Legionella

Legionella is a bacteria that is naturally present in a range of water and soil environments, but can also thrive within building water systems and multiply under ideal conditions that include warm temperature (e.g., 25–45°C), lack of disinfection, and the presence of biofilms. 

Legionella bacteria are also called opportunistic premise plumbing pathogens (OPPP) and can be present in building water systems, water storage containers, pipes, or outlets. Exposure to Legionella bacteria can occur in a range of settings including homes, workplaces, healthcare settings, vacation rentals and hotels, events and festivals, and in the wider community.

Exposure usually occurs when contaminated water becomes aerosolized as mists or sprays and infection occurs when these aerosols are inhaled into the lungs. Occasionally accidental aspiration of contaminated water during drinking or chewing on ice have resulted in infections. Evaporative cooling towers atop buildings are a major source of outbreaks. These systems can allow water aerosols contaminated with Legionella to be dispersed over a large area. 

Illnesses caused by infection with Legionella are collectively referred to as legionellosis. This includes Pontiac fever (a flu-like illness), and Legionnaires’ disease (a severe pneumonia). The case-fatality rate of Legionnaires’ disease is about 5–15% in the general population and up to 40–80% in untreated immunosuppressed persons. Smokers and persons over 50 are at higher risk of infection, and cases occur more often in men than women. The reported occurrence of legionellosis is increasing in Canada, with over 600 cases annually in most years since 2018. Cases are also rising in the United States and Europe.

The majority of outbreak-related deaths are linked to evaporative cooling towers. Major cooling-tower-associated outbreaks in 2025 in London, Ontario and New York City highlight the need to continue to review the drivers of Legionella growth and dispersal in the environment and improve control measures. This includes better maintenance of cooling tower systems, reviewing approaches to building water management, identifying additional tools to support outbreak investigations, and seeking to understand the role of climate factors in legionellosis occurrence.

The resources listed here are intended to highlight:

• Sources of exposure and factors affecting Legionella survival and growth
• Approaches to managing Legionella risks in buildings
• Responses to Legionella outbreaks and the corrective actions
• Emerging areas of study

Background information on Legionella and sources of exposure

• About Legionnaires’ disease (US Centers for Disease Control and Prevention (CDC), 2025)
  This webpage provides general information on legionellosis including causes, people at increased risk, symptoms, diagnosis, treatment, and prevention. Resources specific to clinicians, health departments and laboratories are provided.

• Living with Legionella and other waterborne pathogens (Falkinham, 2020)
  This review article summarizes characteristics of Legionella spp and other OPPPs, their occurrence in premise plumbing, pathways of transmission, monitoring, and remediation.

• Legionella: Who’s addressing the risks in Canada (National Research Council of Canada, Health Canada, and PSPC, 2019)
  This report provides an overview of Legionella, risk factors, symptoms, and treatment, and sets out the roles and responsibilities for managing Legionella risks before, during, and after building construction, including provincial and territorial protocols.

Managing the risks of Legionella in building water systems

The resources below provide information, tips, and examples for controlling Legionella growth and exposure in premises, including how to implement a water management program (WMP).

• Toolkit: Controlling Legionella in common sources of exposure (US CDC, 2025)
  This toolkit provides actions for public health and building managers to control Legionella in common sources of infection such as:
  • Potable water
  • Cooling towers
  • Hot tubs
  • Decorative water features
  • Vacation rentals
  • Other devices that contain non-sterile water (e.g., irrigation, fire suppression systems, safety showers, ice machines, humidifiers etc.

• Legionella training (US CDC and partners, 2024)
  This free training course is designed to assist public health professionals, building managers and others to understand how to assess Legionella hazards and develop a Legionella WMP.

• Guidance for water system risk management (ASHRAE, 2021 and 2023)
  This suite of standards and guidelines from ASHRAE (Standards 514-2023, Standard 188-2021 and Guideline 12-2023) provide minimum requirements and informative guidance for controlling Legionella risks in buildings.

• Developing a water management program to reduce Legionella growth and spread in buildings (US CDC, 2021)
  This toolkit is a plain language translation of the ASHRAE Standard 188 to assist building owners and managers to evaluate whether and how to develop a WMP to control Legionella. 

