Reports of legionellosis cases generally increase during warmer months. While legionella activity has been detected in the spring, the majority of reported cases in Toronto and Ontario over the last four years have occurred between July and October. Cooling towers are one of the main causes of outbreaks, illness and death due to Legionella infection.
For more information on Legionella and Legionellosis, please see this link.
Cooling Towers Defined
The American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) defines a cooling tower as:
A heat transfer device, often tower like, in which atmospheric air cools warm water, generally by direct contact (evaporation)
from American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE). ASHRAE Terminology.
Maintain Your Cooling Tower
- Cooling towers can provide the ideal environment for the growth and spread of Legionella bacteria and need to be actively managed to reduce legionella growth inside them
- A management program for the cooling tower(s) that is documented in a plan is a highly recommended way to deal with Legionella. Important components of the plan are:
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- Qualified staff with clear responsibilities
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- Defined control measures and limits with a monitoring plan
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- Interventions when control measures are not met
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- Verification to ensure the program is running as designed and effective
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- Documentation of the program activities
- There are guidance documents that can help you design a program to maintain your cooling tower to reduce the risk from legionella bacteria.
Industry Best Practices/Standards
These documents are useful tools to help you maintain your cooling tower to reduce the risk of legionella bacteria from your cooling tower (and other at-risk water systems in your building)
- Guidance for water system risk management
This suite of standards and guidelines from ASHRAE (Standard 188 and Guideline 12 and Standard 514) provide minimum requirements and informative guidance for controlling Legionella risks in buildings.
- 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.
Date modified: September 29, 2026
