System-Specific Design, Operation, Maintenance, and Disinfection Guidelines

The following sections provide guidance on effective water system care, including design considerations, operation and maintenance schedules, and other controls to prevent Legionella growth in:

• Cooling towers, evaporative condensers and fluid coolers
• Domestic hot water systems
• Domestic cold water systems
• Heating, ventilation and air conditioning (HVAC) air distribution systems

Cooling towers, evaporative condensers, and fluid coolers use water evaporation, producing ideal conditions for Legionella growth and introduction of bacteria into the air (i.e., aerosolization), if not properly maintained. The water in these systems is likely to have ideal temperature ranges for Legionella growth: 20°-50°C (68°-122°F). The evaporative processes will then release the bacteria into the air, significantly increasing the likelihood for worker exposure. For example, these systems use fans to move air through a recirculated water system where considerable water vapor emission occurs, even when baffles, known as drift eliminators, are used to limit vapor by removing droplets from the air stream before exiting the tower. When these pieces of equipment spray water or generate aerosols, they can disperse Legionella over a wide area, including outside the workplace.

Design Considerations

To prevent Legionella growth and spreading, design water system options to:

• Equip water collection areas, known as "sumps," with drains (sometimes called a "bleed") and supply make-up water to reduce dissolved solids that facilitate Legionella growth.
• Keep sump water temperatures low. Sump water temperatures depend on tower design, heat load, flow rate, and ambient dry- and wet-bulb temperatures (which consider both temperature and humidity). Under ideal conditions, sump water temperatures in evaporative devices approach the ambient wet-bulb temperature (typically a lower temperature than the dry-bulb, or air, temperature because of evaporation of water from the wet bulb surrounding the thermometer used for this type of measurement). The wet bulb temperature is typically low enough to limit Legionella growth.
• Install high-efficiency drift eliminators. These are baffles that remove water droplets from steam and minimize water vapor release from cooling towers. Older systems can be retrofitted with newer, high-efficiency drift eliminators. A drift eliminator specifically designed and fitted for the system can greatly reduce water vapor releases and potential for Legionella exposure.
• Consider other design features that minimize water vapor releases. An enclosed system will prevent Legionella from becoming airborne if it grows in the system.
• Include design features that maximize internal component cleaning. For example, installing easy-access or easily disassembled components will simplify cleaning.

Preventing Legionella Growth in Cooling Systems: Cleaning

Preventing Legionella growth through cleaning will reduce the likelihood for exposure. Considerations for cleaning water systems in the workplace include:

• Cleaning and disinfecting cooling towers at least twice a year. Normally, this maintenance is performed before initial start-up when the cooling season begins and after shut-down in the fall. Systems with heavy biofouling or high Legionella levels in samples may require additional cleaning (see the Outbreak Response page for more information).
• Cleaning and disinfecting systems that are out of service for an extended period of time.
• Cleaning and disinfecting new systems. Construction material residue can contribute to Legionella growth in new systems.

Although cooling towers are among the water systems for which little data exist supporting biocide effectiveness, the types of biocides mentioned in the General Considerations section may help control Legionella growth in cooling towers.

• Chlorine above 0.5 parts per million (ppm) in cooling tower water systems may prevent bacterial growth⁴ if the pH is below 8.0. Usually, free residual chlorine levels are maintained below 1 ppm to prevent corrosion. Maintenance involves frequent monitoring to control the pH and chlorine levels and ensure the chlorine is not combining with organic substances in the water to form hazardous byproducts.
• Bromine is added as a bromide salt and generated by a reaction with chlorine. Bromine's effectiveness is less dependent on pH than that of chlorine. At alkaline pH values (above 7.0), often found in cooling towers and other water system components, bromine may have more bactericidal activity than chlorine.⁵ Bromine is also less corrosive and produces fewer toxic byproducts than chlorine does.
• Fentichlor, 2,2'-thiobis (4-chlorophenol), a non-oxidizing biocide, used weekly for four hours at 200 parts per million (ppm), or bromo-chloro-dimethyl-hydantoin (BCD) in a slow-release cartridge, at an initial 300 ppm concentration, are effective in controlling Legionella growth.⁶ Fentichlor is licensed by the Environmental Protection Agency (EPA) for water treatment in cooling towers, but United States suppliers may not exist.
• Chlorotriazine and sodium bromide salt mixture may be effective when alternated with BCD for controlling Legionella growth.

