Legionella Educational Series: Understanding Building Risk Factors

Author: Stephen Betts

Legionella bacteria live in water and are found in natural aquatic environments.  However, the artificial environments within building systems can provide conditions that encourage the bacteria’s growth, such as ideal temperature, water stagnation, and presence of biofilm. Various water systems have been the source of legionellosis cases including potable water systems, cooling towers, spas, and other artificial systems. Understanding building risk factors is an important part of preventing Legionnaires’ disease.  To cause harm, the bacteria must be present and grow in water, the colonized water needs to be transmitted to a person, and that person must be susceptible to infection.

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Every facility is different in terms of water systems present, water temperature and quality, and ways occupants can be exposed to the bacteria.  Therefore, risk characterization is the first step towards developing a proactive approach to water safety.  While the only way to know if Legionella is present is to test for it, considering building risk factors is part of developing an effective risk management program.

Presence and Proliferation of Legionella in Building Water Systems

Building water systems provide conditions that can amplify Legionella bacteria, so it is important to consider how risk factors may apply to your facility.  According to the US Centers for Disease Control and Prevention (CDC), several factors can lead to Legionella growth within a building[1]. The following summarizes these key factors discussed by the CDC.

pH and Inadequate Disinfectant

Even if water entering a building includes a disinfectant such as chlorine, the water may still contain Legionella bacteria.  In addition to this, processes such as storage and heating reduce disinfectant levels, causing water to have minimal or no disinfectant, which may contribute to Legionella growth.  The pH of the water impacts disinfectant effectiveness, with highest effectiveness in a narrow range (~6.5 to 8.5).

Biofilm, Corrosion, Scale and Sediment

Microorganisms, including Legionella, colonize surfaces such as the interior of piping systems, resulting in a “biofilm”.  This film protects Legionella and provides food that supports bacterial growth.  Scale and corrosion can increase biofilm formation and protect microorganisms from disinfectants.

Water Pressure Changes

Changes in water pressure can dislodge biofilm, potentially colonizing the piping network downstream.

Water Stagnation

Stagnation of water or areas with low flow can promote biofilm growth, reduce water temperatures, and reduce the level of disinfectant in the water.  This can occur in piping or storage tanks that are infrequently used, within dead legs, or at irregularly used fixtures.

Water Temperature Fluctuations

Legionella bacteria have been shown to grow best at temperatures between 25°C and 42°C, although growth can occur outside of this range.  These temperatures are commonly found in building water systems due to settings on hot water heaters and mixing valves, heat loss in distribution piping, stagnation, and other factors.  ASHRAE Guideline 12[2] recommends that where practical, hot water should be stored above 60°C with a return temperature of at least 51°C.


Potable water systems, cooling towers, decorative water features, whirlpools, and other systems generate water in an aerosolized form that may contain Legionella bacteria.  Transmission to people occurs by inhalation of this airborne water.  In addition, building occupants may aspirate water directly into the lungs, a process often referred to as water “going down the wrong pipe”.  Identifying these pathways of potential exposure is an important part of characterizing risk in a building.


Anyone can be affected by Legionella bacteria, but certain people are known to be at higher risk.  According to the ASHRAE 188 Standard, this includes “the elderly, dialysis patients, persons who smoke, and persons with medical conditions that weaken the immune system”[3].  When characterizing risk in a building, it is important to consider which building occupants are served by which water systems, and how people might be exposed.

Risk Factor Examples

Using a couple of examples, let’s consider how risk factors could apply to the design and operation of a building.

Potable hot water systems are a source of aerosolized water, in particular from faucets and showers.  There may be areas of stagnation and low flow.  The ability to achieve ASHRAE best practices with respect to temperature control – storage above 60°C with a return temperature of at least 51°C – will depend on various characteristics of your facility, such as building codes, hot water system configuration, use of mixing valves, and protection from scald risk.  Finally, we should think about susceptibility.  Where are the most at-risk patients, and how are they exposed?
Cooling towers provide another example.  Appropriate water treatment is crucial – these systems should have a water treatment program designed to control microorganisms – but other factors can come into play as well.  For example, does operation of the building lead to periods of stagnant water, such as during shut-downs?  Is there a source of nutrients nearby that might promote bacterial growth?  Have there been recent construction or water pressure changes?  These factors can influence the potential for Legionella to amplify and be transmitted to susceptible building occupants.


