Hayley Bennett deep dives into disinfecting dirty pools

A little over a century ago, taking a dip in a public pool was a filthy business. There was no such thing as a disinfection system and the only means of removing dirt and grime was via filtering, changing the water, or ‘scum gutters’ located around the edge of the pool.

Even health specialists of the time appear to have been startled at the levels of germs in public swimming baths. In 1910, public health officials from Batley, West Yorkshire collected water samples from local swimming baths. On average, they counted over 111,000 microbes in every cubic centimetre of water sampled. ‘We are faced with the fact that this analysis tells us that the water examined was nothing more or less than dilute sewage,’ they wrote. Although not all the organisms they counted were harmful, their results weren’t exactly an endorsement of the water quality.

In your class

There is some interesting chemistry that can be derived from the topic of swimming pools, such as how organic molecules interact with cleaning products. Pique learners’ interest with contexts driven by sport, environment or analysis. Synoptic questions relating to a single context can provide a challenge for learners approaching the end of their chemistry course.

Today, we have chemical disinfection systems designed to kill the microbes in our public pools. Now, though, we have a different water quality problem. There may be less chance of getting a stomach bug – or something more sinister – when you go for a swim, but there is a growing body of evidence pointing to the harms associated with the chemical disinfectants used in pools.

How are pools disinfected?

Chlorine cleaning

There are lots of chemicals that can be used for disinfection. Most modern swimming pools, however, use chlorine. As Stuart Khan, a water researcher at the University of New South Wales (UNSW) in Sydney explains, ‘Chlorine is generally much more effective [than other disinfectants] since it is a very powerful oxidant and inactivates bacteria and viruses quickly.’ Oxidation – the loss of electrons – damages cells and this is what kills all those stomach-churning bugs that used to inhabit public pools. Other types of pools may also employ the disinfecting power of chlorine – in saltwater pools, the chlorine comes from plain old sodium chloride (NaCl).

  • Download this

    Synoptic questions, for age range 16–18

    Explore the topics of structure and bonding, equilibrium, spectrometry and spectroscopy in the context of swimming pools.

    Download the student worksheet as MS Word and pdf, the support worksheet as MS Word or pdf, the teacher notes and answers as MS Word or pdf and a presentation to stimulate discussion as MS Powerpoint or pdf.

Download this

Synoptic questions, for age range 16–18

Explore the topics of structure and bonding, equilibrium, spectrometry and spectroscopy in the context of swimming pools.

Download the resources from the Education in Chemistry website: rsc.li/3wnmMqJ

The reactivity of pure chlorine gas makes it too dangerous to simply pump into a pool. During World War I, thousands of soldiers were killed by chlorine in poison gas attacks. It can cause serious breathing difficulties when inhaled, so the risks of accidental release are too high. Instead, in swimming pools we use solid salts or tablets containing sodium hypochlorite (NaOCl) or calcium hypochlorite, Ca(OCl)2, which react with water to produce hypochlorous acid (HClO).

Hypochlorous acid is strongly oxidising and deals with all the undesirables in the water with less risk to pools’ staff and swimmers. However, even when tablets are used, the disinfectant level must be carefully controlled because sunlight can degrade the HClO, releasing free chlorine that escapes at the water’s surface.

The chlorine or HClO needed to neutralise the biological contaminants is known as the ‘chlorine demand’ and what remains is the ‘residual chlorine’. The residual chlorine has to be kept below a certain level, while at the same time maintaining enough extra capacity in case of accidents – such as noxious leaks from swim nappies.

Disinfection by-products

While public pools need to be disinfected to keep them from turning into the ‘sewage’ baths of Batley, recent research has begun to reveal the potential harms of having a bunch of reactive chemicals floating about in the same space as swimmers.

If there is a problem, then it lies not just with what’s going into pools to keep them clean, but also with what swimmers are adding to the mix. By that, we don’t just mean peeing in the pool, although there could be as many as 75 litres of urine in the average 833,000 litre pool. Any organic compounds that enter the pool on our skin or swimsuits become available to the disinfecting chemicals in the water. Think sweat, sunscreen and even the flame retardants in your swimsuit, which can leach into the water. All these compounds, as well as urine, can react with the residual chlorine to produce cocktails of chemicals known as disinfection by-products (DBPs).

A swimmer using an asthma inhaler

Source: © PA Images/Alamy Stock Photo

Take a breath: elite swimmers may develop asthma due to increased exposure to disinfection by-products

Common DBPs include trihalomethanes like trichloromethane, also known as chloroform, which was used in anaesthetics until the early 20th century, when it was deemed too dangerous. Chloramines such as trichloramine (NCl3) – formed when the urea, CO(NH2)2, in urine reacts with the HClO in the pool – are volatile so escape easily at the water’s surface, causing or irritating asthma symptoms. One recent Norwegian study of 313 swimmers found that while 22% had asthma overall, rates were much higher (36%) in swimmers who spent more than 16 hours a week in the pool. Another common class of DBPs produced in pools is haloacetic acids, which are non-volatile so accumulate in the water instead of at the surface.