CDC also outlines procedures for cleaning cooling towers and related equipment with either of two chlorine compounds, sodium hypochlorite (NaOCl) or calcium hypochlorite, Ca(OCl)₂, calculated to achieve an initial free residual chlorine (FRC) concentration of 50 mg/L.

Additional disinfection protocols are available from the World Health Organization, Legionella and the Prevention of Legionellosis as well as the Cooling Technology Institute, Legionellosis, Guideline: Best Practices for Control of Legionella.^7,8

Additional Considerations

Some states have mandatory inspection, testing, cleaning, and disinfection requirements for cooling towers. Employers should be familiar with applicable laws and regulations in the states where their facilities are located.

Employers in the State of New York and New York City should also be aware of registration requirements that apply to cooling towers and certain other water system components.

Domestic Hot Water Systems

Domestic hot water systems are frequently identified as the source of Legionella during Legionellosis outbreaks. The term "domestic" applies to all non-process water used for lavatories, showers, and drinking fountains in commercial, industrial, and multi-family residential settings.

Water heaters maintained below 60°C (140°F) and that contain scale and sediment may foster Legionella growth. Large water heaters, such as those used in hospitals or industrial settings, frequently contain cool zones near the base where cold water enters. Scale and sediment accumulate in these cool zones where the temperature can provide ideal conditions for Legionella growth. Piping or plumbing altered or capped to prevent water flow (i.e., dead legs) and non recirculated plumbing lines may allow hot water to stagnate, conditions that also promote Legionella growth.

Domestic Hot Water System Design Considerations

When designing domestic hot water systems, consider using options that reduce the chance for Legionella growth. Examples include:

• Ensuring water systems recirculate water and minimizing, if not eliminating, dead legs to reduce stagnation.
• Opting for point-of-use water heaters to eliminate hot water stagnation in infrequently used lines.
• Insulating hot water lines to help maintain water distribution and delivery temperatures.
• Installing heat tracing capabilities in strategic lines to help maintain temperatures or at least 50°C (122°F) in the lines.

Domestic Hot Water System Cleaning Schedule/Controls

One way to control Legionella growth in domestic hot water systems is to raise the water heater temperature to at least 70°C (158°F) for 24 hours and then flush each outlet (i.e., places where water comes out of the system, such as faucets and showers) for 20 minutes. This process is not complete until all taps and other hot water outlets are flushed with the hot water because stagnant areas can introduce remaining bacteria back into the system. Exercise caution, to include warning building occupants or conducting the cleaning when the facility is not occupied, to avoid serious burns from the high water temperatures used in this process.

Another standard Legionella growth control method is to periodically chlorinate the system at the tank. Achieve 10 ppm free residual chlorine, and then flush all taps until a distinct chlorine odor is evident. In-line chlorinators are sometimes installed in the hot water line; however, chlorine is quite corrosive and will shorten the metal plumbing’s service life. To ensure residual chlorine biocidal effectiveness in the system, pH control is extremely important.

Alternative methods to control Legionella growth in domestic hot water systems include using metal ions. Some metal ions (e.g., copper, silver) have a biocidal effect in solution. Ozonization injects ozone into the water and kills microorganisms.

As mentioned in the General Considerations section, ozonization and ultraviolet (UV) radiation are alternative options for controlling Legionella. Commercial, in-line UV systems are effective when installed on incoming water lines or on recirculating systems, but stagnant zones may diminish water treatment effectiveness. Because scale buildup on the UV lamp surface can rapidly reduce light intensity, frequent maintenance is necessary to ensure effective water treatment.

Other ways to minimize Legionella growth in domestic hot water systems include:

• Storing domestic hot water at a minimum of 60°C (140°F) and delivering hot water at a minimum of 50°C (122°F) to all outlets.
• Draining hot water tanks periodically to remove scale and sediment, and cleaning the tanks with chlorine solution. Rinse the tank thoroughly to remove excess chlorine before reuse.
• Eliminating or, at a minimum, periodically removing and cleaning rubber and silicone gaskets.
• Frequently flushing lines—especially dead legs—since they provide nutrients for Legionella bacteria.
• Removing or frequently cleaning fixtures such as aerators and shower heads.
• Running domestic hot water recirculation pumps continuously to avoid stagnation. This may include excluding these pumps from energy conservation plans.

Controls limited to raising the water heater temperature without evaluating the system for stagnation areas, heat loss and gain, cross-contamination, and other factors contributing to Legionella growth are generally not sufficient.

