Legionellosis Outbreaks: Interpreting an Environmental Assessment

Legionellosis Outbreaks – Interpreting an Environmental Assessment

The following presentation explores legionellosis outbreaks in the USA and what to look for when conducting and interpreting an environmental assessment, including evaluation of water sources, methods of disinfection, premise plumbing, cooling towers, spas and ornamental water features. The video, the second in a series published by the US Centers for Disease Control and Prevention (CDC) gives an introduction to environmental health and engineering measures for legionellosis outbreak investigations in the USA.

Legionellosis Outbreaks & Environmental Assessment

This is the second in a series addressing environmental aspects of legionellosis outbreak investigations. During this video we will discuss how to conduct and interpret an environmental assessment.

An environmental assessment should be performed in order to:

  • Identify risk factors for Legionella growth and transmission.
  • Help determine whether interventions like shutting down potential sources are needed and…
  • Determine whether environmental sampling is needed and if so collect information needed to make a sampling plan.

Carrying Out an Environmental Assessment

When you perform an assessment remember Legionella grows best in water that is stagnant warm – between 77 and 108 degrees Fahrenheit, with organic matter and lacking residual disinfectant like chlorine.

CDC has developed a form that can help guide the environmental assessment. This video will not discuss every question on the form but contains useful tips about why it is important to gather certain information and how to interpret this information in the context of preventing legionellosis.

Each section of the form addresses different potential water processes.

Facility Characteristics

Let’s start with the facility characteristics section, which asks about the type and size of the facility as well as the number of people who could have been exposed. In this section you will also determine which aerosol generating devices are present, so you will know which appendixes to complete.

Some questions are included to assess the risk of stagnation that could result due to low occupancy or infrequent use of emergency water systems.

Collecting information about any previous outbreaks at the facility and interventions performed can provide clues to the current problem.

Since Legionella can persist in water systems for decades a facility with a history of outbreaks has a high chance of having another one.

Identifying Water Sources

It’s important to know the source of the water and how it is disinfected before it comes into the facility. Drinking water can come either from a public utility or from a private well. Private Wells are subject to different rules, and typically are not required to provide a residual disinfectant.

Methods of Water Disinfection

It’s important to know how the water is disinfected because you need to know what type of disinfectant residual to measure when you are performing the assessment. For example if the method of disinfection is chlorine, you should measure free chlorine. If monochloramine is used, you should measure total chlorine.

Keep in mind that treatment plants sometimes switch from one type of disinfectant to another based on seasonal changes or one time changes in treatment methods, and this can affect Legionella growth.  You can get this information directly from the water supplier.

It’s also important to know whether there have been any water disruptions because Legionella can be introduced during water disruptions and pressure drops in the premise plumbing system can release Legionella from the biofilm.

Premise Plumbing

The premise plumbing in a building includes all the pipes, devices like water heaters and fixtures like faucets that bring the water to the user.  The building owner or manager is responsible for maintaining water quality within the premise plumbing. It’s important to understand where and how water flows through the building, through hot water heaters, storage tanks and  secondary disinfectant devices to the points of use, because these processes all play a role in bacterial growth.

Having an understanding of the way the premise plumbing is configured and maintained will help you make decisions on where to take samples for Legionella testing, and give you an idea of the extent of the remediation that is needed if samples are positive. For example if you know that all the points of use are fed from the same water heater and there is Legionella in the water heater, you can assume bacteria are being distributed throughout the building.

Heating Water to High Temperatures to Control Legionella Growth

Heating water to high temperatures before distributing it to the points of use reduces the potential for Legionella growth. But water that is too hot will increase the risk of scalding users. There are regulations in place to prevent scalding that set the maximum allowable temperature at the tap, but it is often too low to prevent Legionella growth.

One solution is thermostatic mixing valves [TMVs] that blend hot and cold water to ensure constant, safe temperatures. This section asks about water parameters like temperature that may be routinely monitored by the facility. If the facility does not currently monitor these, they can’t be aware of any potential risk or do anything to modify that risk on a routine basis.

Measuring water parameters is an important step in the assessment because the results can reveal major red flags such as no residual disinfectant in the water, the ph is out of optimal range for the disinfectant used or the hot water temperature is low enough to allow Legionella growth, especially if there is no residual disinfectant. If the facility has a hot water re-circulating system it’s important to measure water quality near the hot water source and from the return. If you measure the parameters and perform sampling the same day you do not need to measure the parameters twice.

