The word legionella strikes fear into everyone associated with the building industry. For most of us, this term makes us think of cooling towers and an outbreak of pneumonia that occurred at an American Legion convention in Philadelphia in 1976. We actively monitor our HVAC systems for standing water that could lead to an outbreak. However, a team of researchers headed by Victor L. Yu, M.D., professor of medicine at the University of Pittsburgh and Chief of the Infectious Disease Department at the VA Hospital in Pittsburgh, has found that the source for most outbreaks of Legionnaires’ disease is not HVAC systems but domestic hot water systems.
Legionella is a naturally occurring bacterium found in aquatic environments. It thrives in warm water conditions. Once legionella enters the public water supply, a facility’s domestic hot water system can become the perfect place for it to grow. The risk to humans occurs when the bacteria is aspirated, typically during showering or bathing. Inhalation of the legionella bacteria can lead to the pneumonia known as Legionnaires’ disease. Those at greatest risk are individuals with suppressed immune and respiratory systems, most commonly the elderly, heavy smokers, cancer patients, and transplant patients. The healthcare industry is, consequently, one of the most susceptible environments for Legionnaires’ disease outbreaks.
“Temperature maintenance is crucial to controlling legionella bacterial growth in domestic hot water systems.”
Temperature maintenance is crucial to controlling legionella bacterial growth in domestic hot water systems. In general, legionella bacterial growth is largely dormant at temperatures below 68°F. Optimum growth occurs between 86°F and 104°F. At temperatures above 113°F, legionella has difficulty surviving and is killed within hours. At temperatures of 140°F and above, 90 percent of legionella is killed in minutes.
All large-storage-capacity domestic hot water systems, especially those in schools, hotels, and healthcare facilities, should be designed to maintain a temperature of at least 140°F in their storage tanks. (For small capacity systems, the rate at which the water is used or changed over is typically faster than the time it takes for the bacteria level to build up.) Mixing valve stations are then used to temper the 140°F hot water down to the traditional domestic supply temperatures. To make sure all water in the system is at some point heated to this temperature, all of the domestic hot water, regardless of the supply temperature, should be circulated back to the main hot water storage equipment where it is reheated to 140°F. In addition, preventing stagnation within the system is crucial. Active use of all plumbing fixtures and seasonal flushing of dead ends and remote points of the hot water distribution system help prevent this.
Temperature maintenance and active use, though not the only means of minimizing the risk of legionella, are very important and effective methods that building owners, facilities engineers, and design professionals can use to control the growth of these dangerous bacteria in domestic hot water systems with large storage capacities.
– Brian D. Walker, PE
Brian is a Project Engineer and LEED Accredited Professional. Please feel free to contact Brian for further details regarding the above information.