The 11th International Conference on Legionella was held in Zurich, Switzerland, September 7–11, 2026, marking 50 years since the discovery of Legionella. The conference presented a broad range of scientific and technical research addressing clinical disease, microbiology, environmental surveillance, genomics, engineering controls, and Legionella prevention and control strategies for building water systems.

This column focuses specifically on sessions addressing building water systems, engineered water environments and practical approaches to Legionella prevention and control. 

A central theme emerging from the conference was the ongoing transition of Legionella prevention from a predominantly microbiological and reactive discipline toward an integrated approach based on engineering, system operation, proper risk assessment by qualified persons who understand building water systems, and long-term Legionella control methods. 

The research presented at the conference demonstrates that microbiological monitoring remains important, especially culture tests, which doctors need to order when patients present symptoms that could be Legionnaires’ disease. These tests allow epidemiologists to later collect samples of building water systems to tie cases in persons to a specific building or location. However, culture results alone do not provide a complete understanding of risk or identify the underlying causes of persistent colonization. 

Many presentations focused on Legionella cases and highlighted that many people assessing building water systems who are not trained in that engineering are making changes to building water system design flows and temperatures without fully understanding the consequences. 

These presentations seemed to show the need for allowing only qualified engineers, contractors or system design professionals who are certified and understand building water system engineering concepts to assess building water systems, identify problem areas and remediate or make changes to system operating conditions. 

I have experienced rogue chemical salesmen or lab technicians who were only there to take water samples, suggesting changes to building water systems that made the systems inoperable and more likely to grow bacteria or scald building occupants because they lacked an understanding of how the building water system components work. 

The conference sessions emphasized several key principles:

• Water management plans are important to control Legionella and many other pathogens and microorganisms in building water systems. 

• Effective Legionella prevention depends on understanding the complete water system rather than focusing solely on testing and chasing the individual positive sample locations. 

• Two kinds of risk assessments were discussed: 

1. One method involved lab techs, microbiologists or scientists who were taking water samples and assessing the risk of Legionella in a building based on developing a chart with the percentage of positive tests for Legionella. This method does not consider the causes or conditions in the building water system that contribute to bacterial growth. They simply classify a building based on positive test data. 

2. The other method was a full evaluation of the building water piping systems, including testing the source water and testing key locations at remote sites and at tank drains, etc. The equipment, valve types and valve positions, dead legs, and water aerosolizing fixtures and equipment systems were also evaluated. 

This type of assessment should be done by someone familiar with the building water system engineering and design intent and with knowledge of Legionella growth conditions and the water system components that can monitor or control flow, temperature and water quality. Then they can recommend adjustments to control conditions conducive to bacterial growth and use testing of water treatment chemical residuals and Legionella to verify and validate the system parameters identified in the water management program.

 • A presentation covered emergency flushing and disinfection and how it should be performed in a specific step-by-step procedure. Disinfection and post-disinfection flushing are important responses to reports of positive Legionella tests or to Legionella cases associated with a building. Flushing and disinfection technologies are most effective when combined with appropriate engineering design and operational controls. 

A new guide, “Emergency Flushing and Disinfection for Legionella,” is for building owners/operators who do not have a water management plan or who have a positive Legionella test in the building’s water. It walks them through the process of flushing and disinfecting the building’s water systems. The guide is available at www.phcppros.com.

• A session covered testing buildings to demonstrate persistent and reproducible Legionella colonization patterns over time, supporting the use of building-specific risk management strategies.

• A study on long-term surveillance and trend analysis that can provide more useful information than isolated sampling events.

• Complex systems may contain poorly treated or hydraulically isolated areas that undermine otherwise effective control programs. 

• New buildings and systems undergoing extended periods of stagnation require specific pre-occupancy water management strategies.

• A presentation covered water conservation and energy-efficiency technologies and their effects on water age, water treatment chemical dissipation, bacterial growth and aerosol exposure.

• A presentation covered how large organizations can benefit from risk-based approaches that prioritize resources according to building characteristics and historical performance. 

• A presentation addressed future prevention strategies that are likely to incorporate advanced monitoring, predictive analytics and artificial intelligence while continuing to rely on fundamental engineering principles.

Collectively, the building water system research presented during the 2026 Legionella Conference supports a comprehensive approach to Legionella prevention in which microbiological monitoring, engineering controls, treatment technologies, operational management and organizational governance function together as components of an integrated water management program. 

50 years since the discovery of Legionnaires’ disease 

The conference provided an opportunity to review current research and practice 50 years after the recognition of Legionnaires’ disease and the discovery of its causative organism after the 1976 American Legion Convention in Philadelphia. 

While Legionella research has historically placed substantial emphasis on microbiology, clinical disease, environmental detection and epidemiology, the 2026 conference demonstrated the growing importance of the operation and maintenance of building water systems as a central area of prevention research.

