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How Quickly Does Legionella Grow and How Can You Prevent it?

20 February 2023

You’ve probably already heard about legionella. And perhaps you’re aware of the dangers it poses. But how quickly can it develop in your water system? And how do you know if your building is at risk of an outbreak?

In our latest blog post, we’ll talk you through everything you need to know about the bacteria and how to prevent legionella from multiplying and causing harm.


What is legionella?

Legionella pneumophila is a bacteria that’s naturally found in freshwater environments like lakes, rivers and reservoirs. 

It generally doesn’t pose any risks to life here because it’s usually low in numbers and doesn’t enjoy the required conditions for it to grow. 

The bacteria becomes a concern when it’s given the perfect breeding ground in man-made water structures such as cooling towers, hot and cold water systems, and spa pools. 


Why is legionella dangerous?

If the water in these systems isn’t stored at the right temperature or it becomes stagnant, the legionella bacteria can thrive and multiply. 

Once they’ve colonised the system, the bacteria can be inhaled through small droplets of water suspended in the air. 

If this happens, it can lead to Pontiac fever, Lochgoilhead fever or Legionnaires’ disease – a potentially fatal type of pneumonia (lung infection). 

We’re all susceptible to infection, but those over 45 years of age and people with pre-existing respiratory or cardiac health conditions are particularly at risk.  


How quickly can legionella develop in water systems?

Many people are often shocked to discover just how quickly legionella can develop in a stagnant water system. In fact, given the right conditions, legionella can grow and become problematic in as little as two weeks

That’s why it’s so important to carry out regular risk assessments and make sure you have the right control measures in place to prevent a legionella outbreak in your water system.  

Remember: the best way to prevent legionella proliferation is to regularly flush your system. If your water system is left stagnant for 2 weeks or more, it’s best practice to call in the experts and test for bacteria. 


Where can legionella grow?

Any purpose-built water system is at risk of legionella growth. But it’s particularly common in systems where the water is kept at a consistent temperature, such as:

  • Cooling towers
  • Evaporative condensers 
  • Hot and cold water systems
  • Spa pools (e.g. jacuzzis)
  • Disused bathroom equipment (shower heads and taps)

The risk is increased if your water system creates or spreads breathable droplets, stores or re-circulates water, or contains nutrients that feed the bacteria like rust, sludge, biofilm, organic matter or scale. 


How can I prevent legionella from growing?

As well as storing water at the correct temperature, the Health & Safety Executive recommends that all commercial water systems are regularly cleaned, flushed and disinfected to reduce the risk from exposure to legionella. 

Why? Because legionella bacteria thrive in stagnant water. 

So, to reduce the risk of an outbreak, it’s really important to: 

    • Regularly flush your system (especially outlets that are used infrequently)
    • Remove any dead legs/dead ends in your pipe system
    • Conduct a quarterly shower head/hose clean
    • Annually inspect cold-water storage tanks and clean when required
    • Drain hot water units to check for any debris or corrosion 


Do I need to carry out a water risk assessment?

Yes. Any organisation in the UK that has a hot and cold water system on site is required by law to carry out a legionella risk assessment. Failure to comply can lead to hefty fines or even prosecution. 

A risk assessment involves a highly qualified engineer visiting your site and surveying your entire system, checking every potential area of risk. 

Once complete, they’ll suggest an action plan based on their findings, designed to resolve any immediate issues, prevent future problems, and ensure your organisation remains compliant. 

Legionella risk assessments are an ongoing process. We’d always recommend a reassessment when there’s been significant physical change to your system or in the way it’s managed.


Water treatment services from H2O

Professional water testing and treatment is the perfect way to reduce the risk of legionella bacterial growth, helping to protect your system, which will save you money in the long term.

Our cleaning and disinfection services are carried out by our in-house, fully trained engineers. We create bespoke method statements for every one of our clients, ensuring that all water treatment work complies with the latest HSG274 and ACoPL8 (including COSHH) regulations. 

If you have any questions or would like a no-obligation quote, don’t hesitate to get in touch with our team today!

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