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.