used higher concentrations of chlorine than you’d find outside of a lab. So, you probably don’t need to panic if you go to a pool party every once in a while.

DBP levels might be an issue at large swim meets, where hundreds of swimmers are probably peeing in the water. Especially if a lot of people are peeing in the same spot – say, at the foot of the diving board – that area will have higher concentrations of DBPs like trichloramine or cyanogen chloride. And researchers are trying to figure out if long-term exposure to DBPs are related to the unusually high amounts of asthma reported among competitive swimmers, which has been documented in multiple surveys.

For now, it looks like DBPs probably aren’t a life-or-death issue in the pool, although they’re not harmless either. So getting out of the pool, drying off, and going inside to use the bathroom is kind of a pain, but your lungs – and probably your friends – will thank you.

Michael: At this point, you might be thinking that maybe you should avoid pools altogether.  You could always swim in a lake after all, but, well, Olivia has some information you might want to consider first.

Olivia: If your summer has ever included cooling off with a nice swim in a lake, you might be acquainted with something called swimmer’s itch. These itchy red bumps appear on the exposed parts of your skin after you take a dip in a lake or pond, and they can last for a week or more.

But swimmer’s itch is more than just an annoying rash it’s actually caused by parasites that burrow into your skin and then die there. Yeah. Ew.

The culprits are tiny worms called schistosomes. Each schistosome species specializes in a specific bird or mammal host. Some schistosome species target humans, causing a debilitating disease called schistosomiasis.

But the ones that cause swimmer’s itch aren’t after you they’re part of a different group of species whose hosts include ducks, geese, muskrats, and raccoons. That itchy rash is what happens when one of these schistosome larvae makes a mistake. Adult parasites live in their host’s blood, and when they lay eggs, the host eventually poops them out.

With a little luck, the eggs end up in water, where they hatch into larvae that swim around in search of the aquatic snails that they need to infect to complete the next stage of their life cycle. The baby schistosomes continue to multiply and develop inside the snail, and eventually the infected snail releases a second type of larvae called cercariae into the water. This is where swimmer’s itch gets its technical name, cercarial dermatitis.

These little guys, each less than a millimeter long, head out to look for a member of their original host species to start the cycle all over again. You’d think it would be easy to tell the difference between a human and a muskrat or goose or whatever, but sometimes the cercariae mess up and burrow into a human swimmer’s skin. It’s a fatal mistake.

They can’t develop there, and so they die. And because dead baby parasites are definitely not something that’s supposed to be in your body, they trigger an allergic reaction as they break down. That’s what all the itching and redness is from.

Because it’s an allergic reaction, some people are more sensitive than others it just depends on how strongly your immune system responds. And if you’re exposed to swimmer’s itch over and over again, you can actually become more sensitive to it over time. You’re most likely to get swimmer’s itch in freshwater, but it’s possible to get it in salt water too, so even at the ocean you’re not completely safe.

To keep your risk small, towel off or shower as soon as you get out of the water, and don’t do anything to attract birds that may host schistosomes to the area. That’s right -- no more feeding the ducks. It’s a small price to pay to avoid parasites dying a slow, itchy death under your skin.

Michael: Basically, there are some downsides to swimming pretty much anywhere, but there are upsides, too.  After all, it is great exercise and it does help you cool off when temperatures spike, and that's more than enough to convince me to follow the sage advice of Dory from Finding Nemo and just keep swimming.  

Thanks for watching this swim compilation and thank you especially to our Patrons on Patreon.  The support of our Patrons is what allows us to make all these videos and we really can't thank them enough, but we can try.  So we're making our next compilation topic a Patron's pick.  We've chosen some fun potential themes and we're leaving it up to our SciShow Patrons to pick which one we go with.  If you want to put your vote in or just learn more about supporting the show, head over to Patreon.com/SciShow.  

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