What is Legionella? A practical guide for UK duty holders

What is Legionella? If you are responsible for hot and cold water systems, this is more than just a technical question – it sits at the heart of health, safety and legal compliance in your building.

Legionella are bacteria that live in water. In the right conditions they can multiply and cause a serious lung infection called Legionnaires’ disease when tiny droplets are inhaled. The UK Health and Safety Executive (HSE) classifies Legionnaires’ disease as a potentially fatal form of pneumonia, and everyone is susceptible, with higher risk in older adults, smokers and people with underlying conditions.
See: https://www.hse.gov.uk/legionnaires/what-is.htm and https://www.nhs.uk/conditions/legionnaires-disease/

For UK facilities, estates and compliance teams, understanding what Legionella is, how Legionella temperature, Legionella risk assessment and Legionella testing fit together – and how modern IoT solutions from AQUAIOT can automate monitoring and testing – is essential for keeping people safe and staying compliant.

This guide walks through the essentials in plain language, so you can join the dots between microbiology, regulations and practical control.

What is Legionella?

“Legionella” is the name of a group (genus) of bacteria that occur naturally in freshwater environments such as rivers, lakes and ponds. In nature, they are usually present at low levels and rarely cause harm. The problem starts when Legionella bacteria enter man-made water systems and find conditions that allow them to multiply.

The species most commonly associated with human disease is Legionella pneumophila. When people talk about Legionella control in buildings, they are usually thinking about this organism, because it is responsible for most cases of Legionnaires’ disease worldwide.

The term “legionellosis” covers several illnesses caused by Legionella bacteria:

Legionnaires’ disease – a potentially fatal pneumonia
Pontiac fever – a milder, flu-like illness
Lochgoilhead fever – a similar but less severe condition

HSE uses “legionellosis” as the collective term in its guidance:
https://www.hse.gov.uk/legionnaires/

How do people catch Legionnaires’ disease?

You do not catch Legionnaires’ disease from person-to-person contact, and it is not usually caught by simply drinking contaminated water.

Instead, people are infected when they inhale tiny droplets (aerosols) of water that contain Legionella bacteria. These droplets can be generated by many everyday systems, including:

Showers and spray taps
Cooling towers and evaporative condensers
Spa pools, hot tubs and hydrotherapy pools
Decorative fountains and water features
Certain types of industrial or process equipment that spray or aerosolise water

NHS guidance explains that Legionnaires’ disease is uncommon but can be very serious and usually requires hospital treatment with antibiotics:
https://www.nhs.uk/conditions/legionnaires-disease/

Symptoms typically include:

High temperature
Cough
Muscle aches
Headache
Shortness of breath
Sometimes diarrhoea or confusion

Early recognition and treatment are critical, which is why preventing growth of Legionella in water systems is a key duty for building owners and operators.

Where Legionella grows in building water systems

In the natural environment, Legionella bacteria usually remain in low numbers. In man-made systems, they can be unintentionally given ideal conditions to multiply, especially where water is stored, re-circulated and warmed.

Risk is highest where you have a combination of:

Temperatures between 20°C and 45°C – the growth zone for Legionella
Stagnant or low-flow areas – “dead legs”, rarely used outlets, oversized pipework
Nutrients – scale, rust, sludge and biofilm on pipework and fittings
Aerosol generation – showers, spray taps, jets, cooling towers and similar

HSE’s technical guidance HSG274 Part 2 – The control of legionella bacteria in hot and cold water systems explains that all hot and cold water systems – from simple domestic installations to complex hospital networks – can present a foreseeable risk if they are not designed and managed properly:
https://www.hse.gov.uk/pubns/books/hsg274.htm

Typical higher-risk assets include:

Cold water storage tanks and calorifiers
Long distribution pipework runs and ring mains
Thermostatic mixing valves (TMVs) serving multiple outlets
Showers, spray taps and rainfall shower heads
Cooling water systems and evaporative condensers
Wet fire-fighting systems and hose reels (in some cases)

Legionella temperature: understanding the critical ranges

One of the most important concepts in Legionella control is temperature. HSE summarises Legionella temperature behaviour as follows:

Below 20°C – Legionella bacteria are present but generally dormant
Between 20°C and 45°C – Legionella can survive and multiply
Above 50°C – growth is greatly reduced
Above 60°C – Legionella bacteria are rapidly killed

Because of this, temperature control remains the primary control strategy for Legionella in hot and cold water systems in the UK. See HSE guidance on preventing and controlling risk:
https://www.hse.gov.uk/legionnaires/what-you-must-do/preventing-controlling-risk.htm

In practice, this means:

Cold water should normally be stored and distributed so that it is below 20°C at outlets (typically after running the tap for up to two minutes).
Hot water should normally be stored at 60°C or higher in calorifiers and delivered to outlets at 50°C or higher within about one minute of running (higher in some healthcare settings).

These target ranges are often referred to informally as “Legionella temperature” thresholds. When water temperatures drift into the 20–45°C band, conditions favour bacterial growth. When they are kept outside this range, it is much harder for Legionella to multiply.

