[Updated July 7, 2020] After Legionella was discovered following the 1976 outbreak of what came to be known as Legionnaires’ disease, tremendous amounts of research ensued to find out why water systems promote growth of Legionella bacteria and how the bacteria are transmitted to people’s lungs. By the 1990s, well over 100 scientific papers had been released, as well as publications (OSHA 1996; Freije 1996) that condensed scientific findings into action steps building operators could implement to reduce risk.


Agreement in Guidelines about the Need for Legionella Prevention in Buildings

In the late 1990s and into the new millennium, many government and industry groups released Legionella position statements and guidance documents. Although the documents varied in details about Legionella preventive measures, they all agreed that waterborne Legionella bacteria can cause Legionnaires’ disease and managing building water systems to minimize Legionella is the best strategy for reducing the risk:

  • EPA 1991: “It is suggested that hospitals, and other institutions with potential for the growth of Legionella, conduct routine monitoring of their hot water systems at least quarterly. …If the presence of Legionella is confirmed, then remedial measures should be taken.”
  • OSHA 1996: “The likelihood of contracting Legionnaires’ disease is related to the level of contamination in the water source. …An acceptable control strategy is to minimize the number of organisms present in a water source.”
  • ASHRAE 1998: “Design and operation and maintenance procedures that prevent amplification and dissemination of Legionella should be formulated and implemented before systems are operated and continued rigidly thereafter.”
  • EPA 1999 and 2001: “Because there is little if any person-to-person transmission of Legionella and no vaccine is available to prevent infection, risk minimization efforts are focused on breaking the chain of transmission between environmental sources of Legionella and human hosts. …For hospitals and other health care settings, regular environmental surveys of both hot water systems and distal sites should be conducted; some health departments have issued mandates for such testing (Allegheny County Health Department 1997).”
  • ASHRAE 2000: “The purpose of this guideline is to provide information and guidance in order to minimize Legionella contamination in building water systems. This guideline provides specific environmental and operation guidelines that will contribute to the safe operation of building water systems to minimize the risk of occurrence of legionellosis.”
  • ASTM 1996, 2003, and 2008: “Concerned employers, building owners and operators, facility managers, and others seek to prevent real and potential health hazards, if possible. Water system operators may identify undesirable situations by monitoring routinely for legionellae and may be able to implement control measures before the bacteria reach amounts sufficient to cause human illness.”
  • EWGLI 2003: “The risk of Legionnaires’ disease can be avoided. Any hotel that does not have an active program to control the growth of legionellae is negligent in ensuring the safety of their guests.”
  • EWGLI 2005: “It must be emphasized that, for the effective prevention of Legionnaires’ disease, risk assessments and control measures must be implemented proactively and not merely in response to a case or cluster of cases of Legionnaires’ disease.”
  • WHO 2007: “Fundamentally, the responsibility for managing the risk of legionellosis belongs to the facility owner or manager.”


Industry Articles About Legionella Prevention

The guidelines were no secret. Magazines written for facility managers and engineers carried articles about Legionnaires’ prevention, some of which specifically noted the availability of Legionella guidelines:

  • Springston 2000: “Fortunately, there are methods readily available for both the detection of Legionella and its control. Establishing a proactive approach of periodic testing, along with proper water treatment in both cooling towers and hot water systems, is the best way to avoid disease outbreaks and potential lawsuits.”
  • Hodgson 2002: “It is possible to operate and maintain a potable water system such that the risk of legionellosis from the system is minimized. This requires a fundamental understanding of system design and operation, careful management attention, and record-keeping.”
  • Cooper 2004: “Fortunately, exposure to Legionella is preventable, but only if health-related risks are assessed and preventive practices are implemented. …This task, however, is incumbent on facility owners or managers and must be performed to successfully manage the risk and liability associated with waterborne pathogens. …This concern has led various industrial associations and government bodies to formulate Legionella control guidelines.”
  • Freije 2006: “Legionnaires’ is contracted by exposure to water contaminated with Legionella It can be prevented at a reasonable cost by minimizing Legionella bacteria in plumbing systems, cooling towers, whirlpool spas, decorative fountains, misters, and other water systems to which hotel guests might be exposed.”
  • McCoy 2006: “Almost all legionellosis is the result of exposure to inadequately maintained building water. Fundamentally, it results from failure to properly manage building water systems. … A hazard analysis and control plan (either WSP or HACCP) should be part of the water management plan for every facility…a duty exists to provide hazard free water.”
  • Turner 2008: “The potential for your building’s hot and cold domestic water systems to harbor Legionella should not be underestimated. …For every water system, there should be a risk assessment, followed by the development of standard operating procedures and a written maintenance plan.”


