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Legionella-Outbreak Environmental Response

In case of a Legionnaires’ disease outbreak, facilities should contact their local health department.   Other actions include:

  1. If not done already, conduct an environmental assessment of the facility (the CDC assessment tool is a good resource).

  2. Evaluate and consider potential points of exposure to aerosolized water such as showers, hand washing sinks, decorative fountains, and mist from cooling towers.

  3. Compare this information to data collected from the environmental assessment.

  4. Evaluate the need to conduct environmental monitoring (i.e. temperatures and chlorine residuals) or Legionella sampling at key locations where exposure may have occurred.  Follow the guidance in the CDC's Sampling Procedure and Potential Sampling Sites and use an ELITE certified laboratory.

  5. Implement water use restrictions if necessary.

  6. Install temporary point of use filtration until short term remediation actions are conducted to prevent exposure.  Legionella bacteria is approximately 2 µm in length and 0.3-0.9 µm in width.  Point of use filtration devices must be capable of removing particles less than 0.3 micron in size at 99.999% efficiency (also known as an absolute filter size rating).

  7. Conduct short term remediation measures including temporary hyperchlorination and superheat and flush.  Restrict access to the water during short term remediation to prevent injury.

  8. Conduct follow up environmental sampling after conducting short term remediation.

Cooling Tower Remediation

If a cooling tower is a suspected source of Legionella, short term remediation steps should include:

  1. Evaluation of cooling tower maintenance.
  2. As necessary, conduct physical cleaning of:
    • Areas that hold water,
    • Areas where water circulates,
    • Ensure that any biofilm present is removed.
  3. Re-disinfection of cooling tower and addition of fresh biocidal agents.

The Cooling Tower Institute provides free publications on control of Legionella in cooling towers at:

Post Outbreak

Post outbreak actions should include:

  1. Develop or update, and implement a water management plan.
  2. Implement and monitor control measures as needed.
  3. If necessary, install long term treatment.
  4. Continue disease monitoring. 

Facilities may also find these CDC resources useful in an outbreak:

Investigation Tools for Single Cases of Disease:

Investigation Tools for Clusters and Outbreaks of Disease:

Considerations When Working with Legionella Consultants:

Factors That Lead to Legionella Growth:

Links to other information and resources can be found on the Legionella-Resources and Trainings page.

Updated 5/30/2018