Because research shows all pools are pee-filled.
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Let's just call out public pools for what they really are: enormous shared toilets we like to splash around and knock back margaritas in. Around one in five people admit to peeing in the pool, which means four out of five lie about it, and the Centers for Disease Control and Prevention has found that 58 percent of communal pools have E. coli (read: poop particles) in them.
Most of us—including professed pool pee-er Michael Phelps—believe it's cool because there's chlorine added to the water to kill the germs, but those sanitation efforts aren't always a sure bet, says Christopher Saddler, an infectious disease physician at the University of Wisconsin School of Medicine and Public Health. A report from the CDC found that eight out of every ten pools incurs at least one health or safety violation (like not having enough disinfectant in the water), and one out of every eight pools gets closed immediately after a health check. So we're being gross, and the pool barons aren't doing enough to combat our blatant disregard for sanitation. But can you really get sick from a spring break dip?
Let's talk about the effects of pee first as a warm-up. There's a lot of it—research shows a commercial-sized swimming pool (222,000 gallons) might contain around 20 gallons of urine. Pee itself has really low levels of bacteria and isn't likely to make you sick, but when it reacts with disinfectants like chlorine, chemical byproducts are formed that can irritate your eyes (you've felt the sting) and lungs—hello pool cough. (Lots of endurance swimmers have asthma, but more research is needed to explore the link.) So the fact that most people whiz out their Coronas into the aquatic abyss is pretty nasty, but it's not really enough to completely undermine your floating minibar dreams.
The facts about poop, however, might give you second thoughts. Excluding babies, most people aren't hiding under a floating swan to take a secret water dump (choose your friends wisely) but the brown stuff can kind of just rinse off you—most people carry around .14 grams of fecal matter into the pool, says the CDC. (The suggestion of showering before taking a dip is starting to feel less insane.) And poop that gets in your mouth—even in really small amounts—can withstand the power of chlorine to wreak havoc on your insides. "Most people who get a short-term gastrointestinal illness from swimming don't even realize where they got it from," says Saddler, who admits he's more comfortable on land after studying this stuff.
The most common poop-related pool parasite is called cryptosporidium, which usually shows up as seven to ten days of diarrhea before it passes on its own. (Crypto is still pretty rare—there were only 874 cases from 2011-2012, the most recent data.) Pools can pass around norovirus, the cruise-ship virus that also causes toilet warfare. Meanwhile, E. coli in pools can produce a toxin called Shiga, which—you guessed it!—causes diarrhea. (A light at the end of this shit-stained rainbow: You can't get HIV or STDs from swimming with an infected individual.) Just the runs, or a hospital stay if you've got a compromised immune system.
So here's the bottom line: Keep swimming because it feels great, but keep your mouth closed underwater—and maybe even during a drunken splash fight. And when you start feeling the effects of those light beers, do everyone a favor and make a run to the real bathroom.
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