October 7, 2021 / Christopher Goulah

Legionella: Post-Shutdown, An Increasing Public Health Threat

Sponsored by EMSL Analytical Inc.

As the United States economy continues to reopen, building owners face a new challenge in the form of stagnation that occurred while properties were left uninhabited during the COVID-19 pandemic. Due to the speed with which the shutdowns occurred, many owners failed to employ preventive measures or provide instruction for their maintenance staff to perform regular flushing of potable water to curb the accumulation of dissolved heavy metals and bacterial colonization that occurs during periods of extended disuse. Consequently, building owners today are returning to find high levels of toxic lead and copper and potentially infectious bacteria systemically contaminating their building water systems.

While a multitude of measures have been put in place by municipal authorities to reduce contaminants below the threshold of risk to public health, allowable amounts of toxins can flow through. Under the right conditions, these small toxins can accumulate to hazardous levels. In the case of biological contaminants, this can result in their amplification and colonization within plumbing systems. One of these hazards is Legionella, which can potentially cause a severe illness known as Legionnaires' disease. Legionella transmission occurs when aerosolized droplets of water—or, less commonly, aspiration of drinking water—containing Legionella bacteria are inhaled and infect the lungs. These types of droplets are formed during everyday activities such as showering and handwashing, as well as during routine operation of hot tubs and decorative fountains.

Building-wide remediation of established bacteria is no small feat and typically requires multiple rounds of treatment to effectively reduce the bio-load to below detectable levels. Legionella can be particularly difficult to combat as they tend to be resilient against hyper-thermal flushing and commonly utilized biocides. Subverting these efforts are a global trend toward green building technology that promotes low or intermittent water flow at outlets in concert with lowering the temperature of hot water tanks. These practices provide discrete opportunities for micro-stagnation. In addition, the continued promotion of remote work is leaving high-capacity buildings under-occupied and their water systems under-utilized, allowing the bacterial colonization to return post-remediation.

A reasonable solution to these issues is the development and implementation of a water management plan (WMP) that incorporates these new variables of accommodating inconsistent building occupancy while maintaining compliance with the latest requirements from company management and recommendations from local health departments. CDC publishes a regularly updated toolkit titled “Developing a Water Management Program to Reduce Legionella Growth and Spread in Buildings” (PDF) that provides a complete overview of what a WMP should contain. Key elements include creating a water management team to oversee all aspects of the program; producing a flow diagram of the building water system that allows identification of areas where Legionella can grow and spread; establishing and monitoring control measures to maintain proper conditions; and setting action levels for when control limits are not met. Essential to the success of the WMP is validation of these measures, which is achieved through a surveillance program to ensure all components are performing as designed. This should include scheduled sampling events to test water quality conditions for parameters such as temperature, pH, and disinfection levels, as well as environmental testing for the presence of Legionella.

Related: Read “Performing Legionella Source Risk Assessments: A Hazard Analysis Tool for Data Interpretation” in the April 2019 Synergist.


AIHA: “Recovering from COVID-19 Building Closures” (PDF, 2020).

American Institute of Architects: “Re-occupancy Assessment Tool.”

CDC: “Reopening Buildings After Prolonged Shutdown or Reduced Operation.”

Christopher Goulah

Christopher Goulah has more than 20 years’ experience in the areas of microbiology, laboratory management, and infectious disease with a focus on human disease. He is currently the National Program Director of Legionella for EMSL Analytical Inc. in Buffalo, New York.


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