Introduction
Legionella is a naturally occurring bacterium found in freshwater environments such as lakes, rivers, and reservoirs. In these natural settings, it typically exists at relatively low concentrations. However, when Legionella enters man-made water systems—particularly those found within buildings—it can multiply under favorable conditions and become a serious public health concern.
The CDC explains that Legionella becomes a health concern when it grows and spreads in human-made water systems, including large plumbing systems, showerheads, faucets, hot tubs, decorative water features, and cooling towers.
Understanding how Legionella interacts with building water systems is essential for property owners, facility managers, and anyone responsible for maintaining safe water conditions.
Building water systems are complex networks that can include:
- Hot and cold water lines
- Storage tanks
- Water heaters
- Fixtures
- Recirculation systems
- Cooling equipment
- Recreational or decorative water features
These systems can create environments where Legionella bacteria thrive, especially when water temperature, flow, disinfectant residual, sediment, and maintenance are not properly controlled.
The primary exposure concern is generally not from simply drinking contaminated water. Legionnaires’ disease is most commonly associated with inhaling small droplets or mist containing Legionella bacteria. The CDC provides additional information about how Legionella spreads.
This makes fixtures and equipment such as showers, certain faucets, spas, fountains, and cooling towers important potential exposure points.
This article provides a comprehensive overview of how Legionella develops within building water systems, where Legionella can grow, and why professional testing and monitoring are important.
What Is Legionella?
Legionella is a genus of bacteria that includes several species, with Legionella pneumophila being one of the most commonly associated with human disease.
It can cause:
- Legionnaires’ disease, a serious type of pneumonia
- Pontiac fever, a generally milder illness
The CDC describes Legionnaires’ disease as a serious pneumonia caused by Legionella bacteria and notes that certain groups have a higher risk of becoming ill.
People at increased risk can include:
- Older adults
- People with weakened immune systems
- Certain people with underlying medical conditions
- Current or former smokers
Unlike many gastrointestinal waterborne illnesses, Legionella infection is generally associated with breathing in aerosolized water containing the bacteria.
Systems that generate small water droplets therefore deserve particular attention.
How Legionella Enters Building Water Systems
Legionella occurs naturally in freshwater environments and can enter building plumbing systems through incoming water supplies.
Once inside a building, whether the bacteria remain at low concentrations or begin multiplying depends heavily on the conditions within the water system.
Important factors include:
- Water temperature
- Water age
- Stagnation
- Disinfectant residual
- Sediment
- Biofilm
- Plumbing-system complexity
- Maintenance practices
Buildings with large or complex plumbing systems can have areas where water moves slowly or remains unused for extended periods.
Disruptions may also affect water conditions. These can include:
- Construction
- Plumbing repairs
- Water-service interruptions
- Extended shutdowns
- Major occupancy changes
- Reduced water use
When occupancy drops, for example, water may remain stagnant within sections of plumbing for longer periods, creating conditions more favorable to microbial growth.
The Role of Temperature in Legionella Growth
Temperature is one of the most important factors influencing Legionella growth.
According to the CDC’s building-water monitoring guidance, Legionella grows most favorably between approximately 25°C and 45°C (77°F–113°F), although growth can occur outside this range under some conditions.
Hot-water systems that are not properly controlled may contain sections that fall within this favorable range.
Cold-water systems can also create risks if temperatures rise sufficiently, particularly:
- In warm climates
- Near heat sources
- In poorly insulated plumbing
- During prolonged stagnation
CDC recommends managing both hot- and cold-water temperatures as part of Legionella control while also considering local requirements for preventing scald injuries.
However, temperature control alone is not enough.
Legionella can persist within:
- Biofilm
- Sediment
- Scale
- Protected areas of plumbing
This is why building water management generally combines temperature management with other control measures and monitoring.
Biofilm and Its Impact on Legionella
Biofilm is a layer of microorganisms and organic material that can develop on the interior surfaces of:
- Pipes
- Storage tanks
- Fixtures
- Water heaters
- Other water-system components
Biofilm can create conditions that support Legionella and make microbial control more difficult.
Within these environments, Legionella can interact with other microorganisms, including amoebae, which can contribute to bacterial survival and replication.
The CDC specifically identifies sediment and biofilm as key factors affecting Legionella growth in potable-water systems.
Biofilm can also reduce the effectiveness of disinfectant because organisms embedded within it may be more protected than organisms freely suspended in water.
