Cooling Towers and Legionella: The Importance of Water Treatment and Testing to Minimize Risks to Human Health

Are your open recirculating cooling systems at risk of harboring Legionella, the bacterium responsible for Legionnaires’ disease?

How can you ensure your cooling tower and all components of your comfort cooling (HVAC) and industrial cooling processes remain safe and efficient?

In the warm and moist environments of open recirculating cooling systems, Legionella bacteria find an ideal breeding ground. Without proper maintenance and water treatment programs, the risk of exposure through airborne water mist increases, posing significant health hazards.

In this guide you’ll find insights to effectively maintain your open recirculating cooling systems. By following cooling tower maintenance and cleaning procedures while implementing effective water treatment programs, you can help mitigate the risk of Legionella and ensure a safer environment for all.

What is Legionella and Legionellosis?

Legionella is a pathogenic gram-negative bacteria that causes legionellosis. Legionellosis varies in severity, from a mild flu-like illness called “Pontiac fever”, to a potentially fatal form of pneumonia known as “Legionnaires’ disease”. 

Legionnaires’ disease can affect anyone. The bacteria are found worldwide in many different natural and artificial aquatic environments including cooling towers and other water systems in:  

Examples of water systems where Legionella may be present include hot tubs, drinking fountains, ice machines, and water displays (fountains). 

Who is At Risk of Legionella and Legionnaires Disease?

Although Legionella can affect anyone, certain populations are at a higher risk of getting sick. These include (1, 2): 

  • Individuals aged 40 and older 
  • Present or past smokers 
  • Those with respiratory problems and chronic lung diseases such as chronic obstructive pulmonary disease (COPD) or emphysema 
  • People who are immuno-compromised 
  • Post-transplant or chemotherapy patients 
  • Individuals battling cancer 
  • Those with underlying health conditions, including diabetes, kidney failure, or liver failure 

Understanding the Risks of Legionella and Cooling Towers

If not properly managed, cooling towers and other water systems can be a breeding ground for Legionella and create health risks for building occupants and the surrounding population. Building owners are obligated to properly maintain cooling tower systems and in some jurisdictions, develop water management plans under local, provincial, and federal regulations.

Water management plans, legionella water treatment programs, and water testing are tools to evaluate risk and ensure precautions are in place to minimize health risks and local health and safety violations. 

The Importance of Cooling Tower Water Treatment

Effective cooling tower water treatment is critical to maintain a clean, efficient cooling tower and minimize Legionella risk. Cooling water treatment involves the addition of chemicals to control corrosion, scale, and microbial growth including Legionella. These measures, combined with regular system assessments and adjustments based on water quality tests, ensure optimal operation and longevity of your open recirculating cooling system.

And, proper cooling tower water treatment and maintenance of your open recirculating cooling system are critical to prevent Legionella and Legionnaire’s disease.

How Legionella Testing Reduces Your Risk

Regular water testing and Legionella bacteria testing are crucial for early detection and prevention of legionella bacterial growth in water systems. Testing helps identify environmental conditions that could foster Legionella, allowing for timely adjustments to water treatment programs. This helps to protect human health by preventing potential outbreaks of Legionnaires’ disease.

With regular water testing you ensure your cooling tower water treatment program is effective and keep your open recirculating cooling system operating efficiently. Proactive testing gives you the ability to respond to small variances before they become big problems. And, with regular legionella testing, you can address minor issues before they escalate to maintain a safe environment.

Guardian Chemicals: Ensuring Safe Water Systems

Guardian Chemicals can provide you with clarity on the risks and requirements related to your cooling systems and Legionella. We design cooling tower water treatment programs to: 

  • Minimize legionella risk 
  • Protect your people 
  • Protect your equipment  

Guardian Chemicals can assist you in the development and implementation of a water management plan. We also provide onsite testing equipment, services to collect water samples, and access to accredited lab culture testing, PCR or rapid field testing for Legionella bacteria.

Benefits of Partnering with Guardian Chemicals

By working with us, you decrease the risk of Legionella cases and liability associated with Legionella for your tenants, operators, contractors, and the public. Our services can help you score LEED and BOMA Best points, increase tenant and owner satisfaction, and provide peace of mind.

Our cooling tower water treatment solutions not only reduce the risk of Legionella, but also help you protect and optimize your cooling system and achieve efficiency at the lowest cost to operate. 

And if you’re curious about how to change water treatment suppliers or need help to find the right water treatment supplier for you, we can help!


