Legionella is a waterborne bacterium that can be inhaled in the form of steam or as droplets by contaminated aerosols. This bacterium lives in all water sources, whether natural or man-made. Due to the nature of Legionella, plumbing and mechanical systems are especially susceptible and must be designed and maintained in accordance with the applicable codes. 

To effectively mitigate the growth of the bacterium, we need to understand what it is and the history behind it. Legionella (also known as Legionellosis, Legionnaire’s Disease, Pontiac Fever, etc.) and all other forms of the polyonymous bacteria were first identified in Philadelphia in 1976 at a convention for the American Legion in the aftermath of a fatal outbreak. 

Once this phenomenon was understood, previous unsolved sporadic epidemics were able to be linked to the newly identified pathogenic bacteria going back to as early as 1943. 

There are two variations of Legionella: Legionnaire’s Disease and Pontiac Fever. Pontiac Fever was the first strain identified in 1968 in a Michigan City Health Department but could not be attributed to Legionella until after its discovery in 1976 (https://bit.ly/3u2qaa7). 

The reason for the discovery of Legionella at the convention was due to its fatality rate compared to Pontiac Fever. While respiratory symptoms were associated with both locations, pneumonia was only observed in Philadelphia, leading to the death of 29 of the 182 infected individuals (http://bit.ly/3ZeyUHN).

Why Legionella is Dangerous

Legionnaire’s Disease is a type of pneumonia or lung infection that impacts an estimated 25,000 people per year in the United States and is usually treated with antibiotics and hospitalization (https://on.ny.gov/3ZaLI1S). While all individuals are at risk of encountering Legionella, healthy individuals are significantly less likely to get sick. 

Individuals at increased risk include those more than 50 years old, current or former smokers, chronic lung disease patients, those who are immuno-compromised, and individuals with other underlying diseases. Currently, no vaccine is available to prevent someone from contracting it.

Legionella does not transfer from person to person outside of extenuating or rare circumstances. This waterborne pathogen exists naturally in moist soil or stagnant freshwater sources such as lakes, slow moving streams or ponds. Within mechanical and plumbing systems, the bacteria may be found in hot and cold-water taps, hot water tanks, cooling towers, evaporative cooler basins, hot tubs and stagnant plumbing systems. 

Luckily, due to the low count of bacteria in the natural environment, being infected with Legionella is difficult. The primary concern is when Legionella becomes established within building systems, which increases the risk of spreading.

Building Types and Systems Prone to Legionella

Common systems in buildings prone to growing Legionella include plumbing systems, cooling towers and evaporative coolers. Buildings being reopened after being shut down for extended periods of time or older buildings with outdated systems are especially prone to its growth. The ideal conditions for Legionella are between the temperatures of
77 F and 113 F in a location with stagnant water and no direct sunlight. 

While disinfectants can be added to systems to prevent the growth of Legionella, standing water does not allow its circulation throughout the system. Stagnant water supports the development of biofilm and allows sediment to settle within the system. 

Once Legionella grows and the associated building system goes back into use, the water from the system may then become airborne as a mist from plumbing fixtures, fountains and cooling towers or simply through evaporation. Once in the air, people may potentially inhale them and begin contracting the disease.

Cooling towers use fans and water to remove heat from the building and if not properly maintained, may cause outbreaks in people exposed to the mist. Water from a cooling tower is circulated through mechanical equipment to remove heat. This warm water is then sprayed over media while fans draw air across to enhance evaporation. Evaporation then cools the remaining water, which can then be circulated back through the system to repeat the process. 

When cooling towers stand idle for long periods of time in warmer climates, there is the potential to have the ideal conditions for Legionella growth. If it becomes established and the tower is then put back into service, the mist from the tower could release the bacteria into the atmosphere.

Domestic hot water systems are another source where Legionella can thrive. If the domestic hot water system is not designed to avoid the temperature range at which it is most likely to grow, outbreaks can occur as plumbing fixtures are used by occupants.

