Mold and Legionella Prevention in Unoccupied Buildings

Michael Tobias
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    The media is currently focused on the new coronavirus (COVID-19), but there are other biological threats that can affect buildings. Mold grows in humid environments, and its spores cause allergic reactions and asthma flare-ups. Many construction materials can also be damaged by the fungal growth itself. On the other hand, Legionella bacteria thrives in stagnant and warm water, and it can cause severe pneumonia with a higher fatality rate than COVID-19.

    Mold and Legionella are not normally a problem in buildings where the HVAC and plumbing systems get proper maintenance. However, the coronavirus outbreak has forced many companies to shut down or reduce their operations. Even if a company operates with remote collaboration and home office, many workplaces have been left empty. A careless shutdown of buildings can create ideal conditions for mold and Legionella, and this becomes a serious issue when reopening.

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    To prevent mold, ASHRAE recommends a relative humidity below 60%. Legionella can be controlled by heating water to 140°F and keeping it in circulation at regular intervals. Also, Legionella and mold spores can both be inactivated with Ultraviolet Germicidal Irradiation (UVGI). This disinfection method is very useful in spots where humidity is unavoidable, such as the cooling coils of air handling units (AHU). Air filtering is also effective against mold spores and bacteria, and the filter should ideally have a MERV 13 rating or better.

    Mold Prevention in Empty Buildings


    Mold is a resilient organism, and it grows almost anywhere if relative humidity above 60% is allowed. Porous materials such as insulation and fiberglass are especially vulnerable, since mold can grow inside them. Complete cleaning is often impossible when this happens, and the affected material must be discarded and replaced. Furniture and carpets with accumulated humidity are also susceptible to mold growth.

    Maintenance personnel can normally deal with small patches of mold, and detailed guidance is available from the US Environmental Protection Agency. However, a major mold infestation requires a professional mold remediation company.

    • If allowed to happen inside your building, this can delay reopening after the coronavirus outbreak.
    • Consider that dirt and other substances can sometimes have an appearance similar to mold. When in doubt, you can contact a mold testing company to analyze a sample.

    The best prevention measure against mold is keeping relative humidity below 60%, as recommended by ASHRAE. Eliminating mold directly is very difficult, and its spores can spread everywhere in building interiors. However, both mold patches and spores are killed when relative humidity is kept below the ASHRAE limit.

    Legionella Bacteria: Risks and Prevention


    Legionella bacteria can cause a serious form of pneumonia called Legionnaires’ disease. This disease is deadlier than COVID-19, having a fatality rate of 10%. Fortunately, Legionella cannot be transmitted from person to person. Unlike SARS-CoV-2, which is a new threat, this bacteria was first detected more than four decades ago in 1976.

    Warm and stagnant water creates an ideal breeding ground for Legionella. In unoccupied buildings, plumbing and HVAC systems must be configured to prevent conditions where Legionella can proliferate. The following are some spots that require special attention:

    • Condensation pans
    • Air handling units
    • Cooling towers
    • Any hot water piping, especially if it has dead legs

    Since Legionella is a living organism, it can be eliminated with UVGI just like viruses and fungi. Air filtering also helps control Legionella, since the bacteria can be spread by airborne water droplets. High temperature can also kill Legionella, and ASHRAE recommends heating water to 140°F as a prevention measure. When a plumbing installation is not being used, the water should be heated and circulated at regular intervals.


    Mold spores are potentially harmful for occupants, and Legionella bacteria are even more dangerous. Before reopening a building, a careful inspection is recommended to rule out both threats. Contractors and maintenance personnel must use personal protective equipment (PPE) when working with equipment or components that may contain mold, bacteria or viruses.

    Preventing mold and Legionella is easier than eliminating them once they have spread. HVAC and plumbing systems should not be shut down completely, or the building can become an ideal environment for these organisms. ASHRAE published extensive guidance on how to manage buildings during the coronavirus pandemic and after, and prevention measures for mold and Legionella are also covered in detail.

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    Tags : indoor air quality mold coronavirus UVGI air filter legionella

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