Hospitals & Healthcare Facilities

Now let’s take a quick look at the appendixes which are not used for every environmental assessment. Appendix A focuses on healthcare facilities which can range from small clinics and doctors’ offices to urgent care centers, hospitals rehabilitation facilities and assisted living homes

Outbreaks often occur in these settings because they house particularly susceptible populations. Healthcare facilities can be large, with premise plumbing systems complicated by construction and new additions.

This section gathers information about how sick and immunocompomised the patients are; however keep in mind that any hospitalised patient is at risk. It also asks whether there has been a previous healthcare associated case at the facility which could indicate an ongoing problem.

Legionella Control in Cooling Towers

An outside contractor frequently maintains cooling towers. You may need to contact the contractor directly to document system maintenance if facilities management does not have in-depth knowledge of these systems.  Every cooling tower is different, and you may need to consult with an engineer or the manufacturer to understand the chemicals used to maintain them. These chemicals are designed to protect the machinery of the tower- not necessarily to kill Legionella. Some red flags to look for include:

  • the location of the cooling tower near fresh air intakes or open windows, or outside areas where people congregate,
  • lack of biocide and
  • intermittent cooling demand or biocide delivery.


Continuous maintenance is the key to keeping spas Legionella free. Some routine maintenance actions may be dictated by state or local public health codes. It is possible for a spa to pass an annual inspection, and still contain Legionella due to lapses in maintenance. Some things to look out for include:

  • lack of a knowledgeable person who has the responsibility of maintaining the spa,
  • lack of maintenance records and broken equipment.

Drain and fill tubs should be assessed in the premise plumbing section if the epidemiology points to this as a possible source.

Ornamental Water Features

Appendix D looks at ornamental water features like decorative fountains, which do not have to be large to cause an outbreak. Red flags to watch out for include:

  • fountains located indoors,
  • lack of disinfectant,
  • lack of a maintenance protocol or
  • presence of an underwater heating source like lighting.

The water quality in large ornamental water features should be measured at multiple points due to variations in temperature and disinfectant residuals.

Construction & Engineering Activities

The important construction activities to pay attention to include:

  • Anything that causes the water to the building to be turned off, and then turned back on or
  • changes to the plumbing in the building or
  • anything that shakes the ground, like blasting or jack hammering. Note that in densely developed areas these activities may affect adjacent buildings.

These can be risk factors because they can cause water interruptions which can cause changes in the water pressure, releasing biofilm and introduce organic matter into the system. Altering the design of the plumbing system by adding pipes and fixtures can decrease the effectiveness of hot water heaters or disinfection systems.

Legionnaires’ Disease Outbreak Investigations in the USA

An environmental assessment is a necessary step for a legionellosis outbreak investigation. Careful completion of this form will help you recognise potential sources of transmission, identify red flags in operation and maintenance, determine whether sampling is needed and if so draft a sampling plan which is discussed in the next video in this series.

Interpreting the Environmental Assessment for Legionellosis Outbreaks – U.S Department of Health and Human Services; Centers for Disease, Control and Prevention

Legionella Control Experts

Water Treatment Services offer a comprehensive range of Legionella control and specialist environmental hygiene solutions including water treatment services for the control of Legionella bacteria, Legionella risk assessments, training courses and water testing for the control of Legionnaires’ disease, Legionella, Pseudomonas and other waterborne pathogens in the workplace.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained engineers, in-field site service specialists and technical experts we can offer professional, cost effective Legionella risk management and ACOP L8 compliance solutions across the whole of the UK, Ireland and internationally.

Contact us today to learn how our expert Legionella control and environmental risk management support solutions can help improve what you do… contact us →

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Further Reading…

More information about the CDC Legionella Environmental Assessment Form – Download … here →

Legionellosis Outbreaks: Interpreting an Environmental Assessment
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Legionellosis Outbreaks: Interpreting an Environmental Assessment
Legionellosis outbreaks and what to look for when conducting and interpreting an environmental assessment including evaluation of water sources, methods of disinfection, premise plumbing, cooling towers, spas and ornamental water features.
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Water Treatment Services
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