Legionella / Legionellosis / Legionnaire’s Disease


Legionella –  A pathogenic, gram negative bacteriam, including species that cause legionellosis, most notably. L. pneumophilia.

Legionellosis – A potentially fatal infectious disease caused by gram negative, aerobic bacteria belonging to the genus Legionella.

Legionella pneumophilia – A ubiquitous aquatic organism that thrives in temperatures between 77 and 113 °F. Responsible for 90% of legionellosis.

Legionnaire’s Disease – also known as “legion fever,” is the more severe form of Legionellosis. Produces high fever and pneumonia.


legionIn July 1976, a mystery disease struck 221 attendees of an American Legion convention in Philadelphia. Thirty four people died in this outbreak. Due to the convention’s high profile, the outbreak garnered wide publicity nationwide. Months later the previously unknown bacterium responsible for the outbreak was identified and subsequently termed Legionella due to the convention.


Legionnaires’ Disease Information

Symptoms: Fever, chills, dry cough which may produce sputum, muscle aches, headache, tiredness, loss of appetite, loss of coordination, diarrhea, and vomiting. Confusion and abnormal heart rate may also appear. Chest X-rays often show pneumonia. Additional testing is required to differentiate Legionnaires’ Disease from other types of pneumonia.

Exposure: Time between exposure and onset of the illness for Legionnaires’ Disease is usually 2 to 10 days.

Transmission: Infection occurs through inhalation of aerosol containing Legionella bacteria.

Prognosis: Fatality rate is between 5% and 30% depending on outbreaks. Rate increases when treatment is delayed.Those at Highest Risk: 50+, former smokers, those with weak immune systems.


Where in building water systems are Legionella found?

If not properly maintained, Legionella bacteria can be found in the following systems:

  • Humidifiers
  • Decorative fountains
  • Cooling towers
  • Pools
  • Spas & hot tubs
  • Conditioners
  • Water storage tanks
  • Faucet fixtures and shower heads
  • Surfaces of inside building plumbing


Why does Legionella grow in building water systems?

Cooling Towers – 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), nutrient sources (algae/scale/other particulates), water vapor drift, and optimum water temperature for bacteria growth of 68-113 F.

Spas/Jacuzzis/Saunas – Whirlpools, spas, and saunas serve as ideal habitats for Legionella because they are maintained at temperatures ideal for their growth and organic nutrients suitable for bacterial growth often accumulate in these waters.

Domestic hot and cold water systems – Legionella bacteria are known to survive and colonize domestic water systems, particularly the hot water system. Legionella bacteria are difficult to 5-1control in the domestic water system because they may hide and survive in the biofilm on the surface of the pipes. Because of the warm temperature requirements for Legionella amplification, hot water systems are particularly susceptible. In addition, dead legs and corners of the pipes and the hot water heater, and temperature stratification in the heater can allow Legionella bacteria to survive and even amplify.

Air Handler Condensate Drip Pans – Condensate drip pans are an ideal growth area for Legionella because of the potential stagnant water that might accumulate in the pans giving proper nutrient and temperature conditions.

Decorative Water Fountains - The bacteria must be present in the supply water or aerosolized Legionella must enter the feature from another source

Ice Machines – Contaminated water supplying ice machine unit


How is Legionella transmitted from building water systems to humans?

Cooling Towers – Transmitted to humans through aerosol drops of water containing the bacterium. Aerosol can be inhaled directly by passer bys or if near air intakes distributed throughout the5-2 building.

Spa Tub/Jacuzzi – Whirlpools and spas can produce water droplets of respirable size that have the potential to transmit Legionella to humans.Centralized Domestic Hot and Cold Water

Systems – As water is dispensed through faucets and showers, a portion of aerosol is released through which the bacteria can infect a human’s respiratory system.

Air Handler Condensate Drip Pans – Particularly in climates with a high humidity and a heavy cooling load, and depending on details of the design and installation of the air handler unit and duct work, there is risk of blowing pathogen-contaminated water droplets downstream inside the air conditioning duct work and thus exposing building occupants.