• Legionella communications factsheet: A guide for health agency staff (Association of State and Territorial Health Officials, 2021)
  This guidance document is intended to provide public health officials with advice on communicating with building and facility personnel about Legionella, and to raise awareness of preventive measures.

• Management of Legionella in water systems (NASEM, 2020)
  This book provides a review of the state of the science on Legionella contamination of water systems, growth, transmission, surveillance, prevention, and control.

• Legionnaires’ disease at temporary events or trade shows – A guide for event coordinators and vendors(Vancouver Coastal Health, 2020)
  This fact sheet provides a guide for trade show coordinators or vendors showcasing equipment that generates water aerosols (e.g., hot tubs, float tanks, misters, fountains, showers etc.) to prevent the growth and spread of Legionella bacteria from their devices.

• Water quality challenges: Morbidity and mortality associated with building water systems (Environmental Science, Policy and Research Institute, 2018)
  This presentation outlines challenges in controlling OPPPs including Legionella, and steps for effective building water management. Case studies for different healthcare facility types are examined. 

• Control of Legionella in mechanical systems MD 15161 (Public Services and Procurement Canada (PSPC), 2016)
  This standard for building owners, design professionals, and maintenance personnel, developed by PSPC for federal buildings, provides a detailed approach to preventing the growth of Legionella bacteria and cleaning and disinfecting contaminated systems.

Outbreak investigation and control

Identifying the source of a legionellosis outbreak is complex. The resources below provide guidance on conducting investigations with examples. Some Canadian jurisdictions, listed below, are now using cooling registries to assist in outbreak investigation and control.

• Legionella and Legionnaires’ disease (legionellosis) (Walkerton Clean Water Centre, 2025)
  This resource collection includes a broad range of information on legionellosis prevention through effective water management, sampling, standards, surveillance, and outbreak investigations.

• Legionella investigation reference document (Ontario Ministry of Health, 2025)This guidance document provides information to support public health in the investigation of Legionella cases and outbreaks.

• Investigating Legionnaires’ disease (US CDC, 2024)
  This resource collection provides information on the key elements of Legionnaires’ disease investigations with specific guidance related to travel-associated outbreaks, investigations of healthcare-associated cases and outbreaks, and community-associated outbreaks.

• Environmental assessment and sampling resources (CDC, 2024)
  These sampling forms, procedures and instructional videos provide tools and advice needed to sample taps, cooling towers, hot tubs (spas), and fountains during a legionellosis cluster or outbreak investigation.

• Legionella outbreak investigation and control (BC Centre for Disease Control, 2023)
  This guidance document provides evidence-based approaches for epidemiological, environmental, and microbiological investigation and management of cases and outbreaks of legionellosis in BC.

• Legionella outbreak investigations: A practical approach (Public Health Ontario Rounds, 2023)
  This presentation slide deck and recording describe environmental risk assessment of a cooling tower, interpretation of environmental testing protocols during outbreak investigations, and interventions and communication strategies to use during a community Legionella outbreak.

• Strategies to combat Legionnaires’ disease. Outbreak investigation and preventive policy (NCCEH, 2020)
  This webinar recording  includes a presentation from Fraser Health on a Legionnaires’ disease outbreak investigation associated with cooling towers in Surrey, BC, and City of Vancouver on the implementation of a registry for cooling towers and decorative water features in the city.  

• Did municipal water distribution system deficiencies contribute to a Legionnaires’ disease outbreak in Quincy, IL? (Rhoads et al., 2020)
  This article reports on a Legionnaires’ disease outbreak, where changes to municipal drinking water supply disinfectant and corrosion control practices may have led to Legionella growth in the distribution system. 

• Cooling tower registries and building water management plans: Strategies to combat Legionnaires’ disease(NCCEH, 2020) 
  This blog provides an overview of cooling tower registries in a Canadian context. For examples of Canadian jurisdictions with regulations for building water systems to control Legionella see:
  • City of Hamilton, Ontario
  • Province of Quebec
  • City of Vancouver, B.C.
  • Province of New Brunswick

• Multiple sources of the outbreak of Legionnaires’ disease in Genesee County, Michigan, in 2014 and 2015(Smith et al., 2019)
  This article reports how a detailed outbreak investigation identified multiple sources of exposure (hospital associated, residential, and cooling tower associated). 

