Understanding and Managing Your Legionella Risk

10/17/2019 - White Papers


News in recent months has highlighted the importance of understanding the risk of Legionella in buildings with large, complex water systems. This June, the Atlanta Sheraton experienced an outbreak of Legionnaire’s disease in June that left one person dead and many more ill. The hotel now faces lawsuits and a hit to its reputation from which it may not recover.

Indeed, most identified Legionella outbreaks are in buildings with elaborate, large scale water systems, such as hotels, long-term care facilities, and hospitals. In responding to a Legionella outbreak, many buildings’ water systems are found to be poorly maintained. Procedures may have been in place, but not implemented appropriately.

In response to increasing numbers of Legionella outbreaks in the US each year, ASHRAE (formerly the American Society of Heating, Refrigerating and Air-Conditioning Engineers) developed a voluntary standard that has become industry best practice.

ANSI/ASHRAE Standard 188-2018

ANSI/ASHRAE Standard 188-2018 (ASHRAE 188) establishes minimum legionellosis risk management requirements for buildings with complicated water systems. It is intended for use by owners, managers, and those involved in the design, construction, installation, commissioning, operation, maintenance, and service of centralized building water systems and components.

ASHRAE 188 describes the types of building water systems covered by the standard, and requires owners of susceptible buildings to carry out surveys of all water system components and to analyze the risks of an event. Monitoring and corrective actions must be applied to high risk components.

Following the standard, it is relatively easy and inexpensive to put a Water Management Program (WMP) in place. By doing so, building managers can prevent the major pain that comes with a positive Legionella response.

For owners, managers, and designers of buildings with sophisticated water systems, this white paper will help you understand:

  • The potential risks in your system
  • Where those risks are greatest
  • How to manage the risks and maintain compliance


Understand the risks

Environmental conditions that promote the growth of Legionella

Legionella is a group of pathogenic bacteria—including the species L. pneumophila—that causes Legionellosis. Several factors impact the growth and virulence of the bacteria, including water quality, the presence of other microorganisms, and biofilms.

Water Quality

The Legionella bacteria is heat tolerant, acid-tolerant, and even salt-tolerant. The water quality conditions they tolerate contribute to their resilience in building water systems.


Legionella multiply where temperatures are between 20-45°C (68-113°F) and nutrients are available. The bacteria are dormant below 20°C (68°F) and do not survive above 60°C (140°F).

Legionella can survive at low temperatures for long periods, proliferating when the temperatures increases. The bacterium is even able to withstand temperatures of 50°C (122°F) for several hours.


Found in environments with pH ranges of 2.7 to 8.3, Legionella tolerates acid conditions. They are even able to withstand a pH as low as 2.0 for short periods.

Other microorganisms


Legionella requires nutrients to grow and propagate. Other species of bacteria or microorganisms can supply nutrients to Legionella, directly or indirectly, through the excess production of organic nutrients or through decomposition of the microorganisms.


The Legionella bacteria parasitizes protozoa, helping to protect the bacteria from chemical and thermal disinfection. This mechanism allows L. pneumophila to survive adverse conditions and to travel in airborne aerosols.


Biofilms help microorganisms, including L. pneumophila, survive and flourish under adverse conditions. Biofilms are complex microbial ecosystems containing bacteria, algae and grazing protozoa. Biofilms flourish in areas where there is little to no flow and the water stagnates.

Legionella growing in biofilms are much more resistant than those growing in the water phase of the system, making the presence of biofilms an important factor for Legionella survival and growth in complex water systems.

In addition, as biofilms grow and develop, portions of the film can slough off into the water flow. This resuspends microorganisms into the water, allowing them to colonize other parts of the system when conditions allow.

Sources of aerosols for transmitting Legionella

Legionellosis is transmitted through aerosols, not through ingestion of contaminated water or food. Thus, the water system components that have the highest potential to harbor Legionella include:

  • Cooling towers and evaporative condensers
  • Whirlpools and spas
  • Ornamental fountains, misters, atomizers, air washes, humidifiers, or other non-potable water systems or devices that release water aerosols
  • Ice machines

In these components, water temperatures can encourage bacterial growth if not managed appropriately.

Potential for exposure to Legionella

Outbreaks of Legionella occur most commonly in buildings with large, complex water systems. Historically, most outbreaks have been in hotels, hospitals, and aged-care facilities, although cruise ships are vulnerable as well. However, not everyone who is exposed to the bacterium develops the diseases. There are other factors involved.

People at high risk for legionellosis include:

  • Those over 50 years old
  • Dialysis patients
  • Smokers
  • Those being treated for burns, organ transplant, chemotherapy, lung disease, or diabetes
  • Those with weakened immune systems


Understand your water systems

Where the water flows - Process flow diagrams

Part of understanding how to manage Legionella is understanding where the water flows (or doesn’t) in the system. That starts with a process flow diagram.

Developing a process flow diagram, as described in the ASHRAE 188, involves identifying and describing all the potable and non-potable water systems in the building and on site. This includes the locations where potable and non-potable water is used, and the locations of any water processing equipment and components. Identify where and how water is received, treated, and stored.

Note possible dead ends with potential for water to stagnate. Identify where low flows may allow biofilm growth. Note monitoring and control points.