Some DBPs have only been identified in swimming pools recently. A 2021 study by Susan Richardson and her co-workers at the University of South Carolina, US, was the first to find iodoacetic acid in pool water. ‘We have very sensitive methods for over 70 DBPs that we have developed, which allow us to see things that other people might miss,’ Susan explains. They found iodoacetic acid in two South Carolina pools. It is one of the most toxic DBPs discovered so far and is thought to form when the iodine in the water source for the pool is oxidised by disinfectants, rather than from reactions with organic chemicals introduced by people.

We should plan for excellent pool maintenance to facilitate lots of healthy swimming

All this may sound worrying but Stuart thinks we need to keep the toxicity of some of these compounds in perspective. ‘The public health risks associated with low levels of exposure to DBPs in swimming pools are much less significant than the public health impacts that would be observed if we simply stopped disinfecting swimming pools,’ he says. Plus, there are things we can do to keep our exposure to DBPs as low as possible.

What can we do?

CSI: pool purification

One thing we can do is keep the organic components that we add to the pool to a minimum. ‘Good maintenance of swimming pools means keeping out dirt and other organic matter that may react with chlorine,’ Stuart says. So, if you thought showering before entering the pool was just for the dirty scoundrels who didn’t have a shower the day before, think again. It’s an important public safety measure to prevent bodily secretions and skin products from combining with pool chemicals to produce compounds that are harmful to our health.

People who swim more regularly, especially elite swimmers who may spend many hours in the pool, are exposed to DBPs more often. But, again, these risks can be reduced by careful pool management. Ventilation helps limit levels of some of the more volatile chemicals that can hang around in the air near the water’s surface, so frequent swimmers should choose to train at well-ventilated pools.

Wild and free

An outdoor pool can contain quite a different chemical concoction to an indoor pool. This is partly because the way that we use the pool changes when we step outside. On holiday, for example, we might put our sun loungers out, rub some sun lotion in, then run around on the grass, dirt or sand and jump in and out of the pool many times throughout the day.

At a crowded holiday resort, hundreds of people might be doing the same thing, leading to lots more grime and sunscreen getting into the pool than at a regular indoor pool. This means there may be more organic chemicals in the mix to react with the disinfectants, producing DBPs. However, there are also differences in the environment that help mitigate the extra risk. For a start, the volatile DBPs that irritate asthma in indoor pools are less likely to be a problem outdoors as they can quickly disperse from the water’s surface into the fresh air. There’s also the added factor of direct sunlight, which is effective at breaking down some of the harmful chemicals.

The situation is different again for natural pools and wild swimming spots, which are untreated and can, therefore, contain some of the harmful bacteria and viruses that we try to get rid of through chlorination. Some well-managed natural pools use gravel filters or purification by plants to keep them clean but, in general, these swimming spots are best used sparingly by swimmers to avoid build-up of pollution.

There are also approaches to disinfection that are known to reduce the levels of DBPs in swimming pools. For instance, Susan’s 2021 study looked at combining more traditional chlorination with a method called copper–silver ionisation (CSI), more commonly used in hospital settings. This method uses antimicrobial copper and silver ions generated by electrolysis, which requires some additional plumbing to put copper–silver electrodes into the pool supply. ‘The copper–silver ionisation takes place near the pool filter, so bacteria are being killed in that small space as the water flows through that area,’ says Susan.

Although this means the pool water must still be chlorinated to immediately deal with nappy accidents – it would take a few more minutes for water to recirculate and be disinfected through the CSI method – the combination approach can reduce the amount of chlorine needed and thereby the levels of DBPs formed. Susan’s team also checked the levels of free copper and silver ions in the pool to ensure they weren’t cause for concern and found that they were well within standards for drinking water.

While we should be aware of the risks associated with disinfecting our swimming pools, there should be no need to cut down on swimming. ‘I would not like to see people attempting to manage this risk by swimming less,’ Stuart says. ‘We should plan for excellent pool maintenance to facilitate lots of healthy swimming!’

More resources

  • Explore the halogens, their salts and how they are linked to swimming with the Chemistry and sport presentation, teacher notes and student worksheet.
  • Find out why the properties of water are vital to the sportsmen and women who compete in, on and around water with your 11–14 learners using Water sports and solutions.
  • Discover how Katty, a Higher apprentice and laboratory analyst, monitors the levels of organic materials in drinking and wastewater.
  • Use the Chemistry and sport resource with your 14–16 learners to explore the halogens, their salts and their relation to swimming: rsc.li/39bnAaj
  • Find out why the properties of water are vital to the sportsmen and women who compete in, on and around water with the Water sports and solutions resource: rsc.li/3L8m6em
  • Discover how Katty, a higher apprentice and laboratory analyst, monitors the levels of organic materials in drinking and wastewater: rsc.li/3FFOXFu

Article by Hayley Bennett, a freelance science writer based in Bristol. Resource by Tim Jolliff, a science education consultant and chemistry teacher, with additions by Dorothy Warren, an independent science education consultant based in the UK