Management of Legionella in Water Systems

Science-Based Guidance for Facilities Managers

The leading cause of reportable waterborne disease outbreaks in the United States today is Legionnaires’ disease, a type of pneumonia caused by the Legionella bacterium. This new Consensus Study Report estimates the number of U.S. cases of Legionnaires’ disease ranges from 52,000 to 70,000 each year. Steps can be taken to limit the growth of this bacterium in water systems to reduce incidence of the disease.

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What Causes Legionnaires’ Disease?

Legionella bacteria reside in many natural environments including rivers, lakes, and soils. These bacteria are also found in a variety of engineered systems that support biofilm growth, including drinking water supplies, image of legionella bacteriacooling towers, hot tubs, fountains, and building plumbing systems and their outlets like faucets and showerheads. These water systems are often characterized by warm temperatures, stagnant water, and a lack of chemical disinfectants—conditions that promote the growth of biofilms and their associated protozoa, which are the hosts for Legionella bacteria. People are exposed to Legionella primarily through inhalation of contaminated aerosols into the respiratory system from these contaminated water systems. Those at higher risk for developing Legionnaires’ disease include the elderly, men, smokers, and the immunocomprised.

Incidence of Legionnaires' Disease Has Increased

Although the Safe Drinking Water Act has been effective in reducing U.S. disease rates of enteric waterborne organisms, it has had little impact on managing Legionella in water systems and buildings. In fact, the incidence of Legionnaires’ disease has increased more than five-fold from 2000 to 2017 (see Figure 1). Facilities managers can play a major role in preventing the growth of Legionella in engineered environments like building water systems.

figure one graph

The reported incidence of Legionnaires’ disease in the United States has increased more than five-fold from 2000 to 2017. Many factors contribute to this increase: more people are at risk because of the aging population, increased use of immunosuppressant drugs, and higher prevalence of comorbid conditions such as diabetes and pulmonary disease. There is also a growing dependence on heating, ventilation, and cooling systems, as well as increasingly complex indoor plumbing systems in large buildings. In addition, efforts to conserve water within buildings are likely to result in an increased risk of Legionella growth due to water stagnation, temperature changes, and loss of disinfectant residual. Higher water temperatures as a result of climate change may also play a role.

Source: Adapted from Shaw et al. (2018) with 2016 and 2017 data from the National Notifiable Diseases Surveillance System.

Determining the Risk of Legionella in a Building

In general, the principles to consider when determining whether a building water system presents a potential risk as a Legionella source and requires control are:

  • Presence of Legionella in the system water;
  • Water temperature between 68-113°F;
  • The system has the means to create and/or spread aerosols;
  • The system stores and/or re-circulates water; and
  • The system is likely to contain a source of food for Legionella, such as contaminants from the surroundings or from the process, including the presence of sludge, rust, scale, organic matter, or biofilm.

Fundamental Factors for Legionella Control in Buildings

Design and commissioning of a large building is a key opportunity to ensure that Legionella control is prioritized, including the appropriate design and implementation of hot and cold water systems and heating, ventilation, and air conditioning (HVAC) features. Furthermore, they should be configured to facilitate the collection of water for Legionella monitoring and for the implementation of maintenance and remediation (such as sampling and injection ports on hot water lines). Hospitals or other buildings where sensitive populations are housed should be designed with the ability to conduct remediation in the case of contamination by Legionella or other pathogens. Unfortunately, the majority of existing large buildings were not designed in this manner and present numerous complex challenges with Legionella control.

It is clear from research and practice that, in most situations, “zero” is not an achievable target for evaluating whether Legionella has effectively been controlled, for several reasons. Some level of Legionella is common in drinking water systems in the absence of an outbreak. The following are the fundamental factors that building managers must consider in controlling Legionella.

Hover over a fundamental factor on the left to learn more.

A fundamental control strategy is to keep hot and cold water systems at temperatures outside Legionella’s preferred growth range of 77-109°F. “Warm” water leaves a water system especially vulnerable to Legionella colonization and growth. Preventative maintenance requires maintaining elevated temperatures (above 140°F at the hot water heater and above 131°F to distal points) across hot water systems. In buildings with populations particularly sensitive to scalding, thermal mixing valves can be used. Curative approaches involve temporarily using very hot temperatures (158°F) to kill existing Legionella bacteria.
A variety of disinfection methods have demonstrated at least some efficacy toward managing Legionella. Those methods include chemical disinfection using oxidizing agents (e.g., chlorine, chloramines, and ozone) and ultraviolet radiation. Of the chemical disinfectants, the use of monochloramine has gained traction in the United States as being the most effective for Legionella control because it is more stable in the water distribution system, minimizes the formation of disinfection byproducts, and can penetrate biofilms better than chlorine.
Managing Hydraulics
Neither heat nor chemical disinfection will be effective unless the water is properly circulated. The flushing of water can have significant benefits. Flushing can reduce total Legionella cell counts in premise plumbing by many mechanisms, such as delivering disinfectants and hot temperatures and dislodging loose deposits and biofilm. Although there is no consensus on optimal flushing frequency, several guidance documents recommend minimum weekly flushing of low-use faucets and showers.
Plumbing Materials
The combination of plumbing materials and building-level water chemistry will uniquely shape the biofilms that grow. However, it remains difficult to identify pipe material that limits Legionella in a given environment. Although copper pipe has well-known antimicrobial properties, it does not universally control Legionella. Polyethylene and other heat-tolerant polymers, popular for their ease of use, are well known to leach organic carbon and can stimulate bacterial growth. Iron pipe is extremely problematic because it enhances biofilm formation and negates the action of disinfectants.
Managing the Distal Portion of the Plumbing
There is strong evidence that concentrations of Legionella in the distal sites of premise plumbing can be significantly higher than in more centralized sections of the plumbing. Delivery devices such as faucets and showerheads provide many surfaces for biofilm growth, and the water in these devices is subject to recurring stagnation. Various actions to prevent biofilm growth include efforts to minimize water volumes at distal points; for example, by using small diameter piping, and maximizing water circulation through improved design and preventive flushing procedures.
Preventing Aerosol Formation
The formation of aerosols is an important risk factor in the transmission of Legionella. Laminar flow of water is preferred, as actions to break the water stream, such as shower nozzles and faucet aerators, can create droplets of less than 5 microns. The report recommends replacing faucets and showerheads to create a laminar flow, as well as taking other steps to limit the inhalation of aerosols, including opening bathroom windows and replacing inefficient bathroom fans.