• Legionella: questions and answers, 2nd edition(Public Health Ontario, 2019)
  This report lists questions and answers on environmental sampling during a legionellosis outbreak, clinical aspects of an outbreak investigation, and infection prevention and control measures.  

Emerging areas of study

• Risk assessment of Legionella pneumophila in drinking water distribution systems (LeChevallier et al. 2025)
  This research article reports on an assessment of health risk posed by Legionella pneumophila in public drinking water across 57 US water utilities. L. pneumophila was detected in 1.19% of samples, and detections were typically <10 MPN/100 ml. A national estimate of risk of exposure from drinking water distribution was found to be exceedingly small.

• Global surge of Legionnaires’ disease in 2024: urgent call for heightened awareness and preparedness (Pareek et al., 2025)
  This article in the Lancet Microbe highlights the need to address crucial research gaps in prevention and control of Legionella transmission.

• From cooling towers to splash pads – Staying safe while keeping cool (O’Keeffe, 2025)
  This NCCEH blog highlights potential sources of exposure to Legionella in cooling infrastructure, considerations for changing exposures due to climate change, and preventive measures for reducing infection risks. 

• Legionnaires’ disease outbreak linked to a cold-water source in Yorkshire, 2022 (Thompson et al., 2025)
  This study reports on a Legionnaires’ disease outbreak in a sheltered accommodation setting in the UK. Dead legs in the plumbing system and heat transfer to the cold-water pipes provided favourable conditions for Legionella growth.

• Quantification of Legionella pneumophila in building potable water systems: A meta-analysis comparing qPCR and culture-based detection methods (Sylvestre et al., 2025)
  This study examines the use of qPCR and culture-based methods for L. pneumophila measurements, aiming to improve understanding of the relationship between the two methods to inform assessment of Legionella associated health risks.

• Two outbreaks of Legionnaires’ disease associated with outdoor hot tubs for private use — two cruise ships, November 2022–July 2024 (Lee et al., 2024)
  This MMWR article documents cases of Legionnaires’ disease associated with private hot tubs on cruise ships and highlights the requirements for public hot tubs that may not be applied in private settings, increasing the risk of Legionella growth in private tubs.

• Weather conditions and legionellosis: a nationwide case-crossover study among Medicare recipients (Wade and Herbert, 2024)
  This article reports on an analysis of records of hospitalizations for legionellosis alongside climate data, finding an association between hospitalizations and precipitation and humidity, and identifying seasonal trends.

• Pinpointing drivers of widespread colonization of Legionella pneumophila in a green building: Roles of water softener system, expansion tank, and reduced occupancy (Joshi et al., 2023)
  This article highlights how a building water system widely colonized by Legionella during a period of low usage, was not remediated by flushing alone, and how water softeners and the expansion tank were identified as a cause of L. pneumophila contamination.

• Effects of the COVID-19 pandemic on Legionella water management program performance across a United States lodging organization (Kunz et al., 2023)
  This article reports on pre- and post-pandemic Legionella test results for buildings with periods of low water use/stagnation. The effect of stagnation on Legionella growth, and the mitigating impact of water management plans and flushing protocols are highlighted.

• Probabilistic analysis of a French legionellosis outbreak shows potential role of wastewater basin (Wallet et al., 2022)
  This article examines possible role of different sources of exposure in a legionellosis outbreak in France, including both an aerated wastewater basin and cooling towers. 

• Legionella occurrence beyond cooling towers and premise plumbing (Schwake et al., 2022)
  This review article examines the occurrence of Legionella in various fresh and marine waters, rainwater, groundwater, wells, drinking water treatment and distribution systems. 

• Climate change and opportunistic pathogens (OPs) in the built environment (O’Keeffe, 2022)
  This NCCEH review article in Environmental Health Review outlines how climate change may affect the occurrence and transmission of OPPPs like Legionella in the built environment.

• Racial disparities in incidence of Legionnaires’ disease and social determinants of health: A narrative review (Hunter et al., 2021)
  This narrative review reports on well documented racial and socioeconomic disparities in the incidence of Legionnaires’ disease in the United States and discusses some of the social determinants that may be contributing to exposure to, or infection by, Legionella.