Legionella is a serious concern for businesses, landlords and facilities managers responsible for maintaining safe water systems. Understanding how to test for Legionella is an important part of preventing Legionnaires’ disease, maintaining compliance and protecting public health.
While a Legionella risk assessment forms the foundation of any management plan, Legionella testing can help identify contamination, verify control measures and highlight areas requiring corrective action. Many organisations use Legionella risk assessment software to record inspections, manage monitoring activities and maintain compliance records across multiple sites.
Legionella bacteria can develop in a wide range of water systems when conditions allow growth. Without appropriate monitoring and control, contamination can spread throughout a system and increase the risk of infection. Understanding how to test for Legionella in water, when testing is required and how to interpret Legionella test results helps organisations maintain compliance and improve water safety.
What is Legionella?
Legionella bacteria are naturally occurring bacteria found in rivers, lakes and other natural water sources. Problems occur when Legionella bacteria in water enters man-made water systems and multiply under favourable conditions.
Warm water, stagnant areas, poor circulation and inadequate maintenance can all contribute to the growth of Legionella bacteria. Once bacteria become established within a system, they can spread through aerosolised water droplets.
When people breathe contaminated droplets, a Legionella infection may occur. The most serious disease caused by Legionella is Legionnaires’ disease, a severe form of pneumonia that can result in hospitalisation or death, particularly in vulnerable individuals.
Common symptoms include fever, coughing, breathing difficulties, headaches and muscle aches. A milder illness known as Pontiac fever can also occur following exposure to certain Legionella species.
Why Legionella testing matters
Legionella testing helps organisations identify contamination before it leads to an outbreak. Testing provides evidence that control measures remain effective and helps determine whether remedial action is required.
A Legionella risk assessment is a legal requirement for businesses. The Health and Safety Executive expects duty holders to identify potential risks associated with Legionella bacteria and implement suitable control measures.
The responsible person must understand their unique water systems and ensure appropriate monitoring arrangements are in place. Risk assessments help determine when Legionella testing is necessary and should consider system usage patterns, occupancy levels and operational requirements.
High-risk areas for Legionella growth include water stagnation zones and fluctuating temperatures. Buildings containing cooling towers, evaporative cooling systems, storage tanks and complex water systems often present a greater risk and may require additional monitoring.
Risk assessments must be conducted by a competent person and should follow the approved code of practice and associated guidance.
How to test for Legionella in water
For organisations looking at how to test for Legionella in water, the process generally involves collecting water samples from selected locations and sending them to a laboratory for analysis.
Legionella testing requires a systematic approach to sample collection and analysis. Improper sampling techniques can lead to false negatives in Legionella testing, making it essential to follow recognised procedures.
The typical process includes:
- Reviewing the risk assessment
- Selecting sampling locations
- Collecting representative water samples
- Recording water temperatures
- Transporting samples correctly
- Laboratory analysis
- Reviewing test results
- Implementing corrective actions where required
This process helps determine whether Legionella bacteria are present and whether further action is necessary.
Selecting sampling Locations
The success of Legionella sampling depends on where the sample is collected.
Common sampling points include:
- Hot and cold water outlets
- Shower heads
- Water storage tanks
- Cooling towers
- Evaporative cooling systems
- Hot tubs
- Air conditioning systems that use water
- Dead legs
- Low-use outlets
- High-risk water sources
Testing for Legionella should focus on areas where contamination is most likely to occur.
Although hot and cold water systems usually do not require routine testing, a risk assessment may identify circumstances where periodic sampling is appropriate.
Collecting water samples correctly
Accurate Legionella testing begins with proper sample collection. Water samples must be collected in sterile containers to ensure accurate results and reduce the likelihood of contamination during handling.
A minimum water sample volume of 1 litre is typically recommended for laboratory analysis. Most specialist laboratories provide containers that contain sodium thiosulfate. Sodium thiosulfate is used to neutralise disinfectants in water samples and prevent treatment chemicals from affecting the final results.
Where appropriate, samples may be collected from both pre-flush and post-flush scenarios for accurate testing. Comparing results from both locations can help determine whether contamination originates within the outlet itself or elsewhere within the system.
Water samples should ideally be processed within 24 hours of collection. Delays can affect bacterial viability and reduce confidence in the findings.
The chosen sampling method should follow recognised industry guidance. In the UK, Legionella sampling and testing should follow British Standard BS 7592:2022. Elsewhere, Legionella testing should follow recognised national guidance where appropriate, such as the CDC Toolkit in the USA.
Temperature monitoring and control
Monitoring water temperatures remains one of the most effective forms of Legionella control.
Temperature control is important because Legionella bacteria multiply most effectively within certain temperature ranges. Maintaining appropriate temperatures throughout water systems helps minimise growth and supports compliance requirements.
