Legionella at swimming pools: what’s the risk?

Time for another instalment in our ‘What’s the risk?’ blog series! Today, Dynamika director Brett Wallace looks at swimming pools; where legionella can be an unexpected danger…

When we talk about health and safety risk areas at swimming pools, what first springs to mind for you?

Slippery floors? Deep waters? Diving board falls?

All valid concerns, but there’s another risk area that you might not have considered; and that’s the risk of legionella contamination.

If you own or manage a pool facility, legionella should absolutely be on your radar. This guide explains how the bacteria can take over your pool space; and what you need to do to safeguard swimmers.

Let’s dive in…

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How can legionella grow and spread at swimming pools?

At first blush, swimming pools don’t seem like the most likely place for a legionella outbreak. Surely the chlorine takes care of any nasty pathogens?

This is true for the most part. Chlorine disinfection is highly effective at killing free-floating legionella in pool water; among other nasty pathogens.

However, chlorine often struggles with biofilms.

You might be familiar with biofilms as the slimy layer of bacteria that forms on the surface of stagnant water, but they can also build up on the walls, floors, filters and pipes of treated pools too.

In fact, the relatively damp conditions of pool facilities can enable biofilms to form on almost any surface; including poolside walkways and benches.

Biofilms develop from the skin, oils and other organic debris from swimmers, and grow in pools with fluctuating disinfection chemical levels and/or warmer waters. They can quickly become a safe haven for legionella bacteria to breed.

Unfortunately, chlorine struggles to break down biofilms; in fact, a study from 2023 showed that chlorine actually promotes biofilm growth.

Meanwhile, water splashes, bubble jets and other pool features create the mechanism to spread the bacteria through the air.

Unfortunately, that’s not the only risk area when it comes to pool facilities.

Many pool owners focus their efforts squarely on pool water quality; but poorly maintained hot tubs, spa pools, steam rooms and changing room showers can also become a problem.

Left unchecked, they often provide the ideal temperature and aerosolisation conditions to enable legionella to grow and spread amongst patrons.

Why is legionella control more challenging for swimming pools?

Even here in the UK, pools and hot tubs are more ubiquitous than you might think.

They show up in a lot of different settings; leisure centres, sports clubs, spas, hotels, resorts, holiday homes, cruise ships, theme parks, hydrotherapy and medical centres, high-rise rooftops, even domestic dwellings!

That means there’s really no one-size-fits-all procedure for legionella control. Different pools may have different designs, different capacities and different technologies; and they can be located indoors or out in the open.

Their usage routines can fluctuate wildly, too. For example, an indoor leisure centre pool naturally caters for large numbers of swimmers each day and might stay open all year.

Meanwhile an outdoor holiday home pool might only serve a handful of patrons per day and go through long periods of seasonal inactivity.