Decorative Water Wall – A water wall feature operates like a cascade fountain pumping the water to the top of the unit and allowing it to flow down over a smooth surface to the sump. Because of the fine sheeting of water created over a large surface area, there is the additional possibility of creating aerosol

Ice Machines: Consumed or inhaled from use of contaminated ice


Legionella Prevention in Building Water Systems:

Cooling Tower/Evaporative Condensers

Legionella bacteria can be found in most cooling towers systems at very low levels and not necessarily the most dangerous type. The likelihood of Legionella infection can be reduced through proper design of HVAC systems and a maintenance program that includes proper water treatment. The following is a list of preventative steps to consider:
  • Clean and disinfect cooling towers before season startup, and whenever they might become contaminated from algae, scale, or debris.
  • Inspect and/or replace drift eliminators when they are damaged or corroded.
  • Maintain proper water treatment program with an effective biocide program.
  • Perform regular water analysis of the cooling tower for Legionella bacteria.
  • Use an automatic water treatment system that continuously controls the quality of the circulating water and feeds the appropriate amounts of chemical.
  • Test daily for water treatment company’s recommended halogen levels and bacteria use dip slides for a barometer of bacteria levels and rough estimate of biocide program’s effectiveness.

Spa Tub/Jacuzzi

The following is a list of preventative steps to consider:
  • Spa should be drained, cleaned, and refilled with fresh water according to local guidelines and frequency of use.
  • Clean according to vendor or manufacturer’s specifications. Cleaning should include cleaning filter cartridges, drains, inlets, jets etc.
  • Maintain proper chlorine/bromine treatment as regulated by vendor and local guidelines.
  • Regularly sample for bacteria

Centralized Domestic Water Systems

  • Legionella bacteria are ubiquitous at low levels in surface water, meaning it is virtually impossible to prevent them from coming into the building and contaminating the water. Therefore, routine and systematic monitoring can serve as an alarm system to determine whether there is a contamination.
  • One preventative step would be to assure stagnant water does not linger, creating a program of opening up less frequently used faucets and shower heads to let hot water flow through.

Air Handler Condensate Drip Pans


  • These trays should be treated with a bromine or chlorine algaecide when the AC is functioning.
  • The AC tabs should be installed at the beginning of every cooling season and replaced after three months of use or according to treatment guidelines.
  • AHUs should be cleaned and disinfected at the conclusion of every cooling season according to the individual unit’s manufacturer specifications.
  • Regularly sample for bacteria.

Decorative Water Fountains 

  • Regular maintenance including biocide treatment will greatly reduce the risk of Legionella growth. Common biocides include ionized copper, halogens, hydrogen peroxide, and polyquat.
  • Some manufacturers do not recommend using halogens and instead recommend weekly cleaning of surfaces with a 50/50 solution of water and white vinegar followed by cleaning of the pump and tubing by pumping a 10% vinegar solution.
  • When not in use for three or more days, features should be drained and cleaned or fed with a high chlorine level (5ppm) prior to operation.
  • If water becomes cloudy or odorous, feature must be cleaned and drained.
  • Enact a visual inspection procedure and log findings
  • Regularly sample for Legionella bacteria

Ice Machines

  • Ice machines need to regularly be maintained and cleaned. As part of this procedure have engineer inspect thoroughly for slime or other bacteria.
  • If a regular cleaning schedule is not in place, create one that includes disinfection similar to procedure explained in corrective action program.


Corrective Actions if there is a Positive Legionella Bacteria Test

In the event of a positive sample result with harmful levels of bacteria:

Cooling Tower/Evaporative Condensers

Super Chlorinate System:
  1. Close bleed valve on infected system.
  2. Add registered biocide to appropriate ppm to kill bacteria
  3. Circulate for minimum of 5 hours.
  4. Re – Open bleed valve.
  5. Retest System
  6. Review Biocide Program to prevent future outbreaks. Legionella bacteria are not 100% preventable, but a proactive biocide program should minimize potential outbreaks.

