**1. Introduction**

*Legionella* is considered an opportunistic human pathogen and these bacteria have been found in up to 70% of building water systems [1]. In recent molecular studies, *Legionella pneumophila* was isolated from 38% of buildings sampled including 42% of residences and 35% of office buildings [2], and in 47% of all taps in a study of buildings and homes in the United States [3]. *Legionella* colonization of potable water systems can pose a public health risk, especially for immunocompromised individuals [1,4].

Several organizations including ASHRAE (formerly known as the American Society of Heating, Refrigerating and Air-Conditioning Engineers), the World Health Organization, the American Industrial Hygiene Association, and the Centers for Disease Control and Prevention recommend the creation of water managemen<sup>t</sup> programs aimed at preventing the growth and spread of *Legionella* and other waterborne pathogens [5–8].

Testing water for the presence of *Legionella* is the most direct means of determining whether the building water system is colonized by *Legionella* [9,10] and its usefulness has been discussed in multiple technical guidelines [7,11,12]. The correlation with disease risk has been well established in healthcare facilities [13–16], but risk has also been demonstrated in hotels and other commercial properties. Rather than recommend testing for the bacteria, some guidelines and standards have suggested that building design or physical and chemical properties of the water can be used as predictors of risk or to demonstrate that water managemen<sup>t</sup> programs have e ffectively controlled the growth and spread of *Legionella* [5,8].

For example, ASHRAE selected certain physical properties of buildings as requisite characteristics for requiring a water managemen<sup>t</sup> program [5]. This included building height (greater than 10 stories including below grade), which had previously been found to have an increased presence of *Legionella* in the buildings' water heaters [17,18]. The Centers for Disease Control and Prevention (CDC) and the Centers for Medicare and Medicaid Services (CMS) recommend that temperature, pH, and free chlorine be tested at numerous outlets when doing a *Legionella* risk assessment [8,19]. Others have suggested that the temperature of the hot water in the recirculation line of a building [20] or other water quality parameters could predict the presence or absence of *Legionella* at the distal outlets (faucets and showers) [20–23].

There is a problem with these recommendations. There is little data to support them. If such monitoring is to be performed and relied upon as part of risk assessments and water managemen<sup>t</sup> programs, the expectation is that this information will have some relationship to either the presence or absence of *Legionella*.

It is important that we better understand these assumptions. Therefore, we performed a large-scale field investigation to evaluate the presence of *Legionella* in premise plumbing systems in 28 buildings in New York City, San Francisco, and New Jersey. The objective of the study was to (1) evaluate the potential of using hot water return line *Legionella* positivity as an indicator of distal site *Legionella* colonization risk in these buildings and (2) evaluate the correlation between water quality parameters and the presence of *Legionella* in water systems.
