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How to Test for Legionella in Water

09 January 2023

Going to work is difficult enough some days without having to worry about whether dangerous bacteria is out there to get you (spoiler alert – it probably is!).

Which is why it’s crucial to be aware of, and test for, legionella bacteria.

A water system contaminated with legionella bacteria poses a serious risk to you and the people you are responsible for, from staff to students.

When it comes to legionella, it’s better to be safe than sorry, which is why we recommend regular testing. While testing is not part of compliance guidelines, periodic sampling will prove that your control scheme is effective and preventing growth of bacteria within your system,

In this article, we’ll explore exactly when is the best time to carry out a legionella test, how they are carried out, and what to do if you detect harmful bacteria in your water system.


When to test for legionella

As previously mentioned, periodic sample testing ensures that your water systems are safe and hygienic, but there are specific occasions when you may want to carry out a test. Here are examples of times when you should collect legionella test samples:

Onsite maintenance 

One of the most crucial times to carry out legionella sampling is after disruption to your usual operation on site. For example, if there’s been any drastic maintenance or construction that might affect your water system, through disuse or damage. 

Any water that may have become stagnant is at significant risk of contamination and should be tested, this could be following a period of low site occupancy such as holiday periods or working away from the usual location resulting in reduced water system use.

Temperature control issues

Another instance where you should carry out Legionella testing is if your temperature control scheme was to fail and water temperatures were outside of the recommended guidelines. This could be your hot or cold water system and temperature outside of control significantly reduces the effect of killing Legionella bacteria.


How to test for legionella

The process for legionella testing is relatively simple. Samples are collected from sources around your site, such as outlets, water storage tanks and water heaters. 

These samples are then sent off for lab analysis at a UKAS-accredited location. The results will then be delivered back to you and a plan can be formed based on the results.


Legionella test results 

What you do after you’ve received your test results back depends on the levels of Legionella bacteria that have been detected in your water system.


This means that no trace of legionella bacteria was identified within your water systems and therefore no action is required.

You can continue to use water on your site without having to worry about infection. A risk assessment can be carried out if necessary, or you can continue to action any remedial work that was determined from the previous assessment.

Detected (variable on levels)

Below 100 bacteria detected

This level of contamination is nothing to be too concerned about. You may find that once you have flushed the taps and tested another sample that no more Legionella is found. 

However, just because it is at a low number does not mean that you’re in the clear and your system will still require monitoring, as legionella is notorious for rapid proliferation if the conditions are right.

>100 – 1000 bacteria detected

Should this number of bacteria be detected, we would recommend remedial works based on HSG274. 


This includes a thorough and complete check of all sources of water and a flush of your system, as well as temperature checks at the time of the sample. Once this process is complete, A review of your risk assessment is necessary to determine areas of concern and whether water treatments are required.

1,000 – 10,000+ bacteria detected

These results are scary and action is required immediately however if the correct processes are followed then these issues can be rectified quickly.

The plan of action depends on the purpose of the building and its occupants and your system may require remedial works, including disinfection.

Don’t worry, our team at H2O can deliver rapid turnaround remedial works in instances like this. Give us a shout if you ever need rapid response water hygiene advice or support. 

Do I need to be worried about legionella? 

This depends on the environment and the purpose of your building.

For example, a building fed directly from mains water with little or no storage of water is less likely to be at risk of contamination, this is due to more frequent turnover of water and therefore less opportunity for bacteria to proliferate However, if your site contains any of the following man-made structures, it can significantly increase the risk of contamination:

  • Water tanks
  • Showers
  • Cooling towers
  • Spray booths
  • Stored hot water

This means that buildings such as care homes, schools, hospitals, spas and hotels are at particular risk and would benefit from regular legionella testing.

We also find that universities and other educational premises are often at greater risk from legionella during half-term breaks, which can result in system stagnation


Legionella risk assessments and testing from H2O Hygiene 

At H2O Hygiene, it’s our passion to ensure that your water systems are safe from the risk of harmful bacteria.

Whether you require legionella testing or a full risk assessment of your water systems, our team of friendly specialists are on hand to help!

If you have any questions about the risks of legionella or would like to arrange for an onsite visit, don’t hesitate to get in touch today!

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