Mitigating Legionella bacteria exposure with evidence-based interventions

Q&A with Chuanwu Xi

Professor, Environmental Health Sciences and Global Public Health

November 28, 2023

Typically acquired by the inhalation of contaminated aerosols, Legionnaires' disease is a concerning waterborne disease and public health issue. The CDC estimates that there were nearly 10,000 cases of Legionnaires’ disease in the United States in 2018, but suggests that the disease is likely underdiagnosed.

While Legionella bacteria can be found in natural freshwater environments, outbreaks of Legionnaires’ disease are more often associated with large water systems in public buildings, cooling towers, and other places where water is stagnant or flows at a low rate.

Chuanwu Xi, professor of Environmental Health Sciences and Global Public Health at the University of Michigan School of Public Health, is an expert in environmental microbiology and health. Here, Xi discusses Legionella exposure, and what we have learned about prevention strategies to limit the spread.

Large and public buildings and spaces like hospitals, hotels, and cruise ships come to mind when you hear Legionella bacteria. What makes these locations a common source for outbreaks?

The first reason is the nature of their plumbing systems. To deliver water to numerous endpoints in these facilities, they need complex plumbing systems. The complexity of these systems, with many areas of stagnant water, can create ideal conditions for biofilm development and bacterial colonization.

The second reason is the warm water temperatures. Large buildings often have water heaters set to temperatures within a certain range, typically between 20°C and 45°C (68°F and 113°F), inadvertently fostering an environment conducive to bacterial growth.

As for facilities like hotels and cruise ships, they often have many showers, fountains, and hot tubs that can create aerosols. The inhalation of aerosols contaminated with Legionella can lead to infection. Additionally, these facilities often experience seasonality. During the “low season,” the dropped water usage and the infrequent maintenance would lead to water stagnation and bacterial growth.

In hospitals, there are many vulnerable patients with weakened immune systems. They are more likely to get infected and have severe symptoms.

According to research, what strategies are beneficial to improving water quality where Legionella bacteria may grow?

There is no known safe level of Legionella in building water systems. Frequent water flushing and disinfection can help to maintain good water quality. Regular monitoring of bacteria in plumbing systems is necessary, especially for facilities that go through long-term lockdowns or low occupancy, as shown through a recent study we conducted.

A water management plan should be established for any large facility. Building water managers should be aware of fluctuations in water quality and bacteria population before and after reopening, as it may take longer than expected for water physicochemical characteristics and bacteria population to return to normal. Managers should also ensure a disinfectant residual is detectable throughout the water system.

The CDC provides resources and guidance, helping managers identify potential risks of Legionella outbreaks. For example, you can use this worksheet to identify buildings at increased risk for Legionella growth and spread. 

Infection cases have been associated with very low levels of Legionella (<10 CFU/mL) in building water systems. According to CDC’s guidelines, water management teams should decide how and when to respond if these bacteria are found in their water systems. 

The National Academies of Sciences, Engineering, and Medicine also provides a comprehensive guide to Legionella management.

What else should people know about legionella bacteria?

There are several things you can do at home to reduce the risk of Legionella infection, including:

• Reduce dead legs and pipe elbows in your plumbing systems.
• If you are using water tanks or other water storage facilities, change the water regularly and avoid using stagnant water for drinking, recreation, or showering.
• Set your water heater to at least 60°C (140°F) to minimize the growth of Legionella.
• Conduct regular cleaning and maintenance for appliances producing aerosols, such as humidifiers, air conditioners, hot tubs, and decorative water features.
• When returning home from vacation, flush out your water system by running showers and faucets for several minutes.
• Monitor water quality seasonally, or monthly if possible. If you notice any changes in water quality, such as strange tastes or smells, or if the water color changes, take action by contacting a professional to check your system.

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