This development reflects an important practical reality. Most human exposure associated with building environments occurs within engineered water systems. Domestic hot water (DHW) systems, cold water systems, cooling towers, showers, plumbing fixtures, hot tubs, swimming pools, decorative fountains, cooling mister systems, healthcare facilities, dental units, storage systems and other components of the built environment can create conditions that support Legionella growth, amplification and aerosol exposure to humans.

As a result, Legionella prevention increasingly requires expertise extending beyond microbiological testing. Effective management requires an understanding of plumbing design, system operation and controls, water temperature, system hydraulics, water age, treatment distribution, stagnation, building occupancy, maintenance practices and system history.

The Zurich conference sessions demonstrated that the prevention of Legionella in engineered water systems is becoming increasingly interdisciplinary. Engineers, microbiologists, medical professionals, public health professionals, building managers, water treatment specialists and facility operators all have important roles in establishing and maintaining safe water systems.

A shift toward systems-based prevention

One of the strongest themes emerging from the 2026 International Legionella Conference is the recognition that Legionella prevention must address the complete building water system.

Historically, a positive Legionella sample often triggered a response centered on flushing and disinfection or remediation. While such flushing and disinfection interventions remain important, the research presented in Zurich suggests that recurring problems frequently result from recognizing that Legionella bacteria are likely to enter a building from the water utility or source during or after events that disrupt the utility water supply (water main breaks, fire events, surge flow events, etc.). 

We must also recognize that building water system conditions that promote growth will amplify bacteria when it is present. Controlling Legionella bacteria should never be a single treatment event. They should be continuously monitored and controlled with a strategy defined after a building water system risk assessment and development of a water management program by a qualified team familiar with building water systems.

These ideal growth conditions may include:

• Inadequate temperatures in the system;
• Inadequate source water chemical residuals to control bacteria growth;
• Inadequate DHW storage, distribution and point-of-use temperature controls;
• Low water turnover;
• Extended stagnation;
• Poor system balancing or hydraulic performance in circulated systems;
• Distal areas with insufficient disinfectant residuals; 
• Isolated branches or loops and specialized system components;
• Inconsistent monitoring;
• Changes in building occupancy;
• Poorly managed pre-occupancy periods;
• Infrastructure deterioration;
• Complex system design.

This systems-based perspective changes the fundamental question from “How can Legionella be removed?” to “What conditions within the water system are allowing Legionella to persist and grow?”

The distinction is significant, and the American Society of Sanitary Engineers has a motto that applies here: “Prevention rather than cure.” Prevent bacteria from growing in the system, and you will not have to deal with curing people who are sick because you failed to control bacterial growth conditions in the piping system. 

A one-time treatment may temporarily reduce Legionella concentrations while leaving the underlying system growth conditions unchanged. In such circumstances, recurring contamination or positive tests in the system should not necessarily be interpreted as treatment failure alone. It may indicate that the water system continues to provide favorable conditions for recolonization. This is because Legionella is often delivered to building water systems through utility water service connections. 

The conference sessions supported the development of water management programs that combine treatment with risk-assessment investigations of the physical and operational characteristics of building water systems. 

Temperature management in DHW systems

Temperature remains one of the most important engineering variables for controlling Legionella in DHW systems.

A major German multicenter study examined the relationship between sampling temperature and Legionella detection using a very large dataset from drinking water installations. The study included more than 450,000 samples collected by multiple testing laboratories.

The research evaluated hot water at various temperatures, identified 131 F as an important benchmark, and demonstrated a strong association between temperatures lower than 131 F and increased Legionella detection.

Samples collected below 131 F were substantially more likely to exceed the relevant action threshold than samples collected at or above this temperature. In the prospective component of the study, samples below 131 F were approximately five times more likely to exceed the action threshold. 

As a side note, ASHRAE in its Standard 188 and Guideline 12 committees has addressed this issue on Legionella growth and kill temperatures and recommended a minimum storage temperature of 140 F for storage-type water heaters and a minimum hot water distribution temperature of 122 F. 

Studies have shown that Legionella bacteria enter a cyst stage and survive between 122 F and 131 F, after which they begin to die. The 122 F temperature has no safety factor, so many engineers use 124 F or 125 F as a safety factor for the target minimum hot water temperature at a temperature gauge at the end of the circulated hot water return near the circulating pump and where the hot water return meets the cold water inlet to the hot water system. (The lowest temperature point in the recirculated system.) 

The findings reinforce the importance of maintaining appropriate temperatures above the Legionella growth temperature throughout DHW systems.

However, the broader engineering significance of the study extends beyond the temperature of water leaving a heater or storage tank. A system can operate with an acceptable source temperature while still developing lower temperatures at distal locations. Potential causes include:

• Long distribution distances;
• Excessive heat loss;
• Inadequate insulation and recirculation;
• Poor balancing;
• Oversized pipe networks;
• Low-use areas;
• Dead-end sections;
• Intermittently used fixtures;
• Complex plumbing configurations.

