Legionnaires’ disease: Can it be found in water systems? - Outbreak News Today | Outbreak News Today Outbreak News Today
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The short answer is yes. Legionella pneumophila, the bacteria that causes Legionnaires’ disease, lives in virtually all water systems. These bacteria exist in the natural environment of lakes, ponds, and rivers but in such small concentrations that they typically pose little threat to humans. The trouble starts when the bacteria survive the conventional treatment provided by municipal utilities and makes its way into the water distribution system. From there, it enters the water systems of the buildings where we live and work.

This image shows Legionella colonies growing on a Petri dish. Image/Otto Schwake

This image shows Legionella colonies growing on a Petri dish.
Image/Otto Schwake

Municipal Potable Water Treatment and Distribution System

Most water treatment plants in the United States use chlorine or chloramine, specifically monochloramine, to disinfect the water. These chemicals are effective at eliminating most bacteria that may be present in the water. Legionella bacteria, however, have shown to be able to survive chemical disinfectants currently being used. The result of this resistance means the bacteria are still present in the water supply when it reaches the water distribution system (treatment, storage, piping) and then into the drinking water of our homes, workplaces and public facilities where use it to consume, bath, play or showcase.  

Some studies have shown chloramine is more effective than chlorine in killing the Legionella bacteria.  In general, more needs to be done to ensure that Legionella is eliminated in source water before it enters the municipal distribution system.

Optimal Conditions for Bacteria Growth

When the public water treatment system fails or the public water distribution system is disrupted from such events as water main breaks, source water changes, or infrastructure upgrades, all types of water contamination can flow into our homes and other buildings. In the case of bacterial contamination, larger buildings are considered higher risk because they contain extensive and more complex plumbing systems where there are more places for bacteria such as Legionella to grow. First, bacteria thrives in warm water, particularly when the temperature ranges between 95 and 115 degrees Fahrenheit. However, they can grow in temperatures as low as 68 degrees and up to 122 degrees. It also likes complex, old plumbing systems like those found in hotels, hospitals, and other institutions, as well as ships. Within the pipes of these facilities, the bacteria can thrive in a biofilm composed of microorganisms such as algae, amoebae, and other bacteria. Biofilm provides nutrients and harbor water borne threats such as Legionella. Additional conditions that promote Legionella colonies include a pH between 5.0 and 8.5, stagnant water, and an accumulation of sediment and scaling in the pipes conducive to the growth of microorganisms.

Controlling Legionella in a Water System

The best place to stop Legionella is in the source water so it never has an opportunity to enter homes or buildings. The next step is to implement a comprehensive building water management and maintenance plan in large, complex buildings based on industry best practices, such as the ASHRAE 188 standard, which the CDC is promoting as part of its recently released Legionella toolkit: “Developing a Water Management Program to Reduce Legionella Growth and Spread in Buildings:  A Practical Guide to Implementing Industry Standards.”  The toolkit guides a building owner on developing and implementing a water management program to reduce the building’s risk for growing and spreading Legionella. It also includes practical resources to help ensure water management programs are comprehensive, effective, and in line with industry standards.

Elimination Methods

If, however, in the rare cases Legionella is still able to grow to dangerous levels in buildings, especially health care facilities, then there are several methods of eliminating it. The most effective methods include:

  • Hyperchlorination: This method involves elevating the chlorine in the water system to a level that will kill the bacteria and keeping it at that level for the time required to kill them. Afterwards, the system must be properly flushed and refilled with fresh water.
  • Superheat and flush: A commonly used method that requires no extra chlorine to be added to the water, although it is frequently combined with hyperchlorination. Water is heated to 170 degrees F, a temperature at which the Legionella bacteria is instantly killed, and circulated throughout a building’s water supply system.  The challenge of this method is getting the high temperature water throughout the building’s water system to kill the Legionella and to effectively flush the biofilm.  Also, Legionella bacteria can reappear after this treatment in a relatively short period of time.
  • Copper-silver ion disinfection: This system is becoming more accepted as an effective method over a longer time period. It is generally installed on the hot-water line before it reaches the heating unit, so the ions are present in the system from its beginning. When the cold-water system continues to carry water containing residual chlorine, these two methods combine with favorable results.

Other methods for controlling Legionella include ultraviolet light, tank-less flash heaters, and chlorine dioxide, a method used successfully in Europe.


It is important to note that Legionella can be effectively controlled. When controlled properly, the bacterium represents a minimal health risk and emergency actions should not be required. The key to preventing an outbreak of Legionnaires’ disease starts with the municipal water system.  Additionally, a good building water management plan, responsibly implemented, should control the bacteria from colonizing to dangerous levels within individual building water systems.


Alliance to Prevent Legionnaires’ Disease
We work with a team of public health representatives, health providers, building engineers, water treatment experts and others to educate the public and government officials about Legionnaires’ disease and the Legionella bacteria that causes it. By sharing peer-reviewed literature along with national and international government agency policies and guidelines, our goal is to dispel the myths about what causes this disease and, more importantly, provide the tools to help protect the public against future outbreaks.


