What is Legionella?

  • Legionella is a bacteria that can cause a very serious type of pneumonia called Legionnaires’ Disease or a milder disease called Pontiac Fever.
  • In nature, Legionella is found in fresh water and soil. It can also grow and spread in human-made building water systems.
  • Legionella multiplies and survives at water temperatures between 77 °F and 108 °F (25 °C to 42 °C).
  • According to the Centers for Disease Control (CDC), Washington State has one of the lowest rates of reported confirmed cases of Legionnaires' disease.


How is Legionella spread?

Legionella can grow in building water systems that have hot water tanks, air conditioning cooling towers, mist machines, decorative fountains, certain medical devices, hot tubs or pipes that are stagnant. People can get sick when they breathe in mist or accidentally swallow water into the lungs containing Legionella. In general, people do not spread Legionnaires' disease to other people.


How can Legionella be controlled in residential homes or buildings?

Residential and commercial customers can reduce or eliminate potential exposure to Legionella by increasing the temperature setting for their hot water tank. Above 122 °F (55 °C) is the optimum temperature for reducing Legionella growth. Always follow plumbing and safety standards to prevent scalding at point of use. Additional information on controlling Legionella in homes and building water systems is available from the CDC and from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 188.


How does Seattle Public Utilities protect against Legionella?

Seattle Public Utilities (SPU) provides disinfection of the water both at the treatment plant and throughout the distribution system. This meets the EPA requirements of the Surface Water Treatment Rule which requires water systems to filter or disinfect water to remove or inactivate microorganisms. The chlorine used for disinfection in the distribution system is very effective against Legionella. Chlorine levels are monitored throughout the system daily. There are no current regulations or monitoring requirements specifically for Legionella.


More information


Public Utilities

Andrew Lee, General Manager and CEO
Address: 700 5th Avenue, Suite 4900, Seattle, WA, 98104
Mailing Address: PO Box 34018, Seattle, WA, 98124-5177
Phone: (206) 684-3000

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Seattle Public Utilities (SPU) is comprised of three major direct-service providing utilities: the Water Utility, the Drainage and Wastewater Utility, and the Solid Waste Utility.