You also need to consider scalding risk, particularly in healthcare and care environments. This is usually controlled using TMVs at or near the point of use, while keeping upstream storage and distribution temperatures high enough to manage Legionella risk. HSG274 Part 2 provides more detail on TMVs and blended water. An extract is available at:
https://www.goodwater.co.uk/wp-content/uploads/2021/05/HSG274-2013-Part-2-Extract.pdf

Legal duties and Legionella risk assessment in the UK

A Legionella risk assessment is the formal starting point for managing Legionella in any building.

In the UK, employers, landlords and those in control of premises have a legal duty under health and safety law to identify and assess sources of Legionella risk. HSE’s “Legionnaires’ disease – what you must do” sets this out clearly:
https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm

In summary, you must:

Identify and assess sources of risk
Manage any risks
Prevent or control those risks
Keep and maintain appropriate records
Carry out any other relevant duties (such as reporting under RIDDOR if required)

The Legionella risk assessment should:

Describe the building’s water systems (schematics, asset lists, operating regimes)
Identify where water is stored, recirculated or can stagnate
Evaluate who could be exposed, with particular focus on higher-risk groups
Review current Legionella temperature control measures
Highlight design or operational issues that increase risk (dead legs, oversized tanks, rarely used outlets)
Recommend control measures, monitoring and Legionella testing where appropriate
Assign responsibilities – including the “duty holder” and “responsible person”
Set out how often the risk assessment should be reviewed

HSE provides specific guidance for landlords at:
https://www.hse.gov.uk/legionnaires/legionella-landlords-responsibilities.htm

More detailed technical information is given through:

ACOP L8 – Legionnaires’ disease: The control of legionella bacteria in water systems
HSG274 Part 2 – The control of legionella bacteria in hot and cold water systems

Background and further guidance are available from HSE and specialist sites such as:
https://www.hse.gov.uk/legionnaires/
https://legionellacontrol.com/legionella-risk-assessment/

Legionella testing and monitoring explained

The term Legionella testing is used in different ways. In practice, Legionella control uses several different kinds of testing and monitoring that work together.

1. Temperature monitoring

Regular checks of Legionella temperature at key points (sentinel outlets, tanks, calorifiers, loops) are a core control measure. They confirm that hot and cold water systems are staying outside the bacterial growth range and that plant is operating as intended.

Traditionally this has meant someone with a thermometer and clipboard doing manual checks according to a written scheme. While this can be effective, it is labour-intensive and generates a lot of paper records.

2. Microbiological Legionella testing

Microbiological Legionella testing involves taking water samples and sending them to a laboratory to check whether Legionella bacteria are present and at what levels.

HSG274 explains when routine microbiological Legionella testing is appropriate – for example in certain healthcare settings, cooling towers or where control measures are not performing as expected. It is not automatically required in every simple domestic-type hot and cold water system, provided that control measures such as temperature control and flushing are demonstrably effective.

When microbiological Legionella testing is carried out, you should:

Follow a suitable sampling plan
Use a competent laboratory
Interpret results in line with HSE guidance and your risk assessment
Record and act on the findings

Public health information on Legionnaires’ disease, including case data and guidance, is also available from UKHSA:
https://www.gov.uk/government/collections/legionnaires-disease-guidance-data-and-analysis

3. System inspections and performance checks

Alongside temperature monitoring and Legionella testing, you also need to monitor system conditions that influence risk, for example:

Visual inspection and cleaning of tanks and calorifiers
Descaling and disinfection of showers and outlets
Verification that unused outlets are being flushed
Checks on pumps, valves, TMVs and other control devices

Together, these elements give you an evidence-backed picture of whether your control scheme is working.

From clipboards to automated Legionella monitoring and testing

Many organisations are finding that traditional “man-in-a-van” routes with handheld thermometers no longer scale – especially across multi-site estates, hospitals, universities or large housing portfolios.

Typical challenges include:

Manual checks are labour-intensive and can be missed during bad weather, staff shortages or access issues.
Spot temperature readings only give a snapshot, not a full picture of trends and anomalies.
Paper logbooks can be hard to search, audit or integrate with other compliance systems.

This is where automated Legionella monitoring and testing using IoT technology comes in.

How AQUAIOT helps automate Legionella testing and monitoring

At AQUAIOT, we deliver a Legionella monitoring system that automates temperature profiles at tanks, loops and sentinel outlets, raises real-time alerts and keeps tamper-evident records for audits:
https://aquaiot.co.uk/service/legionella-monit/

Our approach supports automated Legionella testing and monitoring by:

Installing compact temperature sensors on hot and cold water pipes, cylinders, loops and sentinel outlets
Sending data wirelessly (LoRaWAN, cellular, Ethernet or Wi-Fi) to a secure cloud platform
Continuously checking Legionella temperature thresholds against HSE guidance
Generating alerts when temperatures drift into the risk zone or when flushing is missed
Providing an audit-ready trail that shows that the Legionella risk assessment and written scheme are being followed

Instead of sporadic manual checks, you get continuous data on the conditions that matter for Legionella control. For facilities teams, that means:

