As temperatures rise across the northern hemisphere, millions are flocking to swimming pools to beat the heat. While swimming is widely praised for its physical and mental health benefits, experts are warning that even well-maintained pools may harbour unseen risks—from tropical parasites to opportunistic bacteria.
Cryptosporidium, a microscopic parasite, remains the leading cause of waterborne gastrointestinal illness outbreaks linked to swimming pools in England and Wales over the past 25 years. Transmitted through faecal matter—sometimes shed even after symptoms subside—it can cause severe diarrhoea, vomiting, and abdominal pain that may persist for up to two weeks, with nearly 40% of cases experiencing a relapse.
“Most healthy people recover without complications, but children, the elderly, and immunocompromised individuals can be at greater risk,” says Jackie Knee, Assistant Professor at the London School of Hygiene and Tropical Medicine.
Studies show that swallowing even small amounts of pool water can pose a risk. A 2017 study in Ohio found that adults swallowed an average of 21 mL of pool water per hour, while children ingested more than twice that amount. In another study, Cryptosporidium was detected in 20% of samples from six public pools during the summer, with most positive samples collected during peak hours.
But Cryptosporidium isn’t the only concern. Stuart Khan, head of the School of Civil Engineering at the University of Sydney, notes that bacteria like Staphylococcus can infect the skin, while moist, humid changing rooms may encourage fungal infections. Swimmer’s ear, often caused by prolonged water exposure in the outer ear, is another common complaint.
Rare but serious threats include Acanthamoeba, a parasite that can cause severe eye infections, and Legionella, which, if inhaled through mist, can lead to Legionnaires’ disease, a form of pneumonia.
Chlorine remains the frontline defence in keeping pool water safe. First used in 1903, it’s effective against most bacteria and viruses—including norovirus and Salmonella—but falls short against Cryptosporidium, which can survive for over a week in chlorinated water. “It’s highly chlorine-tolerant due to its spore-like form,” says Khan.
To counter outbreaks, pool operators must carefully monitor chlorine levels, water pH, and filtration systems. In extreme cases, “super chlorination” or enhanced filtration methods may be needed to eliminate resilient pathogens.
Regulations vary by country, with the UK relying on general health and safety laws and the US applying voluntary CDC guidelines. Experts emphasise that while public pools are largely safe, occasional lapses in maintenance or unexpected contamination can still pose health threats.
So while swimming remains a refreshing summer pastime, experts advise avoiding pools when unwell, showering before entering, and minimising water ingestion to reduce the risk of illness.
