From tropical parasites to bacterial pathogens, here’s what else might be sharing the water with you.
While it might simply serve as entertainment for a rainy afternoon, swimming ranks among humanity’s most ancient recreational pursuits. The earliest known swimming pool dates back to 3000 BCE, discovered in the Indus Valley civilization.
Millennia later, during the 19th century, swimming pools emerged across Britain and the United States. However, alongside these aquatic innovations came the formidable challenge of maintaining hygienic conditions. Even today, both public and private swimming pools can transform into breeding grounds for infection if proper maintenance standards aren’t rigorously upheld.
Swimming is widely regarded as exceptionally beneficial for most individuals—delivering a comprehensive full-body workout and cardiovascular enhancement while remaining gentle on bones and joints. Nevertheless, on rare occasions, swimming pools have been implicated in outbreaks of gastrointestinal and respiratory illnesses. Even in meticulously maintained facilities, chlorine works harder to protect us than most swimmers might realize.
So, perfectly timed for the summer swimming season (at least in the northern hemisphere), here’s what else might find its way into the pool water alongside you.
Swimming is considered highly beneficial for most people – providing a comprehensive workout and cardiovascular enhancement while remaining low-impact on bones and joints. However, occasionally swimming pools have been linked with outbreaks of gastrointestinal and respiratory ailments. Even in well-maintained facilities, chlorine tends to be working far harder to protect us than we might prefer to acknowledge.
Throughout the past 25 years, swimming pools have emerged as the most frequent setting for waterborne infectious intestinal disease outbreaks in England and Wales. The primary culprit? Cryptosporidium.
This insidious parasite can trigger a stomach ailment lasting up to two weeks. Affected individuals may experience diarrhea, vomiting, and severe abdominal pain—with approximately 40 percent suffering a relapse of symptoms even after the initial illness appears to have resolved.
Fortunately, most enteric diseases (those causing diarrhea and vomiting) resolve naturally in healthy individuals, explains Jackie Knee, assistant professor in the London School of Hygiene and Tropical Medicine’s Environmental Health Group. However, they pose significantly greater concerns for young children, elderly individuals, and those with compromised immune systems, she emphasizes.
Swimmers can contract cryptosporidium when an infected person experiences a fecal accident in the pool, or through inadvertent ingestion of residual fecal matter from their body, Knee explains.
“And they could continue shedding the parasite afterward, even when they’re no longer experiencing any symptoms,” notes Ian Young, associate professor at Toronto Metropolitan University’s School of Occupational and Public Health in Canada.
You may take extraordinary measures to avoid swallowing pool water, yet evidence strongly suggests that some quantity inevitably enters our systems.
A comprehensive 2017 study conducted at public swimming pools in Ohio involved testing the blood of 549 participants, including both adults and children, following one hour of swimming. On average, adults ingested approximately 21 milliliters per hour, while children consumed roughly 49 milliliters hourly.
When swallowed, the infection risk of this water varies dramatically depending on pool occupancy levels. One study discovered that contracting cryptosporidium becomes significantly more probable during peak swimming times. Researchers analyzed water samples from six pools weekly for 10 weeks during summer 2017, detecting cryptosporidium in 20 percent of samples and at least once in every pool tested. Two-thirds of these contaminated samples occurred during the facilities’ busiest periods—the school holidays.
However, cryptosporidium isn’t the only concern requiring vigilance, explains Stuart Khan, professor and head of the School of Civil Engineering at the University of Sydney in Australia.
Infections from opportunistic bacteria, such as Staphylococcus, can affect the skin, he notes, while fungal infections pose additional risks in swimming pool changing rooms, since these pathogens thrive in warm, humid environments.
Another prevalent bacterial infection contractible from swimming pools is swimmer’s ear, Khan explains, typically caused by water remaining in the outer ear canal for extended periods. Unlike other pool-related infections, this condition doesn’t spread from person to person.
Though uncommon, the acanthamoeba group of parasites also inhabits aquatic environments and can cause severe eye infections that may result in blindness, Khan warns.
Airborne transmission represents another potential infection route. For instance, legionella bacteria may inhabit swimming pools. When inhaled through airborne droplets, these pathogens can cause Legionnaires’ disease, a serious lung infection.
Nevertheless, outbreaks of most infectious diseases linked to swimming pools remain relatively rare. “We don’t observe numerous outbreaks of waterborne illnesses in public swimming pools, which indicates that chlorine disinfection performs adequately most of the time, though occasional outbreaks do occur,” Young observes.
Prior to the 1900s, swimming pools operated without chemical disinfectants. Some facilities filtered or frequently changed their water, while others were constructed on slopes to facilitate drainage or equipped with specialized gutters designed to remove visible contaminants.
“Traditionally, public bathing sites were located either in oceans where water underwent natural refreshment, or in fresh water sources like rivers with tidal movement,” Khan explains.
The inaugural use of chlorine in American pools is believed to have occurred in 1903 at a Brown University facility in Rhode Island, following the chemical’s development as a drinking water disinfectant.
In exceptional cases, bacterial infections from swimming pools remain possible, including pathogens such as Campylobacter, Shigella, and Salmonella. These bacteria typically cause gastrointestinal symptoms including diarrhea and stomach cramping, along with fever. However, they can also precipitate serious complications. Fortunately, chlorine effectively mitigates much of this risk, Khan assures.
Viruses such as norovirus—which can produce diarrhea, nausea, vomiting, and stomach pain among other symptoms—prove somewhat more resilient than most bacteria. While isolated outbreak reports have emerged from swimming pools, these incidents usually correlate with equipment failures or inadequate chlorine levels. The virus generally succumbs readily to proper chlorine treatment, Khan confirms.
Maintaining this level of protection against viruses and bacteria requires meticulous pool management, Khan emphasizes. This involves ensuring optimal water pH and alkalinity levels to maximize chlorine effectiveness, he adds.
Additionally, chlorine requirements fluctuate based on pool occupancy at any given moment. “The higher the chlorine demand, the greater the quantity needed. There’s considerable science involved,” Khan notes.
Regulations governing public swimming pool maintenance vary significantly between countries. In the United Kingdom, no specific health and safety laws exist, though operators must comply with the Health and Safety at Work Act. In the United States, swimming pools face regulation at both federal and state levels. While the U.S. Centers for Disease Control and Prevention (CDC) maintains a health and safety code for pools, compliance remains voluntary.
Even in impeccably maintained pools, cryptosporidium demonstrates remarkable resistance to standard chlorine concentrations.
“The cryptosporidium parasite exhibits extraordinary tolerance to chlorine,” Knee explains. “Most other pathogens perish within minutes, yet cryptosporidium remains alive and active for more than a week under normal chlorine treatment levels.”
This resilience stems from the parasite’s unique structure. “It can enter spore formation, essentially wrapping itself tightly and preventing external contact, which confers resistance to numerous threats,” Khan elaborates.
Infection risk likely peaks from larger, more obvious pool accidents, though this danger can be substantially reduced through immediate response, Knee notes. Pool operators can either employ coagulants and filter the water—provided they possess appropriate filtration systems that don’t process water too rapidly—or implement ‘super chlorination,’ Knee explains. The latter approach involves introducing dramatically higher chlorine concentrations and maintaining them for extended periods.
While these incidents remain fairly conspicuous, people can shed fecal material passively without creating obvious disturbances, Knee observes.
Author: AI
Published: July 1, 2025