Understanding Legionnaires’ Disease and How It Is Spread

Legionnaires’ disease is a serious form of pneumonia caused by exposure to Legionella bacteria. Although Legionella can occur naturally in water, the greatest concern for businesses comes from man-made water systems where the right conditions can allow the bacteria to multiply.Recent events in New York City have once again highlighted how important effective Legionella management is. In July 2026, a Legionnaires’ disease cluster on Manhattan’s Upper East Side was linked to Legionella bacteria found in building cooling towers. (BBC News)

Understanding how Legionnaires’ disease spreads is an important part of protecting employees, customers, residents and visitors from unnecessary exposure. For businesses responsible for water systems, effective Legionella management is not simply about testing water samples – it starts with understanding where the risk comes from and putting appropriate controls in place.

What is Legionnaires’ disease?

Legionnaires’ disease is an infection caused by Legionella bacteria. It can cause a potentially serious form of pneumonia and, in some cases, can be life-threatening.

Legionella bacteria are commonly found in natural water sources such as rivers, lakes and ponds. In these environments, the conditions generally do not allow the bacteria to multiply to levels that present a significant risk to people.

The concern arises when Legionella enters a building’s water system and encounters conditions that allow it to grow.

Certain man-made water systems can provide an environment where Legionella can multiply, particularly where water is stored or recirculated, temperatures are favourable and there is a source of nutrients such as scale, sludge or organic matter.

The risk is therefore not simply about whether Legionella is present. It is about whether the water system creates conditions in which the bacteria can multiply and subsequently become airborne.

How does Legionnaires’ disease spread?

One of the most important things to understand about Legionnaires’ disease is that people generally become infected by breathing in tiny airborne droplets of contaminated water.

These droplets, known as aerosols, can be produced by a variety of water outlets and systems.

For example, a shower can create a fine spray that becomes airborne. If the water contains Legionella and the bacteria have multiplied within the system, inhaling those droplets can potentially result in infection.

Other potential sources include:

• Showers and showerheads

• Taps and other water outlets that produce aerosols

• Cooling towers and evaporative condensers

• Spa pools and hot tubs

• Humidifiers and air washers

• Certain industrial water systems

• Indoor ornamental water features

• Emergency showers and eye-wash systems

  
  

The likelihood of exposure depends on the type of system, how it is operated and maintained, and whether conditions exist that encourage Legionella growth.

For businesses, understanding these potential sources is one of the reasons a professional Legionella risk assessment is so important. A risk assessment looks at the water systems within a premises and identifies where potential hazards and exposure routes may exist.

Does Legionnaires’ disease spread from person to person?

Legionnaires’ disease is not generally considered to spread from person to person.

The primary concern is exposure to contaminated water droplets or aerosols.

This distinction is important for businesses because controlling the risk means looking closely at the water systems within a building or site, rather than treating Legionnaires’ disease like an infectious illness that can be passed between people through normal contact.

Effective water management should focus on identifying potential sources of Legionella, understanding how water is being stored and distributed, and ensuring appropriate control measures are in place.

What conditions allow Legionella to grow?

Legionella bacteria can multiply when several favourable conditions exist within a water system.

Temperature is particularly important. Water temperatures in the approximate range of 20°C to 45°C can provide favourable conditions for Legionella growth. The bacteria can also benefit from stagnant water and the presence of materials such as scale, rust, sludge, organic matter and biofilms.

Some of the factors that can increase the risk include:

1. Water temperatures in the favourable growth range

Poorly controlled temperatures can allow Legionella to multiply.

For hot and cold water systems, temperature management is therefore an important part of controlling the risk. Cold water should be kept appropriately cool, while hot water systems should be operated at temperatures that help control Legionella.

Regular temperature monitoring can form part of an ongoing Legionella control regime, alongside other preventative measures.

2. Stagnant or infrequently used water

Water that sits unused for extended periods can create additional opportunities for bacterial growth.

This can become particularly relevant in buildings with:

• Empty offices

• Unused bedrooms

• Vacant properties

• Seasonal accommodation

• Infrequently used bathrooms

• Redundant sections of pipework

Unused pipework can also create what are commonly referred to as “dead legs” or “dead ends”, where water has limited or no circulation.

Where little-used outlets are identified, appropriate flushing can form part of a wider control programme.

