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Legionella Pneumophilia

What Is Legionnaires' Disease?

"Legionnaires' Disease" was named in 1976 after a respiratory disease affected many delegate's attending a convention in Philadelphia, which was held by the American legion of Pennsylvania. The bacteria that was responsible for the disease was isolated and named as Legionella Pneumophilia.

Legionnaires' Disease and Pontiac Fever are both associated with the Legionella species. Legionnaires' Disease is a severe pneumonia (which can be fatal). Affecting middle aged people, but frequently occurring in people whose resistance to infection has been reduced, it has been estimated to have infected 100,000 people a year in the United States. Pontiac Fever is a mild, non-pneumonia influenza-like illness, only a few outbreaks have been reported in the world, one of them, a large outbreak, was in Canada.

Where is the Legionella Pneumophilia found and how is it transmitted?

Legionella Pneumophilia is present in natural sites in the environment, such as water and soil, but these locations are not usually associated with outbreaks of the disease. Modern building HVAC systems are breeding grounds for the Legionella bacteria. The most common location for the growth of the Legionella bacteria is within the HVAC system cooling tower, however it may also thrive in Humidifiers, Air handling unit dip pans, and any other source of standing water. It appears that the Legionella microbe, in order to produce the disease, must reach the lungs, inhaling contaminated water vapor aerosols deep into the lungs seems to be how this happens.

Why are Cooling Towers mostly associated with the spread of the Legionella' Disease?

The design and operation of cooling towers create conditions that are ideal for the growth, survival and transport of the bacteria.

  • A standing pool of water (tower sump).
  • Algae, scale and other particulates, supply nutrients for the bacteria. 
  • Water Vapor drift which allows the bacteria to become airborne.
  • Optimum water temperature to support bacteria growth (68F - 113F).

Cooling Towers are used to cool water and dissipate unwanted heat to the atmosphere through water evaporation. Warm water flows into the top of the cooling tower, then passes through spray nozzles where tiny airborne droplets are formed, providing maximum contact between the water and the air that is being moved through the tower by fans. The water then cascades through a series of splash bars or what they call tower fill, to help cool the water and prevent the water droplets from fusing into larger ones. While falling through the tower, some of the water evaporates, because evaporation consumes heat, the remaining water is cooled. Air pushed through the tower by fans also cools the water. Some water droplets, known as Drift, are carried out of the tower by the air stream produced by the fans. Drift eliminators positioned at the top of the tower reduce this water loss. The cooled water collects at the bottom of the tower sump, where it is pumped back to continue another cycle.


Prevention of Legionnaires' Disease.


Legionella Pneumophilia can be found in most cooling towers, however at a level that is below what would typically cause human health effects. The likelihood of Legionnella Infection can be reduced and controlled through proper design of HVAC Systems, and the administration of an appropriate maintenance program including proper Water Treatment.



  • Locate cooling towers away from outdoor air intakes (paying special attention to prevailing winds), occupied areas, pedestrian walkways, etc.
  • Design enclosures for cooling towers to eliminate potential drift.
  • Specify sloped drain pans in air handling units.


  • Clean and Disinfect cooling towers before seasonal startup, or whenever they become contaminated from algae, scale or debris.
  • Inspect and/or replace Drift eliminators when they are damaged or corroded.
  • Maintain a proper water treatment program, including biocide treatment.
  • Perform regular water analysis of the cooling tower water for Legionella Pneumophilia. The mere presence should not be cause for alarm.
  • Remove standing water from air handling unit drip pans, and rectify the drainage problem.
  • Develop proper operating and maintenance procedures, and diligently follow them to ensure control of the Legionella bacteria.
  • Ideally, an automatic water treatment system should be used that continuously controls the quality of the circulating water and feeds the appropriate chemicals into the system.

By following the above procedures, you will control the risk of Legionnaires' Disease and in the process, create a more energy efficient system.  


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