Swimming is a cherished summertime tradition, but what are we really diving into? From chlorine-resistant parasites to airborne bacteria and viruses, this in-depth report reveals the hidden dangers of public swimming pools and how to protect yourself and your family. Experts explain the science behind pool hygiene and what it takes to keep these recreational hotspots truly safe.
How healthy are public swimming pools?
From parasites to bacterial pathogens, there are many things that can be swimming in water.
Swimming is perhaps the oldest pastime of humans. The earliest recorded history of swimming pools dates back to 3000 BC and is recorded in the Indus Valley.
Much later, in the 19th century, swimming pools appeared in Britain and the United States. But they also faced the challenge of how to maintain hygiene. Even today, public and private swimming pools can become hotbeds of infection if they are not properly maintained.
Many people consider swimming to be very beneficial – it provides full-body exercise and cardiovascular stimulation, while being low-impact on bones and joints. However, in rare cases, swimming pools have also been linked to outbreaks of stomach and respiratory illnesses. Even in very well-maintained swimming pools, chlorine does more than protect us.
This is a look at what can enter your body from swimming pool water.
Which bacteria are swimming alongside us?
Over the past 25 years, swimming pools have become a common setting for outbreaks of waterborne intestinal diseases in England and Wales. The most common cause is cryptosporidium.
This parasite can cause a stomach illness that can last up to two weeks. During this time, people experience symptoms such as diarrhoea, vomiting and abdominal pain. Up to 40% of cases may recur after the initial illness has subsided.
But in most cases, these intestinal infections (which cause diarrhea and vomiting) resolve spontaneously in healthy people, says Jackie Nee, an associate professor in the environmental health group at the London School of Tropical Medicine. However, she says they can be a major concern for young children, the elderly and people with weakened immune systems.
Nee says swimmers can contract cryptosporidium by accidentally touching an infected person’s stool in a swimming pool or by ingesting fecal matter from their bodies.
“Even when they’re not showing symptoms, they can still shed [the parasite],” says Ian Young, an associate professor at the School of Occupational and Public Health at Toronto Metropolitan University in Canada.
You can avoid swimming in pool water, but evidence suggests that some of it eventually makes its way into our bodies.
A 2017 study in Ohio looked at the blood of 549 people, including adults and children, who swam for about an hour in a public swimming pool. On average, adults drank about 21 milliliters of water per hour, while children drank about 49 milliliters per hour.
The risk of infection from swimming in water varies depending on how busy the pool is. One study found that swimming during busy times of the year was associated with a higher risk of cryptosporidium infection. Two-thirds of the water samples were taken during the busiest times of the year, during school holidays.
In the summer of 2017, researchers tested water from six swimming pools once a week for 10 weeks and found cryptosporidium in 20% of the pool samples, at least once in each pool.
But cryptosporidium isn’t the only thing to worry about, says Stuart Khan, professor and head of the School of Civil Engineering at the University of Sydney in Australia.
Infections caused by bacteria such as staphylococcus can infect the skin, he says, and because these pathogens can survive in warm, moist environments for long periods, fungal infections are a possibility in changing rooms around swimming pools.
Another common bacterial infection that can be contracted at swimming pools is a condition called ‘swimmer’s ear’, which is caused by water remaining in the ear for a long time, Khan says. However, it is not spread from person to person.
Khan says that acanthamoeba, a parasitic amoeba that lives in water, can cause eye infections. If such a condition occurs, it can be very serious and can even lead to blindness.
Infections can also be contracted through inhalation. For example, a bacteria called legionella can be active in swimming pools. When inhaled through air bubbles, it can cause Legionnaires’ disease, a lung infection.
However, even if infections occur during swimming pool use, they are very rare and spread as diseases. “We don’t see a lot of waterborne diseases spreading in public swimming pools, which means it’s been exposed to chlorine for a long time and there are outbreaks,” Young says.
How do bacteria survive in swimming pools?
Before the 1900s, there was no chemical disinfectant for swimming pools. Some pools were filtered or changed frequently, some had a slope to drain the water, and some had drains to remove visible debris.
“Traditionally, public bathing areas were located near a naturally fresh water source, such as the ocean or a river,” says Khan.
The first use of chlorine in a swimming pool in the United States was in 1903 at the Brown University swimming pool in Rhode Island. It is believed that the chemical was developed to be used as a disinfectant for drinking water.