Legionella Mitigation

Over the years, multiple actions and regulations have been put in place to reduce the severity and frequency of the risk of Legionella growth. Document guides, codes and standards may or may not be applied, depending on the type of building and state. The inconsistency of the Legionella management is based on the lack of federal law allowing the states to provide individual requirements if pursued. 

However, European countries have imposed stricter regulations determined to be effective. For one instance, risk assessments are obligatory whereas in the United States, risk assessments and testing are optional. The actions taken to mitigate this bacterium include properly maintained equipment and systems, and a specific system design to not promote its growth.

Routine maintenance, cleaning, long-term building shutdown/re-opening maintenance and continuous monitoring are proper actions to mitigate Legionella. Cleaning and routine maintenance can prevent it by removing biofilm, organic debris and corrosion from water systems. 

When a building is shut down for extended periods of time such as a school over summer break or an organization going out of business, equipment such as the cooling towers should be drained and cleaned. Draining the plumbing hot and cold-water systems may be inadvisable for limited periods of closure, so instead, periodical flushing of the system is recommended to limit stagnant water sources.

Maintenance personnel such as in a school could routinely flush the system by going to each fixture or, if the system is properly designed, flush the entire system at select locations. Sometimes draining and flushing the system during the closure is not practical, in which case additional procedures and efforts can be undertaken prior to reopening. 

Various buildings use a building monitor system that monitors and operates select equipment to avoid generating the conditions for Legionella growth. This may include monitoring the water temperature and periodically circulating systems within the domestic hot water system.

While the above mitigation techniques help to limit the germination of Legionella within the system, a better approach designs the building systems to naturally limit the potential for or kill bacteria that has formed. Selecting heating equipment to maintain hot water at a temperature of 140 F is a start, along with the installation of a recirculation pump to maintain constant flow within the system to prevent the stagnation and cool down to significantly reduce risk. 

While this works for the system at large, there is another form of stagnation in the branch lines that connect to the fixtures without recirculation. These are what are known as dead legs. While it is not possible to remove all dead legs, it is possible to reduce the likelihood of Legionella growth within each dead leg. 

Certain regulations and building codes were created to assist in mitigating Legionella. One such code is the International Energy Conservation Code (IECC), which requires all lavatories have a maximum dead leg of two feet, and for all other fixtures and appliances using domestic hot water have an increased range dependent on the pipe size with a maximum length of 50 feet. 

A regulation used for domestic hot water systems specifically for hospitals is the Facility Guidelines Institute (FGI) for Design and Construction of Hospitals or California Department of Health Care Access and Information (HCAI), formally known as OSHPD for California. The design requirements from both FGI and HCAI require domestic hot water piping dead legs or nonrecirculated branch piping to not exceed 25 feet in length. 

An example of this application is the Ada County Coroner’s Office Facility in Meridian, Idaho. This is a modern facility that provides medicolegal death investigation for 33 counties in Idaho and three Native American reservations in one of the nation’s fastest-growing states (http://bit.ly/3Zf9qu2). The hot water design uses a flow splitter accessory to provide continuous loops to plumbing fixtures such as showers and public lavatories. 

In lieu of providing a full-size loop or requiring the use of multiple balancing valves within the hot water system to comply to the IECC requirement for a 2-foot maximum dead leg, the design allows for smaller piping loops to serve each of the plumbing fixtures. This accessory also has the option to incorporate manual isolation valves to improve maintainability without having to add additional devices. Eliminating the dead leg also shortens the time for hot water to be available at the fixture, lowering energy and water use.

By implementing these actions and being aware of relative regulations, designers and engineers have the tools and knowledge to mitigate the potential of Legionella growth. For additional practices and techniques to prevent its growth, stayed tuned for Legionella prevention in a future issue. 

Megan Hessil is a plumbing engineer at SmithGroup’s Phoenix office and a member of the American Society of Plumbing Engineers (ASPE). Lhymwell Manalo is a plumbing designer at SmithGroup’s Phoenix office. He is a member of ASPE and has six years of experience designing plumbing systems for various building types.