An outbreak of Legionnaires’ disease last year in the Bronx borough of New York City resulted in dozens of people contracting the disease and several fatalities. Students and staff who spend many hours in educational facilities are just as susceptible to the legionella bacteria that causes the disease, so it’s important for administrators to understand how the organism develops and multiplies and how to mitigate the risks
Scope of the Problem
About 6,000 cases of Legionnaires' disease are reported each year in the United States, according to the Occupational Health and Safety Administration. Scientists believe the actual number of cases is much higher, given the difficulty in distinguishing Legionnaires' disease from other types of pneumonia. The most recent U.S. population-based study estimated that 8,000 to 18,000 people are hospitalized each year with Legionnaires' disease.
The legionella organism lives and proliferates in a temperature between 68°F and 122°F. In temperatures below 68°F, it doesn’t die; it simply goes dormant and is less likely to replicate. Additionally, the organism can survive at 122°F for eight hours and doesn’t instantly die until temperatures reach 176°F. This organism is tough and can thrive in many locations.
Plenty has been written about the most common breeding pools for legionella: cooling towers; spas and hot tubs; fountains and similar water features. But there’s been far less discussion about legionella risks in domestic-use water systems.
Three primary conditions are required for legionella growth:
- low water temperature (that is, somewhere between 68°F and 122°F)
- stagnant water
- the presence of biofilm
These conditions may be found in the plumbing systems of school buildings and university residence halls. So let’s examine how each contributes to the risks.
Water Temperature: Depending on the facility or application, the required hot water temperatures in a system will differ. For example, a sink in a residence hall room or a restroom might be limited to 120°F, but laundry and dining facilities might need to have water 130°F or higher. Engineers often solve this issue with mixing valves, but mixing valves can mess with the system equilibrium and may create pockets of stagnating water because of oversized piping, dead piping, or poorly balanced systems.
The water in a typical hot water storage tank is stratified by temperature – some of which is inhospitable to legionella. However, there may be pockets of water where the temperatures are more friendly to the organism.
Water Quality and Movement: To meet the requirements of residence halls, many hot water systems are oversized to accommodate peak-demand times when many students are seeking to use hot water at the same time. Some residence halls even add a backup hot water storage tank for insurance. It’s in this stored (stagnant) water where legionella has the potential to develop if temperatures in the system are not properly maintained.
Water quality in a storage tank is rarely uniform. Sediment of various types gathers at the bottom of the tank; this sediment can be both a food and an insulator for organisms. So, all it takes is for a disruptive event to the tank – for example, a water hammer or fire hydrant used nearby – for the tank to stir and create an environment more favorable for legionella.
Biofilm: Biofilm is simply a polymeric solution that is excreted from certain organisms. It forms a protective and adherent substrate, is a great insulating factor, and is also a food source, making it a popular breeding ground for bacteria. As an insulator, biofilm can exacerbate the stratification in a hot water storage tank, making some areas even more attractive for legionella.
Biofilms can even grow in high-velocity situations where lots of water moves across a surface.
Legionella does not produce biofilm directly, but it does feed on it.
To prevent Legionnaires’ disease, it’s imperative to reduce the risk of legionella growth and transmission in water systems. Schools and universities can achieve this through a combination of design options, operating practices, and maintenance activities. Here are some options to consider:
- Install commercial-grade tankless water heaters. Tankless technology does not eliminate all legionella risks, but it does remove a potential bacteria source from the system.
- Design the piping system so it’s as short and direct as possible. Do not oversize pipes.
- Recirculate water in the system continuously, if possible, to eliminate pockets of stagnant water.
- Remove dead ends and dead legs in existing pipework.
- Properly insulate pipes and water storage tanks to maintain water temperatures that prevent bacteria growth (i.e., lower than 68°F or higher than 122°F).
- Regularly flush out shower heads and faucets that are infrequently used.
- Periodically analyze water samples to check for bacteria.
Sri Deivasigamani is the founder and CEO of Intellihot, a manufacturer of tankless water heaters. He has a bachelor’s degree from Manipal Institute of Technology, India, and a master’s degree in mechanical engineering from Clemson University.
Sidebar: Legionella awareness
Legionella was discovered in 1976 after an outbreak among people attending a Philadelphia convention of the American Legion. Those who were affected suffered from a type of pneumonia that eventually became known as Legionnaires’ disease.
In 2020, the Covid-19 pandemic heightened school administrators’ awareness of potentially dangerous diseases that could spread in their buildings. Thousands of schools and universities across the nation shut down their facilities to combat the coronavirus, but those empty classrooms and campuses created conditions for a different potentially deadly disease.
Empty school buildings meant unused water systems and tanks and pipes with stagnant water—a prime breeding ground for legionella
In the aftermath of the pandemic, several schools discovered the presence of legionella in their water systems as they prepared to reopen their facilities.