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Legionella is a type of bacterium found naturally in freshwater environments, like lakes and streams. It can become a health concern when it grows and spreads in man-made water systems.  This bacterium grows best in warm water (between 25-42°C) and can be found in areas such as:

  • showers and faucets
  • cooling towers (air-conditioning units for large buildings)
  • hot tubs that aren’t drained after each use
  • decorative fountains and water features
  • hot water tanks, reservoirs and heaters
  • large plumbing systems, attached hoses, dead-end lines, etc.
  • rubber washers and fittings, including water hammer arrestors and rubber hoses with spray attachments.

Home and car air-conditioning units do not use water to cool the air and are therefore not at risk for Legionella growth.


After Legionella grows and multiplies in a building water system, that contaminated water then has to spread in droplets small enough for people to inhale. People can get Legionnaires’ disease when they breathe in small droplets of water in the air that contain the bacteria.

Less commonly, people can get Legionnaires’ disease by aspiration of drinking water. This happens when water “goes down the wrong pipe,” into the trachea (windpipe) and lungs instead of the digestive tract. People at increased risk of aspiration include those with swallowing difficulties.


Most healthy people exposed to Legionella do not get sick. Legionnaires’ disease is very similar to other types of pneumonia (lung infection), with similar if not the same symptoms   Symptoms usually begin 2 to 10 days after being exposed to the bacteria but can take up to 2 weeks to exhibit symptoms.  People at increased risk of getting sick are:

  • people 50 years or older;
  • current or former smokers;
  • people with a chronic lung disease (like chronic obstructive pulmonary disease or emphysema);
  • people with weak immune systems from diseases like cancer, diabetes or kidney failure;
  • people who take drugs that suppress (weaken) the immune system (like after a transplant operation or chemotherapy).


There are no vaccines that can prevent Legionellosis.  Instead, the key to preventing Legionella is making sure that the water systems in buildings are maintained in order to reduce the risk of growing and spreading legionella.

Guidelines for reducing the risk of Legionella growth and spread are available for those who maintain and manage building water systems, including systems for potable (water for drinking and showering), non-potable and recreational water from ASHRAE®. 

Recommended Treatment:

  • Cold water should be stored and distributed at temperatures below 20°C.
  • Hot water should be stored at temperatures above 60°C and circulated with a minimum return temperature of 51°C.  Installing preset thermostatic mixing valves will prevent scalding problems.  
  • Systems that incorporate an elevated holding tank should be inspected and cleaned annually and lids should fit snuggly.
  • Hot water heaters and storage vessels for such systems should have drainage facilities at the lowest point and the heating element located at the lowest point of the vessel to mix and prevent water temperature stratification.
  • High-risk applications should incorporate insulated re-circulation loops.
  • Shower systems should be designed to permit mixing of hot and cold water near the shower head and the  warm water section of the pipe between the control valve and shower head should be self draining.
  • Use of copper-silver ionization in the hot water recirc system eradicates Legionella when used in the range of 0.2-0.8mg/L copper and silver.
  • When decontamination of existing hot water systems is necessary, thermal shock treatment (water temp raised to 71-77°C and maintained while progressively flushing each outlet around the system) or shock chlorination treatment (less expensive but may corrode pipes) work best. 
  • In high-risk applications,monthly removal of shower heads and tap aerators to clean out sediment and scale and clean with chlorine bleach solution.
  • Emergency water systems such as safety showers, eyewash stations and fire sprinkler systems are plumbed with the potable water system and have little or no flow resulting in stagnant conditions.  These stations should be run weekly to prevent the build up of bacteria.  Appropriate precautions should be taken when checking the operation of fire sprinkler systems.
  • Cooling towers, evaporation condensers and indirect evaporative air coolers should be maintained clean and use a biocidal treatment program.  It is also recommended that a water treatment specialist is used to define and oversee the treatment.

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