  1. m siwicki says:

    Understand that due to the natural ecology of Legionella and other opportunistic waterborne pathogens it is impractical at the present time to control these organisms w/ conventional approaches at the treatment plant level nor to properly manage their presence w/in the distribution network.

    Suppression and control is best applied at the individual building level and can be effective by the proper implementation of the PROCESS of CSC (Continuous-Supplemental-Chlorination).

  2. Noel Hogan says:

    There are many differing opinions on which methods work best for this ongoing problem. Here in the UK we utilise CL02 which as a gas and being water soluble is 5 times more effective at penetrating amoeba air sacks which conventional chlorine cannot. However Chlorine Dioxide isn’t the answer to all water systems and world peace as many would have you believe. It will eat ferrous metals and wreak havoc with flexible braided hoses which are common place links between delivery copper pipework and outlets or ‘faucets’. I think M Siwicki’s point is valid in that irrespective of what’s happening throughout the network, it’s often much more financially viable to concentrate on treating what’s entering the building rather than carpet bombing millions of acres of pipework with expensive chemicals. I will always state though, never forget that knowledge is power in this industry. If people in water systems management are well informed and have a good understanding of the science behind Legionella, then they can act accordingly to reduce the ‘potential’ for the bacteria to proliferate by eliminating deadleg pipework and flush areas which are little used. Chemicals should always be the last line of defence in my opinion.

  3. Chris Burke says:

    A good article with a lot of useful information. The only thing missing from the elimination methods is the option of using point-of-use filters. They are fitted onto the showers and faucets at the point the water is used. They work well and are improving in lifespan providing a safe and cost effective means of suppressing bacteria. like Mr Hogan, I do not like the use of chemicals very much, but can be used in some cases.

  4. David Crow says:

    One article I read stated CDC no longer sees Silver Copper as Best Practices and there is evidence of immunity buildup. Continuous chlorination and chlorine dioxide are more preferable. As stated above the ClO2 has greater penetrating capabilities as a gas over chlorine which remains in solution. The corrosion issues are a valid point. Chloroamine is especially corrosive to brass and the by product after breakdown – NH4 – can become food for bacteria.

  5. M. Siwicki says:

    Please note, the PROCESS of CSC is a multifaceted, combined, synergistic application of liquid sodium hypochlorite as the disinfectant and a soluble silicate based corrosion inhibitor.

    CSC is intended as additional “insurance” once all reasonable physical measures (i.e. elimination of “dead-legs”, areas of stagnation, water chemistry is balanced, temperatures maintained, cross-connections eliminated etc.) have been applied and CSC is primarily intended for use w/in centralized, recirculated, domestic hot water systems (DHWS’s).

  6. Mark Thorson says:

    Maybe somebody could correct me if I’m wrong, but I thought the major outbreaks were mostly associated with large facility central air conditioning systems in which the source of water is condensation from the atmosphere, not the municipal water system.

    • Noel Hogan says:

      No correction required Mark. It’s quite a common assumption and the blame lies with general media output. ANY water system DOMESTIC or PROCESS has the ability to create an aerosol and cause a risk whereby a person or persons may contract Pontiac fever/Legionnaires’ disease or Lochgoilhead fever.

  7. Noel Hogan says:

    Mark Thorson, I can indeed understand why you would think along these lines as many do. Here in the UK it is an on-going fact that we are never entirely sure ‘just’ how many cases go unreported and to which systems are attributed. However, only recently a large company was fined in the care home sector for a resident and visitor contracting the disease from the domestic water systems. It ‘IS’ likely that given the nature of large systems like evarpourative cooling etc that if there is a known breakoput, it will draw more media attention and thus, we remember those more commonly. In truth, LD can occur litterally anywhere and to stae that you cannot catch it from a typical wash hand basin has been proved wrong too. There is a documented case where a senior citizen here dies as a result of a tap (faucet) being used in a bed and breakfast establishment which wasn’t used prior for some months and had stagnated. The aerosol splash was very limited but just enough to affect the person.

    The Sero group of bacteria which is attributed to Legionnaires’ disease doesn’t know whether its in a stand pipe/shower/cooling tower. All angles must be covered and as a risk assessor, the very worst thing…. the most dangerous enemy if you like to our profession…. is to risk assess your assessment prior to the survey. Assumption is often the bad guy.

    • Mark Thorson says:

      Thanks for the information. I suppose that means that any residence which has not been occupied for some time should have all of its water circuits completely flushed before resuming occupancy. And faucets for outdoor water need to be regularly flushed.

      • Noel Hogan says:

        If i were consulted regarding a vacant building, depending on the systems and circumstances, I would request TVC samples at sentinel points. If the TVCs came back as you would expect, then I’d go down the flushing route with a domestic hot water pasteurisation by elevating the hot water generation to 80 degrees and flushing that too via the outlets.
        If however the TVC reading were not good, I’d go for a disinfection. Silver Peroxide or Chlorine Dioxide. The whole system. But being mindful of the system construction first.

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