For effective control:
- Cold water systems should be maintained below 20°C to control Legionella
- Hot water should be stored at 60°C or above and distributed at temperatures of at least 50°C
- Water temperature should be checked regularly in accordance with recognised guidance
- Domestic hot water systems should be regularly monitored to ensure temperatures remain within recommended limits
Routine monitoring helps identify issues before significant contamination develops and provides evidence that the system remains under control.
Monitoring records should be retained and reviewed regularly as part of wider water safety procedures.
Laboratory testing methods
Once each sample has been collected, it is sent to a specialist laboratory for analysis.
Samples should be sent to an accredited laboratory for valid testing results. Using a United Kingdom Accreditation Service (UKAS) accredited laboratory is considered best practice because UKAS-accredited labs ensure reliable Legionella testing results and provide confidence in the findings.
Several testing methods are available.
Culture method testing
The culture method remains the recognised industry standard for Legionella detection.
Legionella culture testing requires incubation on selective agar for 10 to 14 days. During this period, laboratory technicians monitor growth and identify colonies of Legionella bacteria.
In most situations, culture methods take 10 to 12 days for results. While slower than some alternative approaches, the culture method remains widely accepted because it identifies viable bacteria capable of causing infection.
Many organisations continue to rely on culture testing when undertaking Legionnaires’ disease testing because of its recognised status within the industry.
PCR testing
Polymerase chain reaction (PCR) testing offers a much faster method of detection.
PCR testing can provide results in a few hours by identifying genetic material associated with Legionella bacteria. This makes PCR particularly useful when rapid information is required following a contamination event or suspected outbreak.
Although PCR provides fast detection, results should be interpreted alongside operational information, monitoring records and other test results.
Rapid testing methods
New technologies continue to improve the speed of Legionella testing.
Lateral flow tests can provide quick yes/no results on site, helping maintenance teams identify possible issues before laboratory analysis is completed.
There are rapid swab tests that can detect Legionella in just 25 minutes, and are increasingly used to support routine monitoring programmes.
While these methods can support detection, they do not always replace formal laboratory testing when compliance evidence is required.
How to test for Legionella at home
Many homeowners wonder how to test for Legionella at home.
Domestic properties generally present lower risk than large commercial premises, but contamination can still occur where water remains stagnant for long periods or where water quality issues develop.
Homeowners may wish to test water if:
- A property has been vacant for an extended period
- Water temperatures fluctuate significantly
- There are concerns regarding water quality
- Vulnerable occupants live within the property
- Plumbing work has recently been completed
For reliable results, homeowners should obtain suitable sterile containers, collect representative water samples and submit them to a UKAS accredited laboratory.
However, maintaining suitable temperatures and regularly flushing infrequently used outlets is often more important than routine domestic testing.
How often should Legionella testing be carried out?
There is no single answer regarding testing frequency because every system presents different levels of risk.
Testing frequency depends on risk assessment outcomes, operational requirements and the complexity of the installation.
Examples include:
- Cooling towers should be tested at least quarterly
- Legionella testing may be required weekly if procedures are ineffective
- Hot and cold water systems usually do not require regular testing
- Periodic sampling may be recommended where higher risk conditions have been identified
- Additional testing may be necessary following system modifications, contamination events or poor monitoring results
Regular testing should always be based on identified risk factors rather than arbitrary schedules.
Buildings containing cooling towers, evaporative cooling systems and other higher risk assets typically require more extensive monitoring than smaller installations.
Understanding Legionella test results
Understanding Legionella test results is essential for determining appropriate actions.
Testing results guide necessary actions based on contamination levels and help organisations determine the most appropriate response.
General guidance often follows the categories below.
Non-detected
Non-detected means no action is required beyond maintaining existing monitoring, control and maintenance procedures.
The result indicates that Legionella was not identified within the submitted sample.
Below 100 bacteria detected
In some systems, below 100 bacteria detected requires monitoring.
The organisation should continue routine monitoring activities, review maintenance records and ensure control measures remain effective.
Between 100 and 1,000 bacteria detected
In some systems, more than 100 but fewer than 1,000 bacteria detected requires remedial works.
Investigations should identify the source of contamination, review operational controls and implement improvements where necessary.
Between 1,000 and 10,000+ bacteria detected
In some systems, between 1,000 and 10,000+ bacteria detected requires immediate action.
Organisations should investigate the system, undertake corrective measures, increase monitoring and consider disinfection where appropriate.
Any significant contamination event should be reviewed carefully to determine root causes and prevent recurrence.
Common sources of Legionella contamination
Legionella contamination can develop in a wide range of facilities where conditions allow bacteria growth.
Examples include:
- Hospitals
- Care homes
- Hotels
- Leisure centres
- Schools
- Universities
- Manufacturing facilities
- Commercial offices
The bacteria can be carried throughout a water system and spread through aerosol generation. Once airborne, people breathe contaminated droplets, increasing the risk of disease transmission.
