The United States has one of the safest drinking water supplies in the world. However, the water we use for drinking, swimming and even cooling high-rise buildings still can have adverse health effects. The Centers for Disease Control and Prevention (CDC) reports that about 7.2 million Americans get sick every year from diseases spread through water.

Two of the most prevalent illnesses contracted through water-borne bacteria — Legionnaires’ Disease and Pontiac fever — are caused by Legionella bacteria. Legionnaires’ Disease is a serious type of pneumonia (lung infection), while Pontiac fever is a milder infection that usually gets better without medical care.

According to the CDC, the first identified cases of Pontiac fever occurred in 1968 among workers and visitors at the health department in Pontiac, Mich. (https://bit.ly/3u2qaa7). In 1976, 221 people who had attended an American Legion convention in Philadelphia contracted a type of pneumonia that eventually became known as Legionnaires’ Disease; 34 of the infected people died (https://nyti.ms/3ylotYa). 

Following these incidents, public health officials were able to show that Legionella bacteria causes both diseases.

Present in Most Water Sources

Legionella bacteria are present in most natural water sources, often in very low or undetectable concentrations. Eliminating the uncontrolled growth of Legionella in building water systems is the central mechanism for managing the risk of disease. However, complete elimination of the bacteria is difficult to achieve, if not impossible; it is always being reintroduced from the water system. 

Municipal water is typically dosed with a disinfectant, often chlorine. This is intended to establish and carry a persistent, protective disinfectant residual to reduce the presence of disease-causing organisms. However, treated drinking water is not required to be free of all microorganisms. Treated water flows through the water distribution system, including water mains, where it is re-subjected to biofilm formations and other microbial colonization. As a result, water entering buildings is a source for Legionella. 

Building water systems also have characteristics that can make them prone to biofilm formation, colonization and Legionella growth. Things such as water temperature, no remaining municipal residual disinfectant, water age, cross-connections to other untreated sources, materials, and accumulation of sediment can all contribute to its growth.

For a person to contract Legionnaires’ Disease, several events must occur:

• Entry: The pathogen must gain entry into the water system. This can be from low disinfectant residual levels or water supply disruptions. 

• Growth: Factors such as temperature, disinfectant residual, design, dirt/sediment, nutrients and microbial associations can contribute to the growth of bacteria. 

• Transmission: Legionella must be inhaled to infect a human host, and the disease doesn't spread from person to person. Instead, the bacteria is spread through mist inhaled from sources such as faucets, showerheads, cooling towers, fountains, spas and similar equipment.

• Exposure to a susceptible human host: Not all humans will contract Legionnaires’ Disease. Several factors make it more likely, however, for the bacteria to overcome a host’s defenses. Adults over the age of 50 and people with weak immune systems, chronic lung disease or heavy tobacco use are most at risk. Symptoms of those infected include cough, fever, chills, shortness of breath, muscle aches, headaches and diarrhea. 

Risk Factors for Buildings

While Legionella is present in most water sources, some buildings are more susceptible to outbreaks than others. Several publications provide information on reducing the risk of Legionella; one of the most referenced is ASHRAE Standard 188-2018, Legionellosis: Risk Management for Building Water Systems.

Written with codified language with the intent that regulating bodies will adopt as code, Standard 188 includes steps for design teams to take during the design process and steps for owners to take during operations and maintenance to minimize Legionella risk throughout the building’s life. 

Standard 188 may be applicable to most building types; however, it tends to focus on multiple housing units, health-care facilities or buildings with immunocompromised occupants. 

It also requires owners to establish a risk management plan. This includes a system analysis of hazardous conditions, identification of required control measures and control limits for hazardous conditions (e.g., maximum and minimum values), a system for monitoring the parameters and limits, corrective actions when parameters are outside appropriate limits, and confirmation that the program is being implemented.