For this reason, temperature management must be evaluated as a system-wide performance characteristic. Temperature gauges should be located at the end of every branch on a circulated system, and should be checked at distal fixtures. 

The German study also raised concerns regarding sampling practices and the possibility that limited sampling may fail to adequately represent the condition of a large installation. This reinforces the importance of selecting representative monitoring locations and using sampling as part of a broader assessment of system performance.

Stagnation and preoccupancy water management

Extended stagnation emerged as an important concern, particularly in newly constructed buildings.

A Norwegian study investigated four newly constructed residential buildings during extended periods prior to occupancy. Construction and occupancy delays can leave water systems unused for prolonged periods, creating conditions significantly different from those during normal building operation.

The study compared several management approaches, including continued stagnation, system drainage and strategies involving copper-silver ionized water.

Preliminary results identified early Legionella colonization in stagnant potable water systems.

The significance of this research extends well beyond residential construction.

Extended stagnation may occur during:

• Delayed building occupancy;
• Renovation projects;
• Seasonal closures;
• Partial building shutdowns;
• Changes in building use;
• Vacancies;
• Emergency closures;
• Phased construction and occupancy.

The findings support the need to incorporate water safety (flushing and disinfection) into construction commissioning and building handover procedures.

Traditionally, commissioning has focused heavily on mechanical performance, pressure testing, system functionality and compliance with design specifications. Research from the conference suggests that water quality and microbial management should also be considered during the transition from construction to occupancy.

A newly completed plumbing system should not automatically be assumed to be safe simply because it has not yet been occupied.

Preoccupancy water management plans may, therefore, need to address water turnover, flushing, treatment, temperature, monitoring and verification before occupants begin using the system.

Cooling towers, condenser water systems and the importance of hydraulic design

The cooling tower research presented at the conference provided a particularly clear example of the interaction between treatment and system design. A Canadian investigation examined a cooling tower system experiencing persistent Legionella pneumophila despite substantial disinfection efforts.

Rather than focusing exclusively on increasing or changing chemical treatment, the researchers investigated the system’s hydraulic behavior.

Sampling was conducted at multiple locations, including the cooling tower basin, reservoirs, pumps and a coupon station used for monitoring oxidation. The coupon station was located within a small secondary loop positioned at a substantial distance from the biocide injection point.

The investigation found that this location experienced higher water temperatures and lower disinfectant concentrations than other portions of the system. Both total and culturable Legionella pneumophila concentrations were also higher at this site.

The physical configuration of the system meant that some water circulated through the secondary loop without receiving the same initial biocide exposure as the main system flow.

This finding has broad implications for engineered water systems. A treatment program may appear successful when measured near the point of chemical injection, while significant deficiencies remain elsewhere in the network.

Potential problem areas for condenser water/cooling tower water systems include:

• Lack of maintenance for water treatment equipment/chemicals;
• Secondary loops;
• Bypasses;
• Distal branches;
• Dead legs;
• Low-flow components;
• Monitoring points;
• Storage areas;
• Specialized equipment;
• Poorly mixed sections;
• Piping arrangement and balancing valves.

Research on condenser water/cooling tower water systems demonstrates that effective water treatment requires knowledge of how both water and disinfectant flow through the system and how quickly they can dissipate. 

Chemical dosing alone cannot guarantee effective control if portions of the system are hydraulically isolated or have poor flow balancing with inadequate flow for proper treatment.

The researchers recommended including monitoring locations distant from treatment injection points and specifically adding monitoring/sampling ports for evaluating isolated or secondary loops.

Chemical-free alternatives for Legionella control

The conference included a presentation on research evaluating alternative technologies for controlling Legionella in building water systems.

One study compared chemical-free approaches including dissolved oxygen reduction and charged-membrane filtration with more established treatment technologies.

Dissolved oxygen reduction was presented as a method of modifying environmental conditions within the water system to suppress Legionella growth. In the tested hot water model, maintaining extremely low dissolved oxygen concentrations reduced Legionella pneumophila concentrations.

Charged-membrane filtration addressed a different stage of the water system by reducing microbial loading at the point where water enters the building.

The distinction between these approaches is important.

Point-of-entry filtration may reduce the number of microorganisms entering the building, but it does not necessarily provide residual control throughout downstream plumbing.

Conversely, a treatment approach that modifies conditions within the distribution system may provide broader control but requires consideration of system compatibility and operational performance. This was described as a defense-in-depth approach.

This concept is highly relevant to building water management. No single intervention should automatically be expected to address every aspect of risk.

A comprehensive program may include combinations of:

• Temperature management;
• Hydraulic control; 
• Water turnover;
• Filtration (point-of-entry);
• Chemical treatment;
• Alternative physical or chemical-free technologies;
• Monitoring;
• Maintenance;
• Engineering modifications.

The appropriate combination will depend on the characteristics and risk profile of the individual building.

Next month, we will continue coverage of the 2026 International Legionella Conference.