  • Hot water heater temperatures should be maintained above 140°F (60°C) and the hot water temperature to distal points (the point of connection to a fixture such as a faucet, showerhead, or thermal mixing valve that blends hot and cold water right before they reach the tap) should exceed 131°F (55°C) for all building types.
  • There is growing evidence that, compared to free chlorine, a monochloramine residual better controls Legionella risk from building water systems, although the reasons for the improved performance are not yet clear.
  • Water utilities, plumbers, and building contractors should consider Legionella control when making decisions about pipe and fixture materials, hot water heaters, and plumbing system design.
  • Low-flow fixtures should not be allowed in hospitals and long-term care facilities due to their high-risk occupant populations.

Establishing a Water Management Plan

The report recommends that all public buildings have a Water Management Plan for Legionella. Today, only hospitals and healthcare facilities in New York State, those in the Veterans Health Administration network, and those that receive Medicaid and Medicare funding are required to have such plans.

Elements of a Water Management Plan

1. Establish a program team to carry out all elements of the plan.

2. Describe each water system.

3. Analyze where potential hazards may exist, develop, or propagate.

4. Identify control measures and where they should be applied to stay within limits.

5. Monitor certain parameters (which can include Legionella) to determine if control measures are working.

6. Confirm that the program is being implemented as designed (verification) and that the program effectively controls the hazardous conditions (validation).

7. Document everything.

Monitoring Your System for Legionella

There are many potential locations where Legionella may be present in a large building that should be targeted for sampling. Examples are as follows:

Potable sources
Some building configurations have multiple water mains. A sample should be taken from each source.

Potable tanks
If used, potable tanks should be tested. Water tanks will extend the age of water.

Potable zones
Larger buildings, particularly high-rise buildings, may have multiple building zones as a result of building height and pressures. A sample should be taken from each zone.

Distribution risers
Samples should be drawn from enough risers to provide a good evaluation of all risers. If sampling and testing is done frequently enough (i.e., monthly), a random selection of risers would be possible. Selection should always include risers in places where water use is minimal.

Horizontal distribution
Samples should be taken from enough of the horizontal distribution to be indicative of the entire length. At a minimum, the end point of the horizontal distribution should be sampled to determine whether mitigation is reaching the farthest point of the system.

Potable hot water
All system points listed above should be tested for both hot and cold water systems. Legionella is more likely to exist and propagate in hot water systems where temperatures range from 85-105°F. More hot water points should be sampled than cold.

Potable hot water heat exchangers
Where used, they should be evaluated and sampled.

Potable hot water return piping
Where used, it should be sampled as conditions are often suitable for Legionella development.

Once the locations that will provide a good indication of system performance are identified, the interval for sampling can be determined. In cases where initial testing indicated there was no presence of Legionella anywhere in a facility, and the building use composition indicated no risk of exposure to building occupants, sampling may be done once every 6 months or even once per year. The sampling interval is also driven by the building’s risk tolerance. A hospital with a large immunocompromised patient population and zero tolerance for Legionella may opt for more frequent sampling. In either case, the sampling strategy is dictated by risk and the Water Management Plan parameters.

Determining a Threshold for Environmental Legionella

In order to quantify the relationship between levels of Legionella bacteria and outbreaks of Legionnaires’ disease, the report’s authoring committee reviewed dozens of studies on the occurrence of Legionella in various building types during outbreaks and routine monitoring. The committee found that a Legionella concentration of 5 × 104 colony forming units per liter should be considered an “action level” (i.e., a concentration high enough to warrant serious concern and trigger remediation in a building water system). A lower action level may be necessary to protect those at higher risk for Legionnaires’ disease such as hospital patients, particularly those in intensive care, cancer, and solid-organ transplant units.

Current Legislation, Building Requirements, and Liability

The role of liability in the control and prevention of Legionnaires’ disease has been mixed in the United States. Multi-million-dollar lawsuits are not uncommon for Legionnaires’ disease when the environmental source is tracked to a large building or other entity where the owner and/or other persons are responsible for the safety of those served by an implicated water system. Manslaughter charges have been filed on rare occasions. To protect their clients, some lawyers have advocated that the water facilities considered at risk (e.g., hotels, hospitals) test their water for Legionella as part of a Water Management Plan, while others have advocated that it is better not to test because the results could potentially be used against their client. This latter argument will probably not become entrenched as testing becomes more common, and “not knowing” may hurt rather than help the defense. The growing number of litigants and the large size of settlements may result in the insurance industry pushing many clients with water systems serving the public into improving their prevention programs for Legionnaires’ disease. As Legionella awareness grows, monitoring and protecting systems will most likely be an important step to remain up to date in maintenance management, protect building occupants, and avoid lawsuits.