Joseph G. Allen, Harvard T.H. Chan School of Public Health
Simon Turner, LEED AP, WELL AP, Chief Executive Officer, Healthy Buildings
The causative organism Legionella pneumophila is named after a July 1976 outbreak of pneumonia in Philadelphia which affected 182 people, killing 29.1 Many were members of the American Legion attending a convention at The Bellevue-Stratford Hotel. The organism is commonly distributed in nature and, though positively identified and named only several months after the outbreak of the illness which gave the disease its name (Legionnaires’ disease),2 it has probably been causing infections in humans for hundreds of years – and continues to do so, though the source of an individual infection is commonly obscured.3
Legionella infection is caused by the inhalation of water aerosols containing the bacteria. The numbers of organisms required to induce infection is not known, but varies according to age, general health, and other predisposing factors. The Centers for Disease Control and Prevention (CDC) reports 5,000 cases of Legionnaires’ disease annually in the United States,4 though there are possibly up to 18,000 cases,5 but it is also estimated that the vast majority of cases go unreported.6,7 Though it is possible that healthy people can contract this illness, people with compromised immune systems, respiratory illnesses, and cancer are much more likely to become infected. It is estimated that 10 percent of those infected die,5 but in individuals with compromised immune systems, the fatality rate is likely to be much higher.8
The Centers for Disease Control and Prevention (CDC) reports 5,000 cases of Legionnaires’ disease annually in the United States,4 though there are possibly up to 18,000 cases , but it is also estimated that the vast majority of cases go unreported.6,7
Legionella species occur naturally in soil, rivers, and lakes, and can successfully colonize manmade water handling and storage systems, which often provide ideal nutrition and temperature conditions. Legionella bacteria have an optimum growth temperature of between 95 and 115°F, an optimum pH of between 5.0 and 8.5, and often flourish inside scale and sediment9 where they are protected from hot water and chemical disinfectants. The potential for Legionella to become hazardous to large numbers of people is greatly enhanced by conventional water and air-conditioning engineering methods used in recirculating cooling towers, air-conditioning chill coils and humidifiers, water storage and distribution systems, and aquatic systems (such as whirlpool spa baths); however, the potential for buildings’ domestic hot and cold water systems should not be under estimated. While many of the well-publicized mass cases of Legionnaires’ disease have had their origins in cooling towers, it is thought that the vast majority of sporadic cases are coming from domestic building water systems,10 especially where a spray may be generated from items such as showers, spray faucets, sidewalk or salad bar misters, or ornamental fountains. The bacteria like to lodge where biofilm, scale, and sediment accumulate.
Legionella bacteria have an optimum growth temperature of between 95 and 115°F, an optimum pH of between 5.0 and 8.5, and often flourish inside scale and sediment9 …
Good maintenance of cooling towers and plumbing systems is imperative to keep Legionella at bay, but no water treatment and maintenance system is guaranteed to fully and permanently eradicate the organism. A large water service industry has developed to maintain cooling towers; generally, less attention is applied to domestic plumbing systems. An authoritative reference for risk management is ASHRAE’s Standard 188; Risk Management for Buildings Water Systems which can be applied to most types of buildings.11 Compliance with this standard will also limit legal exposure for building operators. Within this Standard, for applicable water systems, there should be a risk assessment, followed by the development of standard operating procedures and a written maintenance plan. In many parts of the world, risk assessment and monitoring of commercial building water systems are more highly regulated and intensive than they are in the United States, though this may change as this ASHRAE Standard becomes adopted into codes.For cooling towers, the Association of Water Technologies12 recommends the following practices for prevention:
An authoritative reference for risk management is ASHRAE’s Standard 188; Risk Management for Buildings Water Systems which can be applied to most types of buildings.
For domestic plumbing systems, the design should be reviewed; dead legs, storage tanks, and low-flow conditions should be identified. Any water systems set up to allow for low flow or stagnation should be amended to allow for recirculation. The systems should be specified to store the hot water at 140°F and deliver it at a minimum of 122°F at the point of use. This should be achievable within 1 minute of running the outlet. Clearly, this will require effective circulation of the system and good insulation of the hot water supply lines. There is always the concern about scalding – scald protection equipment (such as thermostatic mixing valves) should be specified. Organics leaching from rubber or silicone gaskets are favorite sources for nutrients for Legionella, and designs of hot water systems should eliminate these components where possible. Water pumps in high-rise buildings are often set up with rooftop-mounted header/expansion tanks. The tank can “cook” in the sun and be an obvious source of Legionella, “seeding” building water systems with bacteria. To prevent stagnation, water pumps should run continuously and not be included in energy-conservation programs. There are pros and cons of using point-of-use water heaters – they can minimize stagnation in rarely used lines, but they can also be a frequent source of leaks and floods. Pointof-use heaters with small holding tanks are a common breeding ground for Legionella. Water system temperatures should be regularly assessed. Hot water tanks should be regularly flushed and cleaned with a suitable disinfectant, and should be fully flushed out before being placed back in service.
A widely used metric for the past 30 years that determines risk based on the number of samples testing positive has been debunked and should not be used.12 The screening procedure called for a 30% positivity of Legionella in domestic water systems, which opened the door to both false positives and false negatives. The key to Legionella control is a coordinated risk assessment, identification of assets where control is required, and a systematic maintenance and operations structure appropriate for the specific asset, supported by water sampling, where appropriate.