The CDC’s Support for a Proactive Approach to Legionella Prevention

Although the CDC had years ago been known for advocating a reactive approach to Legionnaires’ disease (treating or testing water only after disease has been identified), CDC officials clarified as early as 2002 that the agency recommends managing building water systems to minimize Legionella bacteria and even emphasized that compliance with ASHRAE Guideline 12 would reduce risk significantly:

  • “While opinions differ as to the role of routine periodic environmental monitoring, there is agreement that all health care facilities should have a control strategy in place” (Fields 2002).
  • “CDC recommends aggressive disinfection measures for cleaning and maintaining devices known to transmit legionellae” (CDC 2003).
  • “CDC recommends a strategy based on proper maintenance of water systems” (Besser 2002).
  • “Adoption of the ASHRAE guidelines could dramatically reduce the likelihood that legionellae will be amplified in a water system, thereby diminishing the risk of transmission” (Fields 2002).


Scientific Evidence that Legionella Control Measures Prevent Legionnaires’ Disease

Backing up the warnings about the need to manage building water systems to minimize Legionella bacteria was scientific evidence that doing so really works, preventing Legionnaires’ disease:

  • Scientific literature is replete with reports of reasonable control measures and disinfection technologies that successfully reduced Legionella in water systems and subsequently prevented new cases of Legionnaires’ disease associated with those systems (O’Loughlin 2007; Sidari 2004; Shelton 1993; Shelton 1994; Stout 2003; Zhang 2009).
  • Data gathered by the European Working Group for Legionella Infections (EWGLI) showed that “since the introduction of the European Guidelines for Control and Prevention of Travel Associated Legionnaires’ Disease in July 2002, the number of hotel cases associated with each identified cluster has reduced, indicating that control measures have been effective in preventing further cases” (WHO 2007).
  • In surveys of spas, Legionella was found only when the disinfectant levels were not adequately maintained (ASHRAE 2000).
  • The Health Protection Agency in London reported in 2006 that every spa it had investigated for Legionnaires’ disease was not maintained according to its guidelines (HPA 2006).


Unprecedented Agreement About the Approach to Legionella Prevention

The June 2015 release of the final version of ASHRAE Standard 188 was a giant step for Legionella prevention in the United States in part because it represented agreement among government agencies and industry groups—not only about the need for Legionella prevention in building water systems—but also about the approach to it.

To comply with ASHRAE 188, building operators must implement a water management program (WMP) for water systems that can harbor and transmit Legionella bacteria. The essential components ASHRAE 188 requires for a WMP are almost identical to those the World Health Organization recommended in 2007 (WHO 2007) and the United States Veterans Health Administration (VHA) has required since 2014 (VHA 2014).

ASHRAE 188 received more attention and support than any government or industry Legionella document previously released in the United States:

  • Just a few weeks after its June 2015 release, because of the tragic Legionnaires’ outbreak that sickened more than 120 people and caused 12 deaths, New York City and State adopted emergency ASHRAE 188-related regulations for minimizing Legionella risk associated with cooling towers. In 2016, the regulations were updated and made permanent.
  • In June 2016, the CDC issued three publications emphasizing the need to comply with ASHRAE 188:

The CDC report in Morbidity and Mortality Weekly Report (MMWR) about a rise in Legionnaires’ disease cases and the need for better building water management caught the attention of the mainstream media, which immediately published stories highlighting the responsibility of building owners and managers.

The CDC provided “Developing a Water Management Program to Reduce Legionella Growth & Spread in Buildings: A Practical Guide to Implementing Industry Standards” to help facility operators “develop and implement a water management program to reduce your building’s risk for growing and spreading Legionella.” Referencing ASHRAE Standard 188, the CDC emphasized “Legionella water management programs are now an industry standard for large buildings in the United States.”

In the Vital Signs article “Legionnaires’ Disease: Use water management programs in buildings to help prevent outbreaks,” the CDC told building owners and managers to “develop and use a Legionella water management program” and state and local officials to “consider changing building and public health codes to include Legionella water management programs.”

  • In July 2016, law.com issued a report concluding, “Compliance with the new ASHRAE standard likely will be viewed by courts as the standard of care in personal injury lawsuits involving exposure to Legionella. …Adopting industry standard practices and complying with applicable law is the best defense. Conversely, failure to follow such standards and legal requirements could expose building owners and operators to potentially significant liability.”


What Else is Needed?

Although agreement among government agencies, experts, and professional societies about the need and approach to Legionella prevention in the United States is a huge step, it is not enough.

Talking about Legionella control measures, writing about them in articles, and even agreeing on an industry standard won’t keep people from getting sick and dying from Legionella infections.

Prevention will happen only to the extent that operators of hotels, hospitals, office buildings, apartment and condominium buildings, nursing homes, industrial facilities, and schools implement comprehensive and effective Legionella control measures in the management of their water systems. The responsibility lies with them, and perhaps with the health departments that advise and regulate them.



ASHRAE. 1998. Legionellosis: Position Paper. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

ASHRAE. 2000. Minimizing the Risk of Legionellosis Associated with Building Water Systems. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

ASTM. 1996. Standard Guide for Inspecting Water Systems for Legionellae and Investigating Possible Outbreaks of Legionellosis (Legionnaires’ disease or Pontiac Fever). West Conshohocken, Pa.: ASTM International.

