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Legionella and Building Water Management

Published 27 June 2026 • By Dr. Megan Tranter

Legionnaires’ disease, a severe and sometimes fatal pneumonia caused by inhalation of aerosolized Legionella pneumophila, is one of the most consequential building-related illnesses confronting modern facilities. Unlike most occupational hazards, the agent is an opportunistic environmental bacterium that colonizes and amplifies within engineered water systems such as cooling towers, potable hot-water distribution, decorative fountains, spas, and complex plumbing. Because exposure occurs through respirable aerosols generated by these systems, Legionella sits at the intersection of microbiology, building-water engineering, and industrial hygiene, and its control demands a systematic water-management framework rather than reactive testing alone.

What You’ll Learn

  • How Legionella grows and amplifies within engineered water systems.
  • Which host factors and building systems drive outbreak risk.
  • What ANSI/ASHRAE Standard 188 requires of a water management program.
  • The difference between verification and validation in a control program.
  • The practical steps an industrial hygienist should take to manage Legionella risk.

Introduction

Legionella control differs from classical chemical-exposure management because the hazard is a living organism whose growth depends on temperature, stagnation, and disinfectant residual. The modern approach, therefore, controls the physical and chemical conditions inside water systems rather than chasing a target bacterial count. This post explains the microbiology, the regulatory framework anchored in ASHRAE Standard 188, and the engineering controls and documentation that define a defensible water management program.

Microbiology and Ecology

Legionella is a Gram-negative bacterium that proliferates within free-living amoebae and biofilms in warm freshwater environments. Growth is favored in the temperature range of roughly 25-42 degrees Celsius, with stagnation, sediment, scale, and biofilm providing both nutrients and protection from disinfectant residuals. Engineered systems that maintain water in this thermal window, such as poorly circulated hot-water loops, dead legs, and warm cooling-tower basins, become amplification reservoirs. Aerosolization at cooling towers, showers, and faucets then delivers respirable droplets to susceptible hosts.

Epidemiology and Risk Factors

Reported cases of legionellosis have risen markedly over the past two decades across North America and Europe. Host risk factors include advanced age, smoking, chronic lung disease, and immunosuppression, which is why healthcare facilities, hotels, and long-term care settings feature prominently in outbreak investigations. Cooling towers have been implicated in some of the largest community outbreaks because they can disperse contaminated aerosols over considerable distances. Because the disease is contracted by inhaling aerosols generated indoors or near buildings, Legionella control is inseparable from broader indoor air quality management.

ASHRAE Standard 188 and Water Management Programs

ANSI/ASHRAE Standard 188, “Legionellosis: Risk Management for Building Water Systems,” codifies a systematic process for assessing and controlling Legionella risk. The standard requires a designated program team to develop a written water management plan that describes building water systems, identifies hazardous conditions and control locations, establishes control limits and monitoring, defines corrective actions when limits are exceeded, and verifies and validates program effectiveness. Notably, the standard emphasizes control of physicochemical parameters such as temperature, disinfectant residual, and maintenance rather than mandating routine Legionella culturing, although monitoring is adopted when the team determines it is warranted. The CDC’s companion toolkit translates these requirements into practical implementation guidance.

Control Strategies and Verification

Effective programs maintain hot water above and cold water below the Legionella growth range, eliminate dead legs and stagnation, manage scale and biofilm, and sustain adequate disinfectant residuals throughout the distribution system. Cooling towers require routine cleaning, biocide treatment, and drift-eliminator maintenance. Verification confirms that control measures are implemented as designed, while validation, often including environmental Legionella sampling, confirms that the program is actually achieving control. Online monitoring technologies, including qPCR-based methods, are increasingly used to shorten the feedback loop between sampling and corrective action. The same logic of controlling building conditions rather than a single number also governs the management of other biological agents in buildings, including bioaerosols and mold in indoor environments.

What Industrial Hygienists Should Do

The industrial hygienist should champion a written, building-specific water management program built on ASHRAE 188 and CDC guidance, ensuring that a competent program team conducts a hazard analysis of every aerosol-generating water system. Establish and document control limits for temperature and disinfectant residual, schedule routine monitoring at representative points, and define unambiguous corrective actions for excursions. During suspected outbreaks, coordinate environmental sampling with public-health authorities and clinical case-finding. Maintain cooling towers on a rigorous cleaning and biocide schedule, flush low-use outlets, and retain thorough records demonstrating both verification and validation. Above all, treat Legionella management as an ongoing engineering-control program rather than a one-time test.

Summary

Legionella is a building-water hazard best controlled by managing the conditions that allow it to multiply: temperature, stagnation, scale, biofilm, and disinfectant residual. ASHRAE Standard 188 provides a framework that requires a program team, a written plan, control limits, corrective actions, and both verification and validation. Treating water management as a continuous engineering-control program, rather than relying on occasional testing, is what prevents outbreaks.

Helpful Resources

Bibliography

American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2021). ANSI/ASHRAE Standard 188-2021: Legionellosis: Risk management for building water systems. ASHRAE.

American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2023). ASHRAE Guideline 12-2023: Managing the risk of legionellosis associated with building water systems. ASHRAE.

Centers for Disease Control and Prevention. (2021). Developing a water management program to reduce Legionella growth and spread in buildings: A practical guide to implementing industry standards. U.S. Department of Health and Human Services.

Cunha, B. A., Burillo, A., & Bouza, E. (2016). Legionnaires’ disease. The Lancet, 387(10016), 376-385. https://doi.org/10.1016/S0140-6736(15)60078-2

Fitzhenry, R., Weiss, D., Cimini, D., Balter, S., Boyd, C., Alleyne, L., Stewart, R., McIntosh, N., Econome, A., Lin, Y., Rubinstein, I., Passaretti, T., Kidney, A., Lapierre, P., Kass, D., & Varma, J. K. (2017). Legionnaires’ disease outbreaks and cooling towers, New York City, New York, USA. Emerging Infectious Diseases, 23(11), 1769-1776. https://doi.org/10.3201/eid2311.161584

Hozalski, R. M., LaPara, T. M., Zhao, X., Kim, T., Waak, M. B., Burch, T., & McCarty, M. (2020). Flushing of stagnant premise water systems after the COVID-19 shutdown can reduce infection risk by Legionella and Mycobacterium spp. Environmental Science & Technology, 54(24), 15914-15924. https://doi.org/10.1021/acs.est.0c06357

National Academies of Sciences, Engineering, and Medicine. (2020). Management of Legionella in water systems. National Academies Press. https://doi.org/10.17226/25474

Occupational Safety and Health Administration. (n.d.). OSHA technical manual, Section III, Chapter 7: Legionnaires’ disease. U.S. Department of Labor.

Plog, B. A., & Quinlan, P. J. (Eds.). (2012). Fundamentals of industrial hygiene (6th ed.). National Safety Council.

Whiley, H., Bentham, R., & Brown, M. H. (2017). Legionella persistence in manufactured water systems: Pasteurization potentially selecting for thermal tolerance. Frontiers in Microbiology, 8, 1330. https://doi.org/10.3389/fmicb.2017.01330

World Health Organization. (2007). Legionella and the prevention of legionellosis. World Health Organization.

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