Pieces of biofilm may eventually detach, potentially releasing microorganisms into other parts of the plumbing system.
Effective water-management programs therefore consider:
- Cleaning
- Sediment control
- Scale control
- Flushing
- Disinfectant management
- Water temperature
- Water age
Stagnation and Water Flow Issues
Stagnation is another major factor associated with Legionella growth.
When water remains in plumbing for extended periods:
- Disinfectant residual can decline
- Temperature can change
- Sediment may accumulate
- Biofilm conditions may become more favorable
Stagnation is particularly relevant in:
- Vacant rooms
- Seasonal buildings
- Low-use fixtures
- Closed wings
- Underoccupied properties
- Dead legs
Dead legs are sections of plumbing with little or no routine water flow.
CDC’s potable-water guidance recommends addressing low-flow areas and regularly flushing infrequently used fixtures as part of Legionella control.
Understanding how water moves throughout a property is therefore an important part of identifying higher-risk sections of the system.
Common Locations Where Legionella Can Grow
Certain parts of building water systems are more likely to create favorable conditions for Legionella.
Common examples include:
- Hot-water storage tanks
- Water heaters
- Hot-water recirculation systems
- Showers
- Faucets
- Cooling towers
- Hot tubs and spas
- Decorative fountains
- Low-flow plumbing sections
- Infrequently used fixtures
The CDC identifies many of these systems as potential locations where Legionella can grow and spread.
A more detailed breakdown of these environments is available in our guide explaining where Legionella can grow in water systems.
Each environment presents different challenges.
For example, storage tanks may experience:
- Water age
- Sediment accumulation
- Temperature stratification
Showers and certain faucets are important because they can create aerosols, while cooling towers can distribute fine water droplets over larger areas if conditions are not properly controlled.
Cooling Towers and Legionella Risk
Cooling towers deserve particular attention because their operation naturally involves water circulation, evaporation, and aerosol generation.
The OSHA Legionella control and prevention guidance explains that cooling towers, evaporative condensers, and similar equipment can create conditions favorable to Legionella growth and aerosolization if they are not properly maintained.
Potential factors include:
- Warm water
- Sediment
- Scale
- Biofilm
- Inadequate disinfectant control
- Stagnation
Cooling-tower management may therefore require specific maintenance and monitoring procedures in addition to general building plumbing controls.
Aerosolization and Exposure Risk
The primary health risk associated with Legionella occurs when contaminated water becomes aerosolized and people inhale the resulting droplets.
Potential aerosol-generating systems include:
- Showers
- Spray faucets
- Hot tubs
- Spas
- Cooling towers
- Decorative fountains
- Certain water-containing equipment
OSHA also identifies exposure to contaminated aerosolized water as a potential occupational Legionella hazard.
Understanding where aerosol generation occurs helps facility managers prioritize water-management and investigation efforts.
The Importance of Professional Legionella Testing
Professional Legionella testing provides direct laboratory information about whether Legionella is detected in specific water samples collected from a building system.
Visual inspection cannot establish whether Legionella is present.
Routine building maintenance is also not a substitute for environmental analysis when sampling is specifically required.
Professional Legionella testing services may involve collecting samples from strategically selected locations such as:
- Water heaters
- Storage tanks
- Showers
- Faucets
- Recirculation loops
- Cooling systems
- Other identified risk points
Sampling locations should reflect the design of the water system and the reason testing is being performed.
It is also important to understand that one negative sample does not prove that Legionella is absent from every part of a complex building water system. Testing provides information about the locations and conditions sampled and should be interpreted as part of the broader system assessment.
How Legionella Testing Works
Professional Legionella analysis requires careful sample collection, handling, and laboratory processing.
Understanding the Legionella water testing process can help facility managers understand how samples move from the building water system to laboratory analysis and reporting.
The exact procedures depend on:
- Sample type
- Laboratory method
- Water-system characteristics
- Purpose of testing
- Applicable requirements
Samples may come from:
- Potable water
- Cooling water
- Spas
- Industrial water
- Biofilm
- Sediment
ISO 11731 and Legionella Analysis
One recognized laboratory standard is ISO 11731:2017 — Water quality — Enumeration of Legionella.
The International Organization for Standardization describes ISO 11731:2017 as specifying culture methods for isolating Legionella and estimating its concentration in water samples.
