Legionella was discovered in 1976 after an outbreak of a pneumonia-like disease at an American Legion convention in Philadelphia; it is normally spread in the form of water vapor breathed into the lungs. Mitigation techniques minimize the impact and the consequences of Legionella growth after its discovery, while prevention seeks to eliminate the conditions in which it can grow and thrive. Proper prevention becomes a primary defense of the bacteria, terminating it when it does occur and ideally eliminating the potential of its growth. 

The bacteria is spread through contaminated aerosols that can impact all types of buildings. Plumbing and mechanical systems are particularly vulnerable and require careful design and maintenance to prevent the formation of Legionella bacteria. It causes two disease variations: Legionnaires’ Disease and Pontiac Fever, with the former being more lethal.

Pontiac Fever is a milder infection with flu-like symptoms that usually resolves on its own. In contrast, Legionnaires’ Disease is a severe lung infection causing symptoms such as coughing, fever and shortness of breath. It affects nearly 25,000 people a year and makes up approximately 7% to 13% of pneumonia cases. The elderly, smokers and those with underlying health conditions are at a higher risk. 

Legionella spreads through contaminated water and soil sources but not person-to-person. Preventing its growth in building systems is crucial. The ultraviolet (UV) light in sunlight breaks down Legionella, making its occurrence in the natural environment rare. The bacteria pose a risk, however, when occurring and multiplying in buildings’ plumbing and mechanical systems. 

Preventing Legionella Growth

Legionella prevention can occur in multiple ways: proper system design, system maintenance and added system solutions. Proper domestic hot water system designs can not only mitigate the bacteria but prevent it from entering the system. A key aspect of that prevention is continuous recirculation and the temperatures at which portions of the hot water system are maintained. 

Understanding the temperatures where Legionella grows, at what point it stops growing, at what temperatures it can survive and the temperatures at which it starts to die is important.

Legionella grows best in dark locations with stagnant water at temperatures between 77 F to 113 F. With water temperatures above 140 F, the bacteria will die within 32 minutes or less. Above 158 F, it will be killed immediately. As a result, domestic hot water systems with storage tank water heaters maintaining 140 F will kill Legionella passing through the tank due to the volume and increased residence time of water in the tank. 

If the domestic hot water system heating source is an instantaneous type, such as a heat exchanger, there is no residence time for the bacteria to be exposed to elevated temperatures. In this case, Legionella prevention increases the outlet temperature to 158 F to provide the instant kill before blending down to the lower loop temperature to maintain outside of the Legionella growth range. 

One example of using a more efficient system in lieu of using more gas or electricity is designing a heating hot water to domestic hot water heat exchanger system. The mechanical heating hot water system can reach up to 180 F, so if the heating hot water system design can provide a high enough temperature to allow for 158 F out of the heat exchanger, Legionella can be terminated from the system instantly. 

A properly designed hot water system, however, is only a part of preventing Legionella growth. Equally important is maintaining the hot water system to prevent Legionella when it is not in operation. During the worst of the COVID-19 pandemic, when offices were shut down and a significant portion of people worked from home, domestic hot water systems were also turned off for extended periods, providing an opportunity for Legionella to grow and become established. 

Another aspect of prevention includes provisions for disinfection. Providing proper system maintenance procedures, such as the injection of chlorine dioxide and monochloramine into the system for disinfection purposes, are considerations to address this. 

Chlorine dioxide is commonly used in hospitals and health-care facilities by direct injection into the hot water system (https://bit.ly/44mpxYA) to break down the biofilm inside the pipe that protects Legionella, releasing it into the system, where it is then eliminated at the heating source. A similar disinfectant is monochloramine, used as a secondary disinfectant after an initial chlorine dioxide treatment. Chloramines penetrate the cell wall of the bacteria and prevent further growth (https://bit.ly/45vKDEx).

However, the chlorine and chloramine variations may produce disinfection byproducts with the potential to corrode the piping. The corrosion causes copper to leach from the pipe into the water. Their presence also deteriorates any natural rubber products (https://bit.ly/45tVPle). While the disinfected water is still potable, studies have identified variable tastes and odors as a result (https://bit.ly/3KGbIwL).

Prevention by System Solutions 

Some system solutions can be added to the hot water systems to prevent Legionella growth, such as ozone, copper-silver ionization and ultraviolet. Ozone is a chemical compound made up of three oxygen atoms, formed when heat and sunlight cause chemical reactions between oxides of nitrogen and hydrocarbons (https://bit.ly/3YEkgdn). 

The ozone molecule is then directly injected into the water system. Once injected, the compound mixes with the water, eliminating Legionella by rupturing the cell walls to a point where the bacteria cannot develop immunity. As the cell walls rupture, the biofilm loosens and is flushed away by the flow (https://bit.ly/3qAJfSp). 

This solution is 50% more effective than using a chlorine disinfectant. Note that while this approach is very effective at killing Legionella, it can be highly toxic and requires extensive monitoring of the systems to ensure safe operation (https://bit.ly/3QKjs50).

Copper-silver ionization treatment uses electrolysis, where an electric current is applied across copper and silver electrodes. This reaction generates positively charged copper and silver ions, which then bond with negative sites on bacteria cell walls and neutralizes them (https://bit.ly/3YLq05m). This treatment is a long-lasting disinfectant and nontoxic and noncorrosive, with copper and silver levels well below the U.S. Environmental Protection Agency guidelines. 

However, elevated levels of copper ingestion can cause health risks, and long-term exposure to high levels of copper can cause liver and kidney damage. Elevated levels of silver may also cause skin discoloration. With proper system implementation, monitoring and design, these risks are minimal.

UV light is used by exposing the building water to specific wavelengths of light, similar to what is found in sunlight. With proper application, these wavelengths can break chemical bonds and kill microbes found in Legionella. This method does not necessarily eliminate the microorganisms but deactivates them and prevents further growth (https://bit.ly/45tXFm8). 

This solution may be used on a larger scale, such as at building service entries or as a local point of use. Note, however, that UV light does not provide any protection after the initial treatment. Any Legionella developing downstream of the UV light will either need another application or a different system solution to address it.

Note that the effectiveness of these treatments, solutions and processes can vary with the different types of buildings and applications. While each of these can be effective at preventing Legionella, they may also have negative effects if applied incorrectly. Understanding all aspects of each solution is critical to effectively protect domestic hot water systems from Legionella bacteria.

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 engineer at SmithGroup’s Phoenix office. He is a member of ASPE and has seven years of experience designing plumbing systems for various building types.