Carefully considering your building’s risk factors is the first step towards managing water safety.  In fact, this is now an industry best practice.  A hazard assessment is part of compliance with the ASHRAE 188 Standard, released in June 2015.  This standard requires certain facilities (including healthcare facilities) to have a risk management program, also called a water management program or Water Safety Plan, for potable and specific non-potable water systems. There is no one-size-fits-all solution with respect to managing risk in building water systems.  The water management program should be informed by an assessment of risk, considering building risk factors and Legionella sampling as appropriate.  It becomes a road map for decision making and implementation of further risk reduction measures if required.  Some of these measures may include secondary disinfection (e.g., monochloramine, copper-silver, chlorine dioxide), temperature changes, or point of use filtration.  However, none of these measures should be implemented unless justified by risk characterization and alignment with the Water Safety Plan. Your water management vendor should be able to guide you through the risk assessment and Water Safety Plan development process. Interested in learning more about Water Safety?  Read Six Steps To Develop A Water Safety Plan And Comply With ASHRAE Standard 188-2015 – Legionella

Stephen Betts has degrees in Chemistry from Queen’s University and Planning from Dalhousie University.

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Legionella Educational Series: Understanding Building Risk Factors

Understanding building risk factors is an important part of preventing Legionnaires’ disease. Every facility is different in terms of water systems present, water temperature and quality, and ways occupants can be exposed to the bacteria. Therefore, risk characterization is the first step towards developing a proactive approach to water safety.

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Dealkalizer Technologies

Some important design considerations for the chloride cycle dealkalizer are:

  • Feed water must be softened
    • Calcium chloride can precipitate and foul the beads
  • Minimal impact on total dissolved solids
  • Potential small decrease in blowdown requirements
  • Relatively low capital cost, reasonably effective, simple to operate


Some important design considerations for the WAC dealkalizer are: 

  • Additional softening required. WAC can remove as much hardness as there is available alkalinity – any residual hardness needs to be removed before the boiler.
  • Efficiency reduction with increasing flow rate, decreasing kinetics.
  • Handling of acid
    • Sulfuric acid – heat of hydration is a concern (can’t have plastic tanks, plastic piping), higher concentrations are available (up to 93%), calcium sulfate precipitation can be a concern for water sources high in sulfate levels)
    • Hydrochloric acid – fumes, plastic can be used, calcium chloride precipitation is not a concern, lower concentrations available (up to 32%)
  • Higher capital cost, very effective, easy to operate, larger footprint

Ion Exchange Explained

A quick review of ion exchange is required to understand dealkalization and we’ll use the water softening process as an example, as most boiler operators are very familiar with this.  Water softeners use strong acid cation (SAC) resin for ion exchange.  SAC resin has an affinity for divalent ions (Calcium, Magnesium) meaning that the resin wants to grab a hold of these divalent ions as they’re passing through the bed and exchange them with the sodium ions. Once resin is saturated and there are no more available free resin beads for ion exchange, a brute force wash of the SAC bead with sodium chloride (salt) brine is required.

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How to Minimize Amine Requirements

Amines should be dosed at the minimum rate required to neutralize carbonic acid, and to maintain pH levels of 8.0 to 9.0 in condensate.

In situations where incoming alkalinity levels are elevated, the concentration of amine required to neutralize the resulting elevated CO2 levels may exceed OTLs or even PELs. A number of alternatives are available to decrease alkalinity levels from incoming water:
  • Reverse osmosis (RO) Weak-acid dealkalization (WAC)
  • Chloride-cycle dealkalization
  • Demineralization (Demin)
RO, WAC and Demin units remove alkalinity from incoming water sources, and are often implemented to reduce energy and/or water consumption in steam plants because they decrease the overall mineral concentration of dissolved solids from incoming water. However, the chloride-cycle dealkalizer is a standout choice if the goal is to simply reduce incoming alkalinity on a budget. It operates much like a softener unit, and can decrease alkalinity levels by up to 95%.

Chloride-Cycle Dealkalizer Operation

Chloride cycle dealkalizers use strong base anion (SBA) ion exchange resin to swap carbonate and bicarbonate ions for chloride ions.  The footprint is similar a sodium softener, and they also use salt as the primary regenerant.  A small amount of sodium hydroxide if also often used to increase the effective capacity per regeneration.

The reduction of alkalinity in the feedwater, reduces the formation of carbonic acid in condensate, thus reducing the required amount of amines to neutralize the carbonic acid to maintain pH levels of 8.0 to 9.0 in condensate.

Implementation of a chloride-cycle dealkalizer can reduce your amine requirement by up to 90%.


There are 2 important concentration guidelines:
  • Permissible Exposure Limits (PELs)
  • Odor Threshold Limits (OTL)
The following table describes the limits set by Occupational Safety & Health Administration (OSHA) and American Conference of Governmental Industrial Hygienists (ACGIH):

Exceeding PELs poses a health risk to occupants. These PELs should never be exceeded for any period of time. See this link for a related article from the Centers for Disease Control and Prevention (CDC).


It is best practice to also follow OTLs to minimize the likelihood of complaints from occupants, especially from those with sensitivities.

A More Detailed Look at the Components of Steam


Liquid water always contains some concentration of oxygen (O2). The solubility of oxygen is primarily determined by the temperature of the water. Higher temperatures reduce the solubility of oxygen in water (see graph).
Because oxygen is extremely corrosive in high temperature water, steam boiler treatment programs use chemical and/or mechanical means of eliminating dissolved oxygen in water. An effectively treated steam boiler, and the steam it produces, will have near-zero dissolved oxygen concentrations.

