As soon as you enter the changing room and make your way to the pool deck, you are hit by a certain smell. Most people think it’s chlorine. Not precisely, it isn’t. Chloramine, a byproduct produced when chlorine reacts with perspiration, urine, and other organic matter already present in the water, is what gives off that acrid, eye-stinging smell. It’s a tiny detail, but it completely alters your perception of that odor. In a way, what you’re breathing in is proof.
The practice of swimming has a longer history than most people realize; archaeological evidence of pools in the Indus Valley dates back to approximately 3000 BCE. The issue of hygiene is a relatively recent development. It wasn’t until the 19th century that public pools became widespread in both the United States and Britain.
Almost immediately, operators had to deal with a problem that hasn’t been fully resolved: how do you keep water clean when hundreds of strangers are sharing it, perspiring in it, and occasionally getting into accidents? It wasn’t until 1903 that chlorine was utilized for the first time at a pool at Brown University in Rhode Island. Prior to that, pools relied on gutters, drainage slopes, or just changing the water frequently and crossing their fingers.
| Category | Detail |
|---|---|
| Topic | Swimming Pool Hygiene |
| Primary Risk | Cryptosporidium parasite, causing gastrointestinal illness |
| Common Pathogens | Cryptosporidium, norovirus, Legionella, Pseudomonas, Shigella |
| Average Water Swallowed | ~21 mL/hour (adults), ~49 mL/hour (children) |
| Key Protective Measure | CDC Healthy Swimming guidance on chlorine and pH balance |
| Regulatory Body (UK) | Health and Safety Executive |
| Regulatory Body (US) | Centers for Disease Control and Prevention |
| Notable Finding | Two-thirds of contamination samples occurred during peak/school holiday hours |
In England and Wales, swimming pools have been the most frequent location for waterborne intestinal disease outbreaks over the previous 25 years. Cryptosporidium, a parasite that can cause vomiting, diarrhea, and cramping in the abdomen that lasts up to two weeks, is by far the biggest offender. After they believe they have recovered, about 40% of those who contract it will relapse. It’s an odd, nearly intransigent illness. The majority of healthy adults will eventually get over it, according to Jackie Knee, an environmental health researcher at the London School of Hygiene and Tropical Medicine. The situation is completely different for children, the elderly, and those with compromised immune systems.
The physical structure of Cryptosporidium makes it particularly challenging to control. It is resistant to conventional disinfection because it can effectively seal itself into a protective shell, a type of biological lockdown. Even after their symptoms have subsided, infected swimmers may continue to shed the parasite, according to Ian Young, an occupational and public health professor at Toronto Metropolitan University. Anyone who imagines a crowded pool on a Saturday afternoon in July will find that unsettling.
Additionally, more people than they’d like to acknowledge swallow pool water. According to a 2017 Ohio study, adults swallow about 21 milliliters per hour while swimming, while children almost double that amount at 49 milliliters. Until you think about what else might be suspended in that water, none of that seems like much. Over the course of a single summer, researchers tested six pools every week and found Cryptosporidium in 20% of samples. Two-thirds of these positive results clustered around the busiest times, especially during school holidays. It turns out that crowded pools contain more than just floating noodles and splashing children.
Although they are typically less severe, bacterial infections also play a role. Skin infections can be brought on by Staphylococcus. Although common, swimmer’s ear is not contagious. It is caused by water that remains in the ear canal. Rather than the water itself, fungal infections typically flourish in the warm, humid corners of changing rooms. Additionally, there are the less common but more terrifying acanthamoeba parasites, which can cause eye infections severe enough to endanger vision, and Legionella, which can cause a serious lung infection if inhaled through fine water droplets. These don’t often appear. They are subtly present in the background of an activity that most of us find soothing.
Viruses can cause problems of their own. Although outbreaks connected to pools are typically associated with equipment malfunctions or excessively low chlorine levels, norovirus is more resilient than most bacteria and more difficult to eradicate. The system tends to function when the chemistry is correct, the pH is balanced, and the amount of chlorine in the water is appropriate for the number of users. When you start delving into the chemistry involved, Khan’s statement that “there’s quite a science to it” almost seems like an understatement.
Countries differ strangely in their regulations. The UK relies on the more general Health and Safety at Work Act rather than having a swimming pool-specific health law. Federal and state authorities in the US share oversight, and even the CDC’s own pool health code is optional rather than required. It’s a patchwork system that primarily depends on individual pool operators acting morally every day, every season.
The risks associated with swimming in open water are entirely different. Chlorine is not used to treat lakes, rivers, or coastlines, and contamination may result from wildlife, sewage, or agricultural runoff. Between May and September, the Environment Agency in England keeps an eye on several hundred designated bathing waters and rates them from Excellent to Poor. Daily pollution forecasts are released following periods of high precipitation. It is more difficult to determine the true risk to humans using waters outside of that designation because they are managed for fish and wildlife rather than swimmers.
Despite everything, Knee and Young appear to come to the same conclusion: swimming is still worthwhile overall. The risk of infection is relatively low in well-managed pools with attentive personnel and sensible maintenance schedules. Things usually go wrong because of the gaps, the malfunctioning equipment, the neglected showers, and the packed weekends. To be honest, I’m not sure if that assurance makes people feel any different when they wade into a crowded public pool this summer.
i) https://www.bbc.com/future/article/20250630-how-hygienic-are-public-swimming-pools-really
ii) https://www.gov.uk/government/publications/swim-healthy-leaflet/swim-healthy
iii) https://www.healthline.com/health/swimming-pool-germs-safety
iv) https://legionellacontrol.com/legionella/diseases-swimming-pools-spas/
v) https://www.cdc.gov/healthy-swimming/prevention/index.html