Now describe all this information graphically in a process flow diagram. Be sure the diagram has sufficient detail to enable the identification, analysis, and management of Legionella risk throughout the water systems.

Assess the risk

The next step is the systematic evaluation of each step in the process flow diagram. Note where hazardous conditions pose a risk and where control measures can be applied. Identify areas where there is a higher probability of infection based on the intended use of the water and the relative vulnerability of patients to Legionellosis.

Apply control

After assessing the risks, control measures are applied to mitigate or control high risks. Control locations are where a control measure like disinfection, heating, cooling, filtering, flushing, or other methods is used to maintain the physical or chemical conditions of water to within control limits. Monitoring at the control locations must be able to detect the physical or chemical conditions of water and warn if the condition exceeds established control limits.


Understand your Responsibility

The ASHRAE standard is a voluntary standard for the most part. However, using compliance with their Code of Participation (CoP) as leverage, the Centers for Medicare & Medicaid Services (CMS) require hospitals, critical access hospitals, and long-term care facilities to have water management policies and procedures in place to reduce the risk of growth and spread of Legionella in building water systems. Compliance with ASHRAE 188 will assure compliance with the CMS CoP.

Relatively inexpensive to develop and easy to implement, a water management program starts with a building survey. A building survey determines the level of compliance required to meet the standard.

Building surveys

The standard recognizes that problems tend to occur in buildings with complicated water systems. Therefore, ASHRAE 188 requires building surveys for each existing building, new building, and any renovation, addition, or modification to an existing building and its water systems. A building survey must be conducted annually, at minimum.

The building survey determines whether the facility has buildings that:

  • Have multiple housing units with centralized hot water systems
  • Are more than 10 stories tall
  • House health-care facilities with patient stays over 24 hours
  • House or treat at-risk occupants
  • Are designed for housing the elderly

Because Legionellosis is transmitted through contaminated aerosols, the building survey will also identify places where the bacteria can proliferate and where there is a risk of aerosols forming. Places in a water system where bacteria proliferate include:

  • Hot and cold water storage tanks
  • Water heaters
  • Water-hammer arrestors
  • Pipes, valves, and fittings
  • Expansion tanks
  • Water filters

Sources of aerosols include:

  • Electronic and manual faucets
  • Aerators
  • Faucet flow restrictors
  • Showerheads and hoses
  • Centrally-installed misters, atomizers, air washers, and humidifiers
  • Eyewash stations
  • Ice machines
  • Hot tubs/saunas
  • Decorative fountains
  • Cooling towers
  • Medical devices (such as CPAP machines, hydrotherapy equipment, bronchoscopes, heater-cooler units)

By identifying high-risk components, the building survey determines the level of compliance and whether a water management program is required.

Water Management Programs

Key to the development of a robust water management program (WMP) is the program team. The team should include the building owner, employees, suppliers, consultants, or others that the building owner has designated to have authority and responsibility for the actions required by the WMP. The team must understand the building water system and its management as it relates to legionellosis.

The development of a water management program ideally follows seven steps. The building survey and process flow diagram provide the basis for the process.

  1. Analyze the building water systems for hazardous conditions.
  2. Determine control locations where control measures are required.
  3. Determine control limits within which a chemical or physical parameter must be monitored and maintained in order to reduce hazardous conditions to an acceptable level.
  4. Establish a monitoring system for the parameters associated with the control limits in Step 3.
  5. Establish the corrective actions to be taken when control limits are exceeded.
  6. Establish procedures to confirm that the Program is being implemented as designed.
  7. Establish documentation concerning all procedures and maintain records appropriate to these principles and their application.

Documentation and Record Keeping

A critical step to remaining compliant with ASHRAE 188 is documentation. Documenting the program itself and all associated policies and procedures assures ongoing implementation. Document procedures for startup/shut down, operation, maintenance, water treatment, and monitoring.

Ensure operational procedures such as water quality monitoring are current and implemented. All data and observations should be recorded, and the records saved.

Schedule a regular review of your WMP for after the annual building survey. Keep the program live!

Then, if the worst happens and despite all your best efforts Legionella is detected in the system, procedures will be in place for responding quickly and effectively.



Following the ASHRAE 188 standard and developing a Water Management Program is relatively easy and inexpensive. Putting a WMP in place can help prevent the major pain that comes with a positive Legionella response.

For owners, managers, and designers of buildings with large, complex water systems, it is crucial to understand:

  • The potential risks in your system
  • Where those risks are greatest
  • How to manage the risks and maintain compliance

By carrying out an annual building survey and risk assessment, and implementing your water management program, you can reduce the risks of a Legionella outbreak in your building.

Our EHS team successfully assists numerous state-of-the-art facilities—including critical care hospitals—in maintaining their water management systems. We're ready to help your team as well. Contact us today to start the conversation.


About the Author

Bob Foster, Business Development Manager In his role at EBI, Bob supports the Environmental Health and Safety team, assisting hospitals and other institutions with EBI’s Environmental Health and Safety, Engineering, and Due Diligence services.

Bob works with a range of clients, some of which focus specifically on support of the Environment of Care. He has over 20 years of experience in business development, product development, marketing, and sales. Prior to joining EBI, Bob focused on business development of both environmental health and safety, as well as energy and sustainability for life science, healthcare, and university clients in the Northeast.


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