3. Scale, sludge and biofilm

Water systems are not always perfectly clean internally. Scale, rust, sludge and organic material can accumulate within tanks, pipes, outlets and other components.

Biofilm – a layer of microorganisms and organic material that can develop on surfaces – can also provide an environment in which bacteria may persist.

Maintaining water systems and carrying out appropriate cleaning and maintenance can therefore form an important part of Legionella prevention.

4. Water storage and recirculation

Systems that store or continuously recirculate water need to be properly designed, operated and maintained.

The more complex a water system is, the more important it becomes to understand how water moves through it and where potential problem areas could develop.

This is one of the areas assessed during a professional Legionella risk assessment.

5. The creation of aerosols

Even if bacteria are present within a system, exposure requires a route for people to come into contact with them.

Water outlets and equipment that generate fine droplets can therefore increase the potential for exposure.

This is why showers, cooling towers, spa pools and other aerosol-producing systems require careful consideration when assessing Legionella risk.

Who is most at risk?

Anyone can potentially contract Legionnaires’ disease, but certain groups are more susceptible to serious infection.

The risk can be greater for people who are older, smokers or heavy drinkers, or who have certain underlying health conditions affecting the lungs, heart or kidneys, diabetes or immune function.

For businesses, this reinforces the importance of considering not only the water system itself but also the people who could potentially be exposed.

A care home, healthcare setting or accommodation provider, for example, may need to take particular care because people using the premises could include individuals who are more vulnerable to infection.

What are the symptoms of Legionnaires’ disease?

The symptoms of Legionnaires’ disease can initially resemble those of flu or other respiratory illnesses.

Symptoms can include:

• High temperature or fever

• Chills

• Headaches

• Muscle aches

• Cough

• Tiredness

  
  

More serious illness can develop, including pneumonia. Some people may also experience diarrhoea or confusion.

Because the early symptoms can be non-specific, Legionnaires’ disease may not immediately be obvious. Anyone who is concerned about symptoms should seek appropriate medical advice.

Where can Legionella risks occur?

Legionella risk isn’t limited to one particular type of business.

Potentially relevant water systems can be found across a wide range of commercial and public premises.

Offices

Office buildings may have hot and cold water systems, showers, drinking facilities and other water outlets that need to be appropriately managed.

Even relatively straightforward systems should not be overlooked, particularly where outlets are infrequently used or parts of a building are left vacant for extended periods.

Hotels and accommodation

Hotels, guest houses and other accommodation can have extensive water systems, multiple outlets, showers and periods where rooms may be unoccupied.

Changes in occupancy and seasonal use can also affect how water systems operate.

Care homes and healthcare premises

These environments require particularly careful management because residents and patients may be more vulnerable to serious infection.

A suitable risk assessment should take into account the characteristics of the building, the water systems and the people who may be exposed.

Leisure facilities

Swimming pools, spa pools, showers and associated systems can create circumstances that require careful assessment and control.

Industrial and commercial premises

Factories, warehouses and other industrial sites can contain more complex water systems, including cooling equipment and specialist processes.

Residential and rented properties

Landlords and those responsible for residential properties also need to consider Legionella risks associated with the water systems under their control.

How can businesses reduce the risk?

There is no single action that eliminates every Legionella risk. Effective control comes from understanding the individual water system and implementing measures appropriate to the risks identified.

A good Legionella management programme can include:

Carrying out a Legionella risk assessment

A risk assessment should identify the water systems present, how they operate, potential sources of exposure and the people who could be affected.

It should also consider factors such as temperature, water storage, circulation, stagnation, system cleanliness and aerosol generation.

Absolute Water Compliance provides professional Legionella risk assessments across the UK, helping responsible persons understand their water systems, identify potential risks and establish appropriate control measures.

Maintaining appropriate temperatures

Temperature control is one of the key ways of reducing the opportunity for Legionella to multiply in hot and cold water systems.

Temperature checks can be incorporated into an ongoing monitoring programme, with results recorded and reviewed.

Preventing stagnation

Unused outlets and redundant pipework can create areas where water remains stagnant.

Businesses should consider whether sections of their water system are being used as intended and whether redundant pipework or infrequently used outlets require additional management.

Where appropriate, flushing programmes can help reduce stagnation in infrequently used outlets.

Keeping water systems clean

Storage tanks, cylinders, showerheads, pipework and other components should be maintained appropriately.