Particular attention should be given to:
- Cooling towers
- Water towers
- Hot tubs
- Shower heads
- Hot water storage systems
- Cold water systems
- Air conditioning systems using water
- Domestic hot water installations
Where bacteria in water systems are allowed to multiply, there is an increased risk of infection and potential outbreak situations.
Effective monitoring, maintenance and control programmes help protect occupants and reduce the likelihood of Legionella bacteria becoming established.
Cost of Legionella testing
The cost of Legionella testing varies depending on the size of the system, the number of sampling locations and the testing method selected.
For many organisations, testing forms part of a wider compliance strategy that also includes inspections, temperature monitoring and risk assessments. Before arranging testing, it is worth understanding the cost of a Legionella risk assessment as both activities are often closely linked.
A single sample may cost relatively little, while larger facilities requiring multiple water samples, ongoing monitoring and specialist laboratory services can incur significantly higher costs.
The overall process should always be proportionate to the level of risk identified within the water system.
Best practice for ongoing Legionella management
Testing should never be viewed as a standalone solution.
Effective Legionella management requires:
- Regular monitoring
- Temperature checks
- Maintenance programmes
- Risk assessments
- Corrective actions
- Record keeping
- Staff training
Good practice involves regularly reviewing system performance, ensuring temperatures remain within target ranges and investigating any unusual test results promptly.
By combining testing, monitoring and preventative maintenance, organisations can manage risk effectively and demonstrate compliance with legal obligations.
Conclusion
Understanding how to test for Legionella is essential for protecting health, maintaining compliance and preventing Legionnaires’ disease.
Whether testing is carried out as part of routine monitoring or following concerns about contamination, the process should always follow recognised guidance and industry standards. Proper sample collection, temperature monitoring, laboratory analysis and interpretation of Legionella test results all play an important role in effective Legionella control.
Although not every water system requires routine Legionella testing, organisations responsible for higher risk installations should ensure appropriate monitoring arrangements are in place. By identifying issues early and responding quickly, duty holders can protect building occupants, improve water safety and reduce the likelihood of an outbreak.
Frequently asked questions
Is a Legionella risk assessment mandatory?
It is a legal requirement to assess the risk from Legionella, such as by completing a Legionella risk assessment in businesses that control premises with water systems where a risk may arise. The assessment helps identify risks and determine appropriate control measures.
Who should carry out a Legionella risk assessment?
Risk assessments must be conducted by a competent person with sufficient knowledge and experience of Legionella management and water systems.
What does a risk assessment consider?
Risk assessments should consider system usage patterns, occupancy levels, water temperatures, stagnation risks, equipment condition and any factors that could contribute to Legionella growth.
How quickly must samples be analysed?
Water samples must be processed within 24 hours of collection to ensure reliable results and accurate detection.
Why are sterile containers used?
Water samples must be collected in sterile containers to ensure accurate results and prevent external contamination from affecting laboratory analysis.
What is sodium thiosulfate used for?
Sodium thiosulfate is used to neutralize disinfectants in water samples. This prevents residual treatment chemicals from affecting the final test outcome.
Why are pre-flush and post-flush samples collected?
Where required, samples should be collected from both pre-flush and post-flush scenarios for accurate testing. This helps determine where contamination may be originating within the system.
What standards should Legionella testing follow?
Legionella testing should follow established guidelines, such as the British Standard BS 7592:2022 in the UK or CDC Toolkit recommendations in the USA.
Are UKAS laboratories important?
Samples must be sent to an accredited laboratory for valid testing results. A UKAS accredited laboratory provides confidence that recognised quality standards have been followed.
How long does culture testing take?
Legionella culture testing requires incubation on selective agar for 10 to 14 days. In most cases, culture methods take 10 to 12 days for results.
How quickly can Legionella be detected?
PCR testing can provide results in a few hours. Lateral flow tests can provide quick yes/no results on site, while rapid swab tests detect Legionella in just 25 minutes.
What happens if Legionella is detected?
The response depends on contamination levels. Non-detected means no action is required. Below 100 bacteria detected requires monitoring. More than 100 but fewer than 1,000 bacteria detected requires remedial works. Between 1,000 and 10,000+ bacteria detected requires immediate action.
Do hot and cold water systems need regular testing?
Hot and cold water systems usually do not require regular testing. Testing frequency depends on risk assessment outcomes and whether concerns have been identified during monitoring.
How often should temperatures be checked?
Depending on the system and risk assessment, water temperature may need to be checked monthly. Cold water systems should generally remain below 20°C, while hot water systems should store water at 60°C or above and distribute it at temperatures of at least 50°C as part of effective control measures.