Spa Tub/Jacuzzi:

  1. Close the spa to bathers immediately and shut down jets, circulation pumps, but do not drain the water.
  2. Obtain samples of contaminated water for possible use later. Some state or local public health agencies require their own testing of contaminated spas.
  3. Drain all water from the spa. Dispose of the water to waste or as directed by local regulatory authority.
  4. Scrub all surfaces, skimming devices, and circulation components with free chlorine at a minimum concentration of 5ppm to remove any biofilm or slime. After scrubbing rinse the spa with clean water and flush to waste.
  5. Replace any filters or filter media, bag and dispose as normal solid waste.
  6. Inspect the hot tub thoroughly for any broken or poorly functioning components such as valves, sensors, tubing, or disinfectant feeders.
  7. Refill the hot tub with clean water.
  8. Hyperchlorinate using 20 ppm free chlorine.
  9. Keep the jets off and let water circulate for 1 hour in all of the components including surge tank, filter housing and piping.
  10. Turn on the jets to circulate for 9 additional hours ensuring that the 20 ppm of free chlorine is maintained.
  11. Flush the entire system to remove hyperchlorinated water from all equipment prior to repeat sampling.
  12. Take repeat samples for laboratory testing to confirm bacteria is gone.  If positive, repeat steps until testing is negative.
  13. Ensure halogen and pH levels meet local and state standards before reopening.
  14. If the hot tub is associated with an outbreak, conduct a test every 2 weeks for 3 months, then every month for 3 months to ensure complete elimination.

Centralized Domestic Water Systems

There are three distinct options:
1. Super-Chlorination
  • Introduce free chlorine gas into the water system and allow the increased Cl levels to circulate in the entire water system for a few hours.
  • All outlets are opened and flushed so proper disinfection can be achieved.
  • A free Cl concentration of greater than 5 ppm is recommended.
  • It may be necessary to install a chlorine gas injector to control contamination.
2. Super-Heating
  • Raise the hot water temperature to at least 140 F or 60 C, preferably to 160F and 70 C. The hot water should circulate and flush the entire system and outlets for a period of time. There is no standard duration for allowing the flush. 5 – 30 minute duration is suggested.
  • We recommend trickling water out of outlets to allow the boilers to make up the loss of super-hot water during treatment.
  • We recommend treating and flushing the system for one hour for every ten years of system age.
3. Drying and Flushing
  • This is a new method in which the water system is disconnected, drained, and hot air is blown through the pipes. After the system is dry, reconnect it to the water source and flush the system.
  • Water-borne bacteria are susceptible to desiccation, thus the hot air likely kills the bacteria.
  • The drying process helps remove the biofilm and scale deposits. Dried biofilm will be flushed out.
After all disinfection procedures it is recommended to retest two, four, and eight weeks after.

Air Handler Condensate Drip Pans

  1. Suspend use of unit.
  2. Follow manufacturer’s procedure for proper disinfection using a halogen if permitted by manufacturer.
  3. Re-Install AC drain pan biocide tab.
  4. Commission unit and retest.

Decorative Water Fountain

  1. There is no standard procedure for disinfecting decorative water fountains as there are different metals and components in every fountain.
  2. Follow manufacturer’s disinfecting guidelines thoroughly and retest.
  3. We suggest a similar method to spa/tub. Block area to public, drain feature, physically clean, refill and circulate with manufacturer recommend biocide at appropriate ppm of product, drain and refill with fresh water.

Ice Machines

Follow manufacturer’s disinfection procedure.Below are links to two prominent disinfection procedures for removable and non-removable ice contact surfaces.
  1. Disinfecting Commercial ice-making machines with removable ice contact surfaces
  2. Disinfecting Commercial ice-making machines with non removable ice contact surfaces

Warnings and Disclaimers: This web page is designed to provide information regarding the subject matter presented. It is produced with the understanding that neither GWT nor the contributors are rendering legal, medical, engineering or other professional services. Neither GWT nor the contributors shall be liable for damages, in any event, for incidental or consequential damages caused, or alleged to be caused, directly or indirectly, by the use of any information disclosed on this web page, including the use of any recommendations, methods, products, services, instructions or ideas.