In rare cases, pathogenic bacteria, including Campylobacter, Shigella and Salmonella, have been found in swimming pools. In most cases, these bacteria cause diarrhea and abdominal pain, as well as fever and stomach-related illnesses. They can also cause serious complications. Fortunately, chlorine has minimized many of the risks, Khan points out.
Viruses such as norovirus, which can cause diarrhea, nausea, vomiting and abdominal pain, among other symptoms, are significantly more severe than most bacteria. There have been reports of them spreading in swimming pools. However, such cases are associated with factors such as equipment failure or too low chlorine levels. Viruses are usually killed well by chlorine, Khan says.
To maintain this level of protection against viruses and bacteria, a swimming pool needs to be well-maintained, says Khan. This includes ensuring that the water is at the correct pH and alkalinity (to neutralize acids) for chlorine to be effective.
The amount of chlorine needed also depends on how many people are in the pool at any given time. “You need to add chlorine when the demand is high. There’s a science to it,” says Khan.
Regulations for the maintenance of public swimming pools vary from country to country. There are no specific health and safety laws in the UK, but operators must maintain them in accordance with the Health and Safety Act. In the US, swimming pools are regulated at both the federal and state levels. The U.S. Centers for Disease Control and Prevention has a health and safety code for swimming pools, which is voluntary.
But even in properly maintained swimming pools, Cryptosporidium is resistant to normal chlorine levels.
“Cryptosporidium is extremely resistant to chlorine,” says Nee. “Most other pathogens are killed in minutes, but Cryptosporidium can survive and thrive in normal chlorine concentrations for more than a week.”
This is because of the way the parasite is structured. “It goes into spore formation, where it’s tightly encapsulated and prevents anything from coming in contact with it, and it’s also resistant to a lot of things,” Khan says.
The risk of infection from larger and more obvious hazards in the pool can be high, but this risk can be minimized if you respond quickly, says Nee. Pool operators can use a flocculating agent and filter the pool water if they have the right filtration system that doesn’t do instant filtration. If not, Nee says, the process of adding significantly more chlorine than normal can be called ‘superchlorination.’ Then, a large amount of chlorine is added to the water and left for a long time.
But while these incidents may be very visible, people can still passively excrete waste without making a big deal of it, says Nee.
Are there other risks in swimming pools?
You might be surprised to learn that the chlorine smell you may be picking up from your changing area and when you enter the pool isn’t actually chlorine.
“This smell occurs when chlorine reacts with other substances in the water, especially ammonia concentrations like urine and sweat,” says Khan. This ammonia reacts with chlorine to form chloramines, which create an odor.
Swimming in other environments
The presence of many unwanted guests around swimming pools may tempt you to consider choosing a natural swimming spot. Sometimes it could be a swimming pool in a lake, river or sea. But even those places carry the risk of infection that we all know about.
Some water bodies may contain sewage, and some may be contaminated with animal feces.
“So the smell suggests that there may be bodily fluids in the pool that are reacting with the chlorine,” says Khan.
These chloramines float on the surface of the water and can irritate our throats and eyes when inhaled, says Young. “Chemicals that cause inflammation and reduce the quality of chlorine in the pool can affect everyone’s health,” he says. “Even a short exposure can affect you.”
A small group of studies has suggested that people who are exposed to chloramines on a daily basis, such as swimming instructors and lifeguards, are at higher risk of developing conditions like asthma.
What can you do to reduce your risk when swimming?
Young says that making sure everyone showers before entering the pool can reduce the risk of chloramines building up on the surface of the water, which can help eliminate fecal matter. Showering can also help reduce the risk of disease transmission and infection, says Nee.
Yong also emphasizes the importance of good ventilation around swimming pools.
Another important way to avoid getting sick from swimming pool water is to avoid swallowing pool water, says Nee. Diarrhea-causing pathogens are transmitted by ingesting water contaminated with feces.
It’s important to notify pool personnel immediately if a pool is contaminated and to get out of the pool immediately, says Nee.
The Centers for Disease Control and Prevention recommends that pool operators reduce the risk of infection by regularly draining and refilling with clean water, maintaining chlorine and pH levels within certain ranges, and thoroughly scrubbing the pool surface to remove slime.
Both Nee and Khan agree that the health and social benefits of swimming outweigh the risk of infection.
“Properly maintained, properly maintained swimming pools, and operators who are knowledgeable about how to handle a contaminated incident, have minimal health risks in terms of infectious disease transmission,” says Nee.
So, there are plenty of reasons to hit the pool this summer. But make sure to shower first.
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