Swimming is often seen as a relaxing, healthy activity, especially during the summer months. It provides numerous benefits – from muscle engagement to improved lung capacity. Yet, even clean-looking swimming pools may contain hidden threats to our health. Behind the sparkling water surface may lurk parasites, bacteria and chemicals that pose a danger if not properly managed.
Key facts to keep in mind when swimming:
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Over 20% of tested pools have contained Cryptosporidium.
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Children swallow up to 49 mL of pool water per hour.
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Even well-maintained pools can carry parasites resistant to chlorine.
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Contamination can occur via inhalation, ingestion or direct contact.
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One of the biggest risks comes from seemingly healthy swimmers.
Below we uncover the facts about what can truly swim alongside us in pools, how it spreads, and what is being done to stop it.
Cryptosporidium in swimming pools in Anglia and Walia
Over the last 25 years, public pools in England and Wales have been the most frequent site of waterborne gastrointestinal outbreaks. The primary culprit is the Cryptosporidium parasite, which causes a highly contagious stomach illness. Symptoms include:
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Diarrhoea
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Vomiting
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Abdominal cramps
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In some cases, a relapse after initial recovery
Around 40% of those infected experience a return of symptoms even after seeming to get better. Jackie Knee, a researcher from the London School of Tropical Medicine, explains that while healthy people often recover without treatment, small children, the elderly and the immunocompromised are especially vulnerable.
Contamination often occurs when infected individuals unknowingly release faecal particles into the water – even microscopic amounts. Professor Ian Young from Toronto adds that symptomless people can still shed the parasite after recovery.
A 2017 study in Ohio tested 549 swimmers’ blood and showed how much water was swallowed during just one hour in a pool:
| Age group | Average water swallowed (mL) |
|---|---|
| Adults | 21 mL |
| Children | 49 mL |
The chance of contracting Cryptosporidium is significantly higher during peak hours, such as school holidays. Researchers found the parasite in 20% of pool samples across six locations, most frequently when the pools were busiest.
Other pathogens in pools - bacteria and fungi
Cryptosporidium is far from the only concern. Professor Stuart Khan from the University of Sydney warns about opportunistic bacterial infections such as:
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Staphylococcus aureus, causing skin infections
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Pseudomonas aeruginosa, linked to swimmer’s ear
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Fungal infections, especially from wet floors in changing rooms
Acanthamoeba parasites, though rare, pose another threat. These can infect the eye and even lead to blindness, particularly dangerous for contact lens wearers.
Legionella bacteria are also present in some cases. If inhaled in water droplets, they can cause Legionnaires' disease, a severe form of pneumonia. While such infections are uncommon, the presence of even trace amounts of dangerous pathogens is reason enough for caution.
How swimming pools are disinfected and why chlorine isn’t always enough
Before the 20th century, pools relied on flowing water or manual cleaning. In 1903, chlorine was first introduced in the USA at Brown University to chemically disinfect pool water. Today, it's the most common treatment, but not all threats are eliminated by chlorine.
Pathogens killed by chlorine:
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Shigella
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Salmonella
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Norovirus (in most cases)
However, Cryptosporidium is highly chlorine-resistant. According to Knee, it can survive for over a week even in properly chlorinated water. This resistance comes from its tough outer shell that shields it from chemical contact.
To counter severe contamination, pool operators may apply:
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Super chlorination: Increasing chlorine concentration for a longer period.
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Filtration with coagulants: Helps trap and remove the parasite more effectively.
Standard vs. Emergency Pool Treatment
| Treatment type | Chlorine level | Duration | Target |
|---|---|---|---|
| Standard chlorination | 1-3 ppm | Constant | Most bacteria & viruses |
| Super chlorination | 10 ppm+ | Several hours | Resistant parasites (e.g., Cryptosporidium) |
Urine, sweat and chemical reactions
The distinctive "chlorine smell" is actually not chlorine itself, says Khan. It's a sign of chloramine, a compound formed when chlorine reacts with ammonia from urine or sweat. These chloramines hover above the water surface and can irritate the:
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Eyes
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Throat
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Lungs
Professor Young warns that constant exposure to chloramines might increase the risk of asthma in pool workers and regular swimmers. Even short-term exposure can cause discomfort, especially in poorly ventilated areas.
Natural waters also pose dangers
Swimming in rivers, lakes or seas may seem like a safer, more “natural” alternative, but these environments are also risky. Contaminants include:
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Untreated sewage
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Animal faeces
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Industrial run-off
These sources can contain Escherichia coli, Leptospira or parasites, all of which can cause serious health issues.
Preventing pool infections
Proper hygiene is the first defence against contamination. Knee and Young recommend the following to reduce risk:
For swimmers:
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Shower thoroughly before entering the pool
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Avoid swallowing pool water
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Do not swim when sick with diarrhoea
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Report any contamination immediately
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Limit pool time during crowded hours
For pool operators:
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Maintain correct chlorine and pH levels
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Regularly monitor pool water quality
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Super chlorinate when needed
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Scrub pool surfaces to remove biofilm
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Ensure effective ventilation systems
Widden dangers don’t outweigh the benefits
Despite the presence of pathogens, public swimming pools remain safe when properly maintained. Outbreaks of infectious diseases are rare, thanks to modern water treatment technologies and hygiene standards. As Knee puts it, “Properly managed pools pose minimal health risks.”
Swimming offers both physical and mental health benefits. With informed habits and good pool management, the risk of infection can be kept low. This summer, remember to shower, avoid swallowing water, and stay alert – and you can enjoy the water safely.
Source: BBC