ASTM. 2003. Standard Guide for Inspecting Water Systems for Legionellae and Investigating Possible Outbreaks of Legionellosis (Legionnaires’ disease or Pontiac Fever). West Conshohocken, Pa.: ASTM International.

ASTM. 2008. Standard Guide for Inspecting Water Systems for Legionellae and Investigating Possible Outbreaks of Legionellosis (Legionnaires’ disease or Pontiac Fever). West Conshohocken, Pa.: ASTM International.

Besser RE. 2002. “Legionnaires’ disease in the Unites States: Opportunities for Prevention.” In: Marre R et al, eds. Legionella. Washington, D.C.: American Society for Microbiology, 391-397.

CDC. 2003. Guidelines for Environmental Infection Control in Health-Care Facilities: Recommendations of CDC and the Healthcare Infection Control Practices Advisory Committee. Atlanta: Centers for Disease Control and Prevention. Available at http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5210a1.htm.

Cooper A. 2004. Assessing Risk of Legionella. ASHRAE Journal. April.

EPA. 1991. Guidance Manual for the Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources, March 1991 Edition, Parts One and Two, Appendix B, “Institutional Control of Legionella,” pp. 201-206. Washington, DC: U.S. Environmental Protection Agency

EPA. 1999. Legionella: Human Health Criteria Document. Washington, DC: U.S. Environmental Protection Agency. EPA-822-R-99-001, November 1999.

EPA. 2001. Legionella: Drinking Water Health Advisory. Washington, DC: US Environmental Protection Agency. EPA-822-B-01-005, March.

EPA. 2016. Technologies for Legionella Control in Premise Plumbing Systems: Scientific Literature Review. EPA 810-R-16-001 September

EWGLI. 2003. European Guidelines for Control and Prevention of Travel Associated Legionnaires’ Disease. London: European Working Group for Legionella Infections

EWGLI. 2005. European Guidelines for Control and Prevention of Travel Associated Legionnaires’ Disease. London: European Working Group for Legionella Infections

Fields B, Benson R, Besser R. 2002. Legionella and Legionnaires’ Disease: 25 Years of Investigation. Clinical Microbiology Reviews 15; 3; 506-526

Freije M. 1996. Legionellae Control in Health Care Facilities: A Guide for Minimizing Risk. HC Information Resources Inc. (hcinfo.com)

Freije M. 2006. Ebb & Flow: Ten Ways to Minimize Stagnation in Domestic Water Systems. Health Facilities Management, January.

Freije M. 2006. Legionnaires’ contamination preventable at reasonable cost. Hotel & Motel Management, April 17th.

Hodgson M. 2002. Water management practices: controlling legionella in water systems, Parts I and II. The Synergist, April and May.

HPA. 2006. Management of Spa Pools: Controlling the Risk of Infection. London: Health Protection Agency.

McCoy WF. 2006. Legionellosis: Why the problem continues. ASHRAE Journal, January.

O’Loughlin RE, Kightlinger L, Werty MC, et al. 2007. Restaurant Outbreak of Legionnaires’ Disease Associated With a Decorative Fountain: An Environmental and Case-Control Study. BMC Infectious Diseases 7:93

OSHA. 1996. Technical Manual; Section II, Chapter 7. U.S. Department of Labor, Occupational Safety and Health Administration.

Shelton BG, Morris GK, Gorman GW. 1993. Reducing risks associated with legionella bacteria in building water systems. Presented at the 4th International Symposium on Legionella, 1992. In: Barbaree JM, Breiman RF, DuFour AP, eds. Legionella: Current Status and Emerging Perspectives. Washington, DC: American Society for Microbiology, 1993;296-297.

Shelton BG, Flanders WD, Morris GK. 1994. Legionnaires’ disease outbreaks and cooling towers with amplified legionella concentrations. Current Microbiology 28;359-363.

Sidari F, Stout J, Vanbriesen J, et al. 2004. Keeping Legionella out of water systems. Journal AWWA 96:1;111-119.

Springston J. 2000. Legionella lives. Engineered Systems, January.

Stout J, Yu V. 2003. Experiences of the first 16 hospitals using copper-silver ionization for Legionella control: Implications for the evaluation of other disinfection modalities. Infection Control and Hospital Epidemiology 24:8;563-568.

Turner S, Handley D. 2008. Find and prevent Legionella in your building water systems. Buildings, January.

Zhang Z, McCann C, Hanrahan J, et al. 2009. Legionella control by chlorine dioxide in hospital water systems. Journal AWWA 101:5;117-127.

VHA. 2014. Prevention Of Healthcare-Associated Legionella Disease and Scald Injury from Potable Water Distribution Systems. VHA Directive 1061. Washington, DC: Department of Veterans Affairs

WHO. 2007. Legionella and the prevention of legionellosis. Geneva: World Health Organization.



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