ASHRAE 188 also provides a checklist to help determine if a building is at a higher risk for Legionella bacteria. Key building risk factors include: 

  

1. Hot water system design temperature. Building codes often require water temperatures at lavatories and showers to be 115 F or less, which is right in the Legionella prime growth range. Key temperature benchmarks to know:

• Below 68 F: Legionella survive but are dormant

• 68 F to 122 F: Legionella thrives in this prime growth range

• 131 F: Legionella bacteria die within five to six hours

• 140 F: Legionella bacteria die within 32 minutes

• 151 F: Legionella bacteria die within two minutes

• 158 F: True disinfection level — instantaneous kill of bacteria

Understanding these temperature ranges helps establish system designs. Many domestic hot water systems are designed to store and circulate at 140 F. Locating the mixing valves at the point of use instead of at the source minimizes the volume of the system in the prime growth range. 

2. Infrequently used pipes, also called dead legs, with low-flow or no-flow conditions. As the water ages, the disinfectant can lose its effectiveness, sediment can build up and the water can reach room temperature — all of which can promote Legionella growth. 

Designs should eliminate dead legs to reduce the age of the water. Strategies include properly sizing the pipe to increase water velocity, including purge or drain valves, and adding recirculation through piping, storage and expansion tanks.

3. Ornamental water features. These are often exposed for occupants to enjoy, but they easily operate at prime Legionella growth temperatures. Building owners should discuss the design and operation of the features with the manufacturer or designer to minimize the risk of growth and transmission of the bacteria. Appropriate design features may include chilling, filtering or chemically treating the water. 

4. Cooling towers. These evaporative systems are used to reject, to the atmosphere, the heat transferred to the circulating water from equipment used to cool a building. Growth and transmission of Legionella in evaporative heat rejection systems are linked to the quality of the supply water, the quality of circulating water, stagnate conditions during periods of intermittent operation (systems that are idle without draining), and periods when water treatment is stopped or absent. 

Legionella bacteria can grow on wet surfaces and in the basins of cooling towers, which generally operate in the prime growth range of 68 F to 100 F. The bacteria can become airborne by the fan circulating air through the tower. The risk can be minimized by ensuring:

• Water has little opportunity to remain stagnate in the basins;

• Basins have a positive slope to the outlet;

• Chemical treatment including disinfection as well as corrosion protection;

• Chemical treatment feed system is functioning properly;

• Regular testing of the water;

• Placement of towers away from air intakes and immunocompromised occupants.

Another ASHRAE publication, Guideline 12-2020, when used in conjunction with Standard 188-2018, can provide prescriptive guidance for operators of water management systems to control Legionellosis in building water systems.

Reducing Risk to Occupants

Maintaining appropriate water temperatures, limiting stagnate water, reducing bacteria food sources and ensuring circulation pumps are operational are all critical steps for reducing the risk of Legionella to occupants. Reducing water age also is important; seldom-used branches should be periodically flushed through end-of-line fixtures or through dump valves. (Dump valves may be a hose valve or manual valve that may be opened to discharge water.)

Water and fixtures should be tested regularly. If the tests reveal control limits are exceeded, owners should follow through with corrective actions as set forth in their established risk management plan. Corrective actions may include thermally shocking the entire system with hot water (e.g., 160 F), actively flushing the entire system, chemically cleaning fixtures or introducing supplemental disinfection to the system. 

A comprehensive approach to water safety — including the steps set forth in this article, from design through operational corrections — can minimize any building’s risk of Legionella bacteria and the potential contraction of Legionnaires’ Disease by occupants.

Dave Bodenschatz, PE, LEED AP, is the director of mechanical engineering for IMEG Corp.’s Technical Operations Team, which provides the firm’s designers with the latest industry knowledge, standards and tools.

Wade Ross, PE, is a project executive with IMEG Corp. He is a mechanical engineer with expertise in the design of infrastructure systems and master planning for health-care, government and industrial facilities, including central utility plants.