Scale, sludge and other deposits can contribute to conditions that support bacterial growth, making regular inspection and maintenance an important part of an overall control strategy.

Implementing a Legionella control regime

Once the risks have been identified, businesses need to ensure appropriate control measures are actually carried out.

A Legionella control regime can include activities such as flushing little-used outlets, temperature monitoring, shower and outlet descaling, cold water storage tank inspections and other monitoring or maintenance tasks appropriate to the individual system.

The exact requirements will vary from one premises to another, which is why control measures should be based on the findings of the risk assessment.

Does Legionella testing prevent Legionnaires’ disease?

This is an important distinction for businesses.

Legionella testing does not, by itself, prevent Legionnaires’ disease.

Water sampling and microbiological testing can provide valuable information about the condition of a water system, but testing should form part of a wider risk management approach.

The appropriate frequency and type of sampling will depend on the water system, the level of risk and the findings of the risk assessment.

Absolute Water Compliance provides Legionella water sampling and testing as both a standalone service and as part of ongoing monitoring programmes. Samples can be submitted to a UKAS-accredited laboratory for analysis and interpretation.

This is why it is important not to view a negative Legionella test result as proof that a water system will remain safe indefinitely. Conditions within a system can change, and effective prevention relies on ongoing management.

Why is a Legionella risk assessment important?

A Legionella risk assessment provides a structured way of identifying where exposure could occur and whether existing controls are adequate.

It should consider the whole water system rather than focusing on an individual tap or shower.

This can include identifying:

• Water storage tanks and cylinders

• Hot and cold water distribution systems

• Showers and other outlets

• Dead legs and redundant pipework

• Temperature control measures

• Areas of low or intermittent usage

• Sources of water stagnation

• Potential aerosol-generating equipment

• Cleaning and maintenance requirements

• Monitoring arrangements

• Responsibilities for managing the system

A properly completed risk assessment gives the responsible person a clearer understanding of the system and the actions required to manage potential risks.

If you are unsure whether your existing assessment is still suitable, Absolute Water Compliance also provides a free Legionella risk assessment checklist that can be used as a starting point for reviewing the arrangements in place.

What happens if a Legionella risk is identified?

Finding a potential risk doesn’t necessarily mean that a building needs to be closed or that the entire water system needs replacing.

The appropriate response depends on the nature and severity of the risk.

Depending on the circumstances, corrective measures could involve improving temperature control, addressing stagnation, cleaning or disinfecting parts of a system, removing redundant pipework, improving monitoring or introducing additional control measures.

Where microbiological testing has identified Legionella, the result should be interpreted alongside information about the water system and its existing controls rather than considered in isolation.

This is why competent assessment and professional advice can be particularly valuable when a potential problem has been identified.

Legionella prevention is an ongoing responsibility

Managing Legionella is not something that should be treated as a one-off exercise.

Buildings change. Water systems are modified. Businesses expand or reduce their operations. Rooms can become vacant, equipment can be replaced and water usage can change.

Any of these changes could affect the way a water system operates.

For this reason, Legionella risk assessments and control measures should be reviewed when there are changes to the system or circumstances that could affect the validity of the existing assessment.

An assessment should also be reviewed if monitoring or inspection identifies that existing control measures may no longer be effective.

Keeping accurate records is another important part of demonstrating that appropriate control measures are being implemented and monitored.

Protect your business from Legionella risks

Legionnaires’ disease is a serious but manageable risk when water systems are properly understood, maintained and controlled.

The key is not simply to ask whether Legionella is present. Businesses should consider how their water systems operate, where conditions could allow bacteria to multiply, how aerosols could be created and who could potentially be exposed.

A professional Legionella risk assessment can help identify potential hazards and provide a clear basis for managing them.

From risk assessments and ongoing Legionella control regimes to water sampling and testing, Absolute Water Compliance can support businesses with their ongoing water hygiene and Legionella management requirements.

With more than 20 years of experience in water compliance, Absolute Water Compliance works with organisations across the UK to help them understand their responsibilities and manage water system risks effectively.

If you’re responsible for a commercial premises and you’re unsure whether your Legionella controls are adequate, contact Absolute Water Compliance to discuss your requirements or arrange a professional Legionella risk assessment.

Need help managing your Legionella responsibilities?

Absolute Water Compliance provides professional Legionella risk assessments, water sampling, control regimes and water hygiene services across the UK.

Request a free consultation to discuss your requirements with the team.