Carbon Dioxide

Carbon dioxide (CO2) is released by the heating of carbonate (CO32-) and bicarbonate (HCO3-) in boiler water. These ions are naturally present in water from lakes, rivers and underground wells, and their concentration determines the alkalinity of the water source. The amount of carbonate alkalinity entering the boiler is proportional to the volume of carbon dioxide gas that will be in the generated steam. Carbon dioxide eventually forms carbonic acid in condensate. Higher alkalinity values result in greater carbonic acid concentrations.

The Release of Carbon Dioxide

The above reactions describe the release of carbon dioxide gas from sodium bicarbonate (1) and sodium carbonate (2).

The heat energy in boiler water is sufficient for the first reaction to proceed to 100% completion.  The completion of the second reaction is dependent on increasing pressure and temperature.

Higher carbonate and bicarbonate levels in boiler feedwater will lead to proportionally higher concentrations of CO2 in steam.


The amine compounds used in boiler water treatment are selected based on their boiling point, and their distribution ratio. The distribution ratio is a measure of how far the amine will travel before condensing. An optimal blend of amines will protect the entire condensate piping network (near and far). Amines are considered volatile organic compounds, and their concentration must be monitored to prevent exposure to levels beyond permissible limits.

Lesson about Amines to Impress Your Water Treatment Professional

Amines are a functional group in organic chemistry, and are derivatives of ammonia. They are separated into three main groups, primary, secondary and tertiary amines. These groups are defined by the number of hydrogen atoms replaced by organic substituents.

The most commonly used amines for neutralizing carbonic acid in condensate are:
  • cyclohexylamine (CHA)
  • diethylaminoethanol (DEAE)
  • morpholine
These amines are selected for their availability, basicity (ability to neutralize acids), boiling points, and most importantly, distribution ratios.

Distribution ratios (DR) are a measure of the how far amines will travel with steam before condensing. A proper blend of amines will include low DRs to protect condensate piping closest to the boiler, and high DRs to protect piping in longer and more complex condensate networks. Below is a table with the properties of the amines discussed above.

Other Types of Humidification Systems

Pan Humidifiers:

Pan humidifiers are essentially small shallow basins filled with water. The basins are heated with electric elements or steam, with the intent of evaporating water.

Pan humidifiers are found in smaller HVAC systems, and are susceptible to biological and corrosion fouling.

Water Spray Humidifiers:

This design uses an array of nozzles to atomize liquid water directly into the air stream. The phase change from liquid to vapour causes a noticeable drop in air temperature.

This type of system is most susceptible to biological and corrosion fouling. Facilities with year-long continuous cooling loads requiring high RH are best suited for this technology.

Steam to Steam or Clean Steam Generators:

These systems are small steam boilers, specifically designed to produce steam from high purity water sources, such as demineralization, or reverse osmosis. The energy input comes from steam raised elsewhere in the facility by a traditional steam boiler.

This design is typically more costly, and adds complexity, but produces steam with no boiler water treatment compounds.

Clean steam generators can only produce steam at low pressures.  The packaged heat exchangers rely on the higher energy content of higher pressure steam.

Water purity is critical for clean steam generators.
  • Low hardness levels (>3ppm of calcium, magnesium, or iron) will lead to fouling of heat exchange surfaces.
  • Water with even moderate alkalinity levels will release CO2 gas which will corrode any condensate piping components.
  • Moderate levels of total dissolved solids (TDS) will lead to priming or carry over, which may damage the steam control valves and/or contaminate the steam.
Therefore, Reverse Osmosis (RO) systems are ideal for humidifier makeup.  These units are designed to remove nearly all of the minerals from incoming water sources, and produce water with TDS concentrations of 0-5 ppm.

Steam to steam generators do cycle up.  Despite high purity makeup, there are always some dissolved solids.  If the generators do not purge some volume of water regularly, the bulk water will concentrate beyond acceptable levels, causing water discolouration and may lead to fouling and/or corrosion to system components depending on materials of construction.

Effects of Humidification on Occupant Comfort and Building Materials

RH levels have a direct impact on the health of patrons in a facility.

When humidity is too low occupants will get dry skin, irritated sinus, throats and eyes.

When humidity is too high mold/mildew problems can occur in the building, thus increasing the risk of illness to occupants. These health impacts are of increased concern with health care facilities who treat immunocompromised patients.

RH levels also have an impact on building materials.

The amount of moisture the material can hold will determine the extent to which it shrinks and swells with fluctuations in humidity. The effect is especially pronounced in wood and drywall, where gaps and cracks will form over time.

Windows are also prone to condensation in cold climates because they generally have little insulation value. The likelihood of condensation on windows increases as the indoor relative humidity rises, and the outdoor temperature decreases.

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