Domestic Cold Water Systems

Water stored below 20°C (68°F) is generally not an environment in which Legionella will grow. However, ice machine water lines have been shown to have high bacteria levels.⁹ Heat from the condenser coil is likely the reason for this bacterial growth. Legionnaires’ disease has also been linked to grocery store ultrasonic produce misters and dental water lines, which are now recognized as water sources commonly contaminated with high microbial concentrations, including Legionella.^10,11

Design Considerations

Design cold water tanks to reduce storage time to a day or less.

Eliminate water tanks that allow water to remain stagnate for long periods of time.

Cover water tanks to prevent contamination.

Protect water tanks from temperature extremes and insulate cold water lines that are close to hot water lines to prevent water from reaching a temperature favorable for Legionellagrowth.

Protect all connections to processed water with a plumbing code-approved device (e.g., backflow preventer, air gap) to reduce the risk for cross-contamination between the domestic cold water system and other systems.

Cleaning Schedule/Controls

Hyperchlorination can eradicate Legionella in cold water lines with significant contamination using the following procedure:

• Add chlorine to the cold water system, maintaining free residual chlorine levels between 20 to 50 ppm;
• Continue water treatment for one hour at 50 ppm or two hours at 20 ppm;
• Run faucets until the chlorine/chloramine odor is present; and
• Allow the water to remain in the lines for approximately two hours before flushing.

Well-maintained and properly operated HVAC systems are not normally favorable to Legionella growth. They are unlikely to have the water necessary for bacterial growth. It is more likely that an air handling system would transfer or disseminate Legionella-contaminated aerosolized water from other sources within and outside the facility into the workplace environment. The common examples of these sources discussed in this section represent different paths for potential air handling system contamination.

Potential HVAC Contamination Sources within a Facility

Leakage from Pipes

Contaminated water leaking from pipes into air ducts can become aerosolized and spread by the air handling system. When evaluating the source for Legionella in these cases, consider pipes in domestic water systems, fire sprinklers, and refrigeration condensers as potential contaminated water sources.

Improperly Maintained Humidifiers

Improperly maintained HVAC humidifiers can lead to Legionella growth and dissemination. Commonly used humidifier types in commercial facilities, industrial settings, and multi family residential buildings include:

• Heated pan humidifiers use a heat source to evaporate water from a pan into the air. Intermittently using the device coupled with allowing warm water to stagnate in pans may support Legionella growth.
• Direct steam-type humidifiers inject boiler-generated steam directly into the air stream. These systems normally operate above 70°C (158°F), inhibiting Legionella growth. Avoid using raw steam from the central heating boiler due to contamination with corrosion inhibitors and anti-scaling chemicals.
• Atomizing humidifiers use mechanical devices or pneumatic air to create a water mist that evaporates into the air stream. Use contaminant-free water in atomizing humidifiers since their internal piping or tubing can encourage biofilm formation.

In general, steam or atomizing humidifiers are less likely to support Legionella growth than humidifiers using recirculated water.

Although personal or residential-style humidifiers used in a commercial or industrial facility are not part of its HVAC system, they may still be sources of Legionella that can spread in the work environment. These types of small, free-standing, portable units with refillable water tanks use an internal fan and wet media to disseminate a wet air stream. The sumps are frequently contaminated with Legionella. Daily cleaning is necessary to maintain acceptable water quality. Because these units seldom receive appropriate maintenance, avoid using them in commercial or industrial workplaces.

Intermittently Using Evaporators as Humidifiers

Direct evaporative air coolers are often used as humidifiers. These devices mix water and air to create a cool, wet air stream by evaporation. When used intermittently as humidifiers, direct evaporative air coolers can collect stagnant water in the sumps and promote Legionella growth. Cleaning systems prior to using direct evaporative air coolers as humidifiers prevents stagnant water accumulation in the sumps and Legionella growth.

Using Warm, Stagnant Sump Water

Many air handling systems designed for dryer climates use direct evaporative air-cooling. Wet evaporative coolers, slinger air coolers, and rotary air coolers are common in commercial applications. When these systems use 100-percent outside air in a dry climate, the water sump temperatures are generally low and do not represent a significant Legionella hazard. Properly operating and maintained systems ensure sump water temperatures remain cool and the water does not become stagnant.

Leakage from Coils

Systems designed for dryer climates use indirect evaporative air-cooling. One common design circulates cool water from a cooling tower sump through a water coil in the supply air stream. When a coil develops a leak, the pumped cooling tower water is injected directly into the supply air stream with potentially harmful effects if the sump water is contaminated with Legionella. Frequent and proper maintenance checks help to ensure that coil leaks are found and repaired.