The standard can apply to different water matrices, including:
- Potable water
- Industrial water
- Natural water
- Wastewater
- Biofilm
- Sediment
No single laboratory method detects every possible Legionella organism under all circumstances, so the selected method and interpretation should match the sampling objective. ISO itself notes limitations associated with culture-based recovery.
Building Water Management Programs
Building water management programs provide a structured approach to reducing the risk of Legionella growth and spread.
The CDC’s Water Management Program guidance describes water management as the primary strategy for controlling Legionella in relevant building systems.
A typical program may involve:
- Establishing a water-management team
- Mapping the building water system
- Identifying potential areas where Legionella could grow or spread
- Establishing control measures
- Monitoring those controls
- Defining corrective actions
- Verifying that the program operates as intended
- Documenting activities
CDC outlines these steps in its guidance for developing a Water Management Program.
Testing can form part of verification, validation, investigation, or other program-specific activities depending on the building and management strategy.
ASHRAE Standard 188
ASHRAE is another important source of building water-management guidance.
ANSI/ASHRAE Standard 188-2021 addresses Legionellosis risk management in building water systems.
ASHRAE states that Standard 188 establishes minimum requirements for Legionellosis risk management and covers areas including the design, construction, commissioning, operation, maintenance, repair, and expansion of relevant building water systems.
The standard supports development of structured water-management programs rather than relying on isolated testing alone.
Laboratory Analysis and Testing Standards
Professional laboratories follow documented analytical and quality-control procedures when performing Legionella analysis.
Depending on the laboratory and jurisdiction, relevant requirements may involve:
- Approved sampling procedures
- Laboratory quality assurance
- Method-specific controls
- Documentation
- Laboratory accreditation or certification where applicable
ISO 11731 provides a recognized culture-based framework for Legionella enumeration.
When choosing a laboratory, facility managers should consider:
- Laboratory capabilities
- Analytical method
- Relevant accreditation
- Experience with Legionella samples
- Reporting practices
- Sample collection requirements
For answers to common questions about sampling and laboratory analysis, facilities can review the water testing FAQ.
Interpreting Legionella Test Results
Legionella culture results are commonly reported as colony-forming units, or CFU, per unit volume.
Interpretation should consider more than the numerical result alone.
Important factors include:
- Sample location
- Water-system type
- Building population
- Previous results
- Water-management program
- Applicable local regulations
- Reason for sampling
There is no single universal action threshold that applies identically to every building, system, and jurisdiction.
A result from:
- A cooling tower
- Hospital water system
- Hotel
- Residential building
- Spa
may require different interpretation.
Laboratory data should therefore be evaluated within the context of the specific building water system.
Local and Regional Considerations
Legionella risk management can also be influenced by:
- Climate
- Incoming water conditions
- Building age
- Plumbing configuration
- Local regulations
- Occupancy
- Water usage patterns
Properties seeking geographically relevant support can review available Legionella and water testing service locations.
Local health departments and regulatory authorities may also have requirements or recommendations that differ by jurisdiction.
Ongoing Monitoring Is Important
Building water conditions can change over time.
Potential changes include:
- Occupancy levels
- Water temperature
- Plumbing modifications
- System repairs
- Seasonal conditions
- Disinfectant levels
- Water usage
- Equipment operation
A building that has acceptable conditions at one point in time should therefore not be assumed to remain unchanged permanently.
Ongoing water-management activities help facility managers identify these changes and respond appropriately.
Additional Legionella and water-testing articles can help property owners understand monitoring, sampling, water-system conditions, and laboratory testing in more detail.
Conclusion
Legionella is a complex and potentially serious concern within building water systems, but risk can be reduced through a combination of:
- Understanding system conditions
- Water-management programs
- Temperature control
- Managing stagnation
- Biofilm and sediment control
- Proper maintenance
- Appropriate monitoring
- Professional laboratory testing
CDC identifies water-management programs as a central strategy for reducing Legionella growth and spread, while ASHRAE Standard 188 establishes minimum risk-management requirements for relevant building water systems.
Professional Legionella testing provides objective data about the water samples and locations analyzed. It can support facility managers in understanding system conditions, investigating potential problems, and evaluating water-management efforts.
No single measure can completely eliminate every Legionella risk. Effective management depends on understanding the entire building water system and using multiple preventive controls.
For buildings that require environmental analysis, professional Legionella testing services can provide laboratory data to support informed decisions.
If you need assistance determining appropriate sampling locations or testing requirements for a property, you can contact a professional water testing laboratory to discuss your building type, water system, and testing objectives.