Stronger evidence for compliance
Fewer site visits solely for temperature checks
Faster, more targeted responses when the system drifts out of tolerance

To see how Legionella monitoring fits within wider smart water management, you can also read:

Legionella Monitoring UK – Smart Compliance That Cuts Costs:
https://aquaiot.co.uk/legionella-monitoring-uk-smart-compliance/
Smart Water Management in the UK:
https://aquaiot.co.uk/smart-water-management-uk-iot-innovations/

Practical steps to reduce Legionella risk

Once you understand what Legionella is and how it behaves, control becomes a structured process rather than a mystery. For most UK duty holders, the building blocks look like this:

Appoint a duty holder and responsible person
Make sure clear responsibility for Legionella control is assigned and documented, as set out by HSE:
https://www.hse.gov.uk/legionnaires/what-you-must-do/duties.htm
Complete and maintain a Legionella risk assessment
Ensure you have a current, suitable and sufficient Legionella risk assessment for every relevant water system. Review it when systems change, occupancy patterns shift, or at regular intervals.
Design out risk where possible
Use the risk assessment to identify design issues – oversized storage, dead legs, rarely used outlets – and plan remedial works to remove or reduce them.
Maintain effective Legionella temperature control
Keep hot and cold water within the recommended Legionella temperature limits and verify this through routine checks or automated monitoring.
Manage stagnation and usage patterns
Flush little-used outlets, especially after shutdowns or seasonal closures. Factor vacancy, refurbishments and hybrid working patterns into your Legionella control plan.
Implement appropriate Legionella testing and inspections
Where justified, carry out microbiological Legionella testing, tank inspections and other checks, following HSG274 Part 2 and any sector-specific guidance.
Keep good records
Document all aspects of your Legionella control scheme – risk assessments, schematics, temperature logs, test results, remedial actions and training. HSE provides guidance on monitoring and keeping records here:
https://www.hse.gov.uk/legionnaires/what-you-must-do/keeping-records.htm

AQUAIOT’s continuous monitoring platform is designed to support these steps with live data and audit-ready evidence:
https://aquaiot.co.uk/

FAQs: quick answers about what Legionella is

What is Legionella in simple terms?

Legionella are bacteria that live in water. In buildings, they can grow in hot and cold water systems, cooling towers and similar equipment if conditions are right. When tiny droplets of contaminated water are inhaled, they can cause Legionnaires’ disease, a serious lung infection.

Is Legionella found in all water?

Low levels of Legionella bacteria can occur in many natural freshwater environments. The main concern is not their presence in the wild but their growth and amplification in man-made systems where water is stored, warmed and recirculated, and where aerosols can form.

Can you get Legionnaires’ disease from drinking water?

Legionnaires’ disease is usually caused by inhaling contaminated droplets, not by drinking water. Swallowing water contaminated with Legionella is not considered a common route of infection in otherwise healthy people.

Do small buildings still need a Legionella risk assessment?

Yes. HSE guidance makes it clear that all systems supplying hot and cold water services can present a foreseeable risk of Legionella, regardless of size. Smaller, simpler systems may have simpler control schemes, but you still need a Legionella risk assessment and a plan to control risk.

Do I always need Legionella testing, or is temperature monitoring enough?

For many straightforward hot and cold systems in low-risk environments, robust Legionella temperature control and system maintenance may be sufficient without routine microbiological Legionella testing. In higher-risk systems or sectors (healthcare, cooling towers, complex process systems), regular sampling and testing may be required. The decision should be based on your Legionella risk assessment and the relevant HSE and sector guidance.

Conclusion: how AQUAIOT can help with Legionella control and automated testing

Understanding what Legionella is is only the first step. The real challenge for estates and facilities teams is turning that understanding into consistent, auditable control across real-world buildings and portfolios.

In the UK, that means:

Designing and operating systems to keep water outside the Legionella growth range
Carrying out a robust Legionella risk assessment and keeping it up to date
Maintaining effective flushing, cleaning, inspection and Legionella testing regimes
Holding reliable records that show what you did, when and why

Traditional manual checks and paper logbooks can make this difficult to achieve at scale, especially across multiple sites. That is why many organisations are moving towards automated Legionella testing and monitoring – using sensors and IoT platforms to record temperatures and key parameters continuously, rather than in occasional snapshots.

AQUAIOT specialises in exactly this kind of automation. Our Legionella monitoring solution:

Tracks Legionella temperature profiles automatically at tanks, loops and sentinel outlets
Provides real-time alerts when conditions drift into the risk zone
Supports your Legionella risk assessment with hard data and clear reporting
Creates tamper-evident records to demonstrate compliance with HSE guidance

If you want to move from clipboards and guesswork to continuous, evidence-based Legionella control, we can help.

To find out how AQUAIOT can support your Legionella monitoring and automated testing, get in touch with the team via our contact page:
https://aquaiot.co.uk/contact/

Whether you manage a single building or a national estate, we can help you design and deploy a practical, scalable Legionella monitoring strategy that keeps people safe and makes compliance more manageable.