When drained properly, water captured in the condensate pans located below cooling coils is normally not a Legionella source because the water temperature is so low.

Heat Exchanger Leaks

Indirect evaporative air-cooling is also found in air-to-air heat exchangers. The heat exchanger on one side is an evaporative-cooled wet air stream. The other side supplies air for the conditioned space. If the heat exchanger leaks, the wet air stream can mix with supply air and cause problems if the wet air stream contains Legionella. The potential for heat exchanger failure requires the design to account for the wet systems to mix with the air distribution systems.

Air Conditioning Units in Infrequently Used Rooms

Air conditioners in rooms with limited use such as computer rooms are also considered possible Legionella sources. Computer room air conditioners typically include humidifiers and may contain a sump with contaminated water. Frequent maintenance and cleaning are critical to ensure the sump remains free from contaminated water.

Potential HVAC Contamination Sources outside of a Facility

Sources external to the building or air handling system may emit contaminated aerosolized water. This aerosolized water can then be drawn into an air handling system as described in this section.

Fresh Air Intakes

Water vapor or mist discharged from cooling towers, evaporative condensers, and fluid coolers can enter HVAC fresh air intakes depending on factors such as prevailing wind direction and velocity, building effects, architectural screen walls, and distance from the water vapor discharge point to the fresh air intake. For example, low pressure zones on leeward building sides (i.e., the side sheltered from the wind) and on the roof could result in Legionella contamination in HVAC systems.

When designing air handling systems, locate HVAC fresh air intakes where they will not draw any water vapor or mist from a cooling tower, evaporative condenser, or fluid cooler into the HVAC system.

Fresh Air Intake Areaways

Fresh air intake areaways, typically concrete plenums (open areas through which air moves) located at grade level, supply fresh air to air handlers in the basement or lower building levels. These fresh air intake areaways can become contaminated as they collect organic material (e.g., leaves, dirt) and stagnant water from rain or irrigation—conditions that may facilitate Legionella growth. The routine maintenance and cleaning protocol for air handling systems should include checking these areaways to remove standing water and organic debris as necessary.

Direct Paths

Contaminated air from a cooling tower, evaporative condenser, or fluid cooler can easily enter a facility directly through open windows or doors and contaminate HVAC systems. Although this route of contamination may be more difficult to control systematically, employers should be aware of this possibility as they conduct routine inspections and maintenance of the HVAC equipment, as well as when designing new facilities or HVAC system modifications.

HVAC Maintenance, Cleaning, and Other Considerations

Operating all HVAC systems as originally designed and maintaining them to perform as designed will reduce the risk for Legionella contamination. This includes testing all HVAC equipment periodically to ensure performance as designed. Maintenance failures can produce contaminated, stagnant water, potentially leading to an ideal environment for Legionella growth, especially if heated (e.g., by sunlight).

The following steps are necessary to minimize Legionella contamination in HVAC systems and resulting worker exposure to Legionella:

• Minimize water reservoirs, sumps, and pans. Chemically untreated, stagnant, warm water sources provide an ideal environment for Legionella growth.
• Ensure there is a way to drain water sumps when not in use (e.g., an electric solenoid valve on the sump drain). When an HVAC sump is used during the hours a building is occupied, drain the sump during unoccupied hours.
• Ensure there is a bleed for water sumps so that dissolved solids do not form sediments.
• Drain and bleed inactive sumps to prevent sediment accumulation.
• Slope drain lines and drain sumps away from the bottom of the water collection pans so that all water can drain and allow the pan to dry.
• Follow a maintenance schedule and/or checklist detailing the procedures to use to ensure HVAC systems have been fully inspected and properly maintained.

For More Information

The CDC Developing a Water Management Program to Reduce Legionella Growth and Spread in Buildings: A Practical Guide to Implementing Industry Standards toolkit can help employers develop and implement a water management program to reduce the risk for growing and spreading Legionella in buildings. The toolkit also provides direction on assessing and strengthening existing water management programs.

The American Industrial Hygiene Association offers guidelines in its Recognition, Evaluation, and Control of Legionella in Building Water Systems publication.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Guideline 12-2000, Minimizing the Risk of Legionellosis Associated with Building Water Systems 2000, provides information on the Legionella ecology and guidance to minimize and remediate colonization in building water systems. Also see ASHRAE Standard 188: Legionellosis: Risk Management for Building Water Systems that establishes minimum risk management requirements for building water systems to prevent Legionellosis.