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How Do Germs Get into the Swimming Pool? You Might Not Want to Know

The views expressed are those of the author and are not necessarily those of Scientific American.

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cryptosporidium, infection, pool, swimAs the summer winds down and Labor Day weekend approaches in the U.S., beaches and public pools will be filling up with swimmers looking to take one last dip outdoors before the season ends. Most people will hit the water without worrying about the microscopic organisms they’ll be swimming with. Maybe that’s for the best, considering what those organisms are and how they’re introduced to swimming holes.

The protozoan organism Cryptosporidium, one of the most frequent causes of waterborne disease, has become a major problem in swimming pools, says Charles Gerba, a microbiologist at the University of Arizona who’s spent decades studying how pathogens are transmitted. Crypto is a microscopic parasite with a tough outer shell that allows it to survive for days even in properly chlorinated pools.

The germ generally causes prolonged bouts of diarrhea if swimmers drink contaminated water. How crypto contaminates the water in the first place is another matter. “Besides what the average person puts in the pool when they go swimming”—such as sweat, sunscreen and even urine—“there is also the problem of AFR’s, or accidental fecal releases, by bathers, which occur on a regular basis in pools which have children in them,” Gerba says.

But such accidents are not the only way water becomes contaminated. One of Gerba’s studies from a decade ago revealed that bathers naturally shed an estimated 0.14 gram of fecal matter during a swim. Another Gerba study indicated the average bather releases 50 milliliters of urine and a liter of sweat per hour into the water during a recreational swim.

Although Cryptosporidium outbreaks in swimming pools certainly are not a new phenomenon, one area of central Idaho reported 21 cases over 10 days earlier this month and late July. That same region normally sees maybe 10 cases annually, according to the state’s Central District Health Department (CDHD).

Overall, a well-maintained pool poses no major infection risk to healthy individuals. Still, there are ways to cut down on crypto in swimming areas. One is for parents to take their kids out of the water frequently for bathroom breaks and to check diapers for infant AFRs. The U.S. Center for Disease Control and Prevention also recommends that all swimmers wash themselves thoroughly with soap and water prior to entering the pool. Swimmers should also check the pool water for signs of trouble before jumping in. If you can’t see the bottom of the pool, or if the edge of the pool is foamy, you might want to find an alternate way of cooling off.

Image courtesy of Slobodan Vasic, via

Larry Greenemeier About the Author: Larry Greenemeier is the associate editor of technology for Scientific American, covering a variety of tech-related topics, including biotech, computers, military tech, nanotech and robots. Follow on Twitter @lggreenemeier.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. Na g n o s t ic 8:26 pm 08/21/2012

    Needlessly inflammatory article. Most pools already provide showers and signs indicating the expectation that prospective bathers shower prior to pool entry.

    Of course, we could ban pools. I’m sure there’s somebody in the government this moment dreaming of a pool-free utopia and how they can hasten its arrival.

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  2. 2. elderlybloke 1:43 am 08/22/2012

    Plenty of Signs,but does anybody read them or take any notice of them?

    Answer- Occasionally.

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  3. 3. BillR 8:58 am 08/22/2012

    Add a requirement for everyone to get an enema before getting into the pool. Great business opportunity! Set up your enema shop next to the pool and clip on a tag to the wrist good for 4 hours of certified safe swimming….

    Seriously though, I do not use public pools unless the water is clear and I see someone making measurements of the water quality. I also do not bother getting into a pool full of kids. I will relax on the sidelines and watch them having fun and get some quality reading time in.

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  4. 4. Ulysses Susan Grant 2:15 pm 08/22/2012

    Accidental fecal release?!!! I’m sorry, but I’m laughing at this one. Humans must be the only dim-wits to swim where they poo. We deserve any pathogens that come our way — afterall — we made them.

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  5. 5. scientific earthling 8:31 pm 08/22/2012

    Ulysses Susan Grant: You are wrong about humans being the only animals to swim where they poo. Ever heard of fish, don’t they poo? and so do all the Hippos and other animals you see spending time in the water.

    By the way one organisms poo is another organisms yummy yummy food. Shock horror humans too, ever ate cheese? or should I call it bacteria poo and corpses. The Oxygen you breathe today was the pollution waste of lifeforms that lived in the sea at a time there was no Oxygen in the air. Don’t worry our planet will get back to that state, and we humans are rushing it along.

    Oxygen is the second most reactive element after Fluorine. As Oxygen it will react to oxidise Carbon, Nitrogen, Sulphur etc it reacts with everything on the opposite side of the periodic table, and if it was not for the pollution of our atmosphere we would not be around. Wont be around anyway when we succeed to return our planet to its pre polluted state.

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  6. 6. jgrosay 11:03 am 08/29/2012

    An old joke: a man goes towards a swimming pool, someone stops him at the entrance: “Mr. Marquis, you’re banned from entering the pool” -”I made the decision to build this pool, what’s the reason why?” -”you pee in the pool” -”everybody pees in the pool” -”yes, bu you do it from the top of the trampoline”

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  7. 7. andrewf 11:04 am 08/29/2012

    I wonder if UV sterilization of pool water would help with these chlorine-resistant pathogens. Two modes of action are better than one.

    But yeah. I do not swim in very busy public pools, especially the ones at amusement parks. I get grossed out enough by the thing I can see (bandaids, boogers, clots of hair) never mind the things I don’t.

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  8. 8. Quinn the Eskimo 5:52 pm 09/2/2012

    Okay, I get all this. However, a properly maintained swimming pool is chlorinated. Now, think about your favorite lake or pond. Public beaches, which never ever see chlorine.

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  9. 9. jhonraymagno 5:20 am 11/8/2012

    Hi! I definitely agree to the author. As a pool service staff to one of the most popular pool cleaning service in Palm Beach, [www.//] almost all our main inquiries comes from commercial pool owners who are asking for a cleaning service prior the summer season because they fear of getting bad reviews about the situation of their pools. Nevertheless, its still a part of their responsibility to have an annual maintenance. We are only acting as a tool to prevent germs and diseases from spreading. The final decision still lies in their hands.

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  11. 11. jamesv 3:44 pm 12/12/2012

    Germs in a swimming pool are no laughing matter. has a lot of great information about pool related illnesses. Happy swimming

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  12. 12. johnmarry89 8:37 am 07/19/2013

    During summers swimming pool is heavily used by one and all. So, it easily becomes home to harmful bacteria and algae. Thus, it becomes crucial to clean the pool water effectively and at regular intervals to ensure shimmering and germ free pool. With the use of high-quality pool filters and pool chemicals swimming pool can remain safe and healthy! has a great collection of pool filters for your swimming pools.

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  13. 13. swimingpool 5:52 am 02/19/2015

    Summer is hotter than ever and your swimming pool continues to be the most reliable place to cool off and relax. Unfortunately, the scorching temperatures mean an increase in droughts and now cities around the nation are placing greater emphasis on water conservation.

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  15. 15. nancy84 10:34 am 04/18/2015

    Shelley Hill Pools has over 40 years of experience providing cool pools in rochester ny, . Our focus on quality and aesthetics has made us one of the leading providers of high-end pools in the community, which is why we are the top choice for many of your neighbors. you can look us up at

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  16. 16. darrencharles 1:15 am 04/25/2015

    Our skin, sweat from our skin and saliva can boost the population of germs inside a swimming pool which can prove to be really fatal. In case a pool is properly maintained, most germs in it can be killed by certain chemical. You can also get the swimming pool remodeling done if you want to completely sanitize your pool.

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