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Legionella - Environmental

Reports of Legionnaires’ disease in Ohio is increasing annually, with approximately 600 cases reported in 2015. Legionella are bacteria that grow naturally in the environment and especially in warm water. It occurs in freshwater lakes and streams, however, the quantities in natural water bodies are generally insufficient to cause disease.

Growth or amplification of Legionella can occur under different environments in water systems.  Conditions that promote amplification include:

  • Water stagnation
  • Warm temperatures (25 - 51° C [77° - 124° F])
  • Presence of scale and sediment
  • Presence of organic matter (biofilms)
  • Presence of protozoa in the water
  • Lack of residual disinfectant

The bacteria grow well in warm water (25 - 51° C [77° - 124° F]) and in the presence of biofilms or organic material. In building water systems, it is likely to be found when there are low levels of disinfectants (ex. Chlorine), after construction, dead ends in the piping, stagnant water, in cooling towers, thermostatic mixing valves, decorative fountains, ice machines.


Legionella is dispersed through aerosolization of water droplets containing the bacteria.  Sources of aerosolized water droplets include:

  • Showers and faucets
  • Cooling towers
  • Hot tubs
  • Decorative fountains
  • Large, complex water systems

If a person inhales water droplets or mist containing the bacteria, they may get an infection. An infection of the lungs causing pneumonia is called Legionnaires’ disease, and a milder infection of the upper respiratory tract is called Pontiac Fever.

Building owners and managers can take action to reduce the risk of Legionella growth in water systems and cooling towers.  Click on the navigation links to the left or visit the CDC Legionella website for more information and technical resources:

Updated 5/30/2018