Legionella and cooling towers don’t have to go hand in hand. The risk posed by Legionella in cooling towers can be managed with an effective cooling tower water treatment program and Water Management Plan. The plan surveys the use of water at each point in the building. A proper water management plan covers the entire facility. It includes guidelines on water treatment, periodic environmental testing, system monitoring, maintenance and operating best practices proven to reduce risk.

Learn more about cooling tower maintenance, read the article below:

Start Your Journey to a Safer Cooling Tower Today

When you’re ready, contact us to reduce your Legionella risk and improve the safety and efficiency of your cooling tower system. Together we can create a better environment to protect your employees, customers, and your business reputation. 

FAQ: Legionnaires Disease, Legionella Testing and Cooling Tower Water Treatment

Legionnaires’ disease is a serious lung infection caused by Legionella bacteria, often found in water systems like cooling towers. Regular Legionella testing and water treatment are essential to prevent outbreak of the disease. This FAQ section addresses common concerns about Legionnaires’ disease, the importance of testing for Legionella, and best practices for cooling tower water treatment to ensure public health and safety.

What is the most effective cooling tower water treatment for preventing Legionella?

Effective cooling tower water treatment for Legionella prevention involves a comprehensive approach. This includes the use of biocides to control microbial growth; maintaining water quality to prevent the conditions that foster Legionella proliferation; and proper cleaning and maintenance schedules. By doing this, you can ensure your cooling tower operates efficiently and safely, reducing the risk of Legionella outbreaks and protecting public health. 


How often should cooling towers be tested for Legionella to ensure safety?

When in operation, cooling towers should be tested for Legionella monthly. This frequency will vary based on local regulations, the usage of the cooling tower, your water management plan and the results of risk assessments. More frequent testing may be necessary in high-risk situations or if previous tests indicate the presence of Legionella bacteria. Weekly dip-slide testing is also recommended as an early indicator of potential problems. Regular testing is crucial for early detection and effective management of Legionella risks. 
Learn more about test requirements as recommended by PWGSC in the chart below.

cooling tower testing and maintenance schedule from PWGSC

What are the key components of a water management plan for cooling towers?

A comprehensive water management plan for cooling towers includes risk assessment to identify areas vulnerable to Legionella growth, establishing control measures to mitigate these risks, continuous monitoring and maintenance of the cooling water system to ensure proper functioning and cleanliness, and regular Legionella water testing. Your water management plan should include an action plan in case Legionella is detected in your open recirculating cooling system. Plans should be reviewed and updated regularly to adapt to new guidelines, findings, or system changes. 

Learn more about water management plans. 

How does Legionella grow in cooling towers, and what conditions favor its proliferation?

Legionella bacteria thrive in warm, stagnant water, typically between 25°C and 45°C (77°F and 113°F). Cooling towers provide an ideal environment for their growth due to the presence of water and suitable temperatures. Factors that favour Legionella proliferation include the accumulation of scale, sediment, and biofilm, as well as poor water flow and inadequate disinfection. Regular maintenance and cooling tower water treatment are essential to control these conditions and minimize the risk of Legionella growth. 

What are the health risks associated with Legionella in cooling towers, and who is most at risk?

The health risks associated with Legionella in cooling towers primarily include Legionnaires’ disease, a severe form of pneumonia, and Pontiac fever, a milder illness. These diseases occur when individuals inhale droplets of water contaminated with Legionella bacteria. While Legionnaires’ disease can be serious and require hospitalization, Pontiac fever is usually self-limiting. The bacteria thrive in warm water environments, making cooling towers potential breeding grounds if not properly maintained. 

Certain populations are more susceptible to illness from Legionella exposure, including people aged 40 years or older, current or former smokers, those with chronic lung diseases (like COPD), individuals with weakened immune systems (including those on drugs that suppress immune function), cancer patients, and people with underlying conditions such as diabetes, kidney failure, or liver failure. 

It’s important to maintain cooling towers through regular cleaning, disinfection, and monitoring to mitigate the risk of Legionella growth and spread. Implementing control measures, such as maintaining water quality, ensuring proper disinfectant levels, and regularly testing for Legionella, are critical steps to prevent outbreaks of Legionnaires’ disease and ensure the safety of building occupants and the surrounding community. 

Can regular maintenance of cooling towers significantly reduce the risk of Legionella outbreaks?

Yes, regular maintenance significantly reduces the risk of Legionella in cooling towers. This includes filtration, annual disinfections, monitoring for disruptions, and adhering to Legionella testing guidelines. Such practices prevent conditions favorable for Legionella growth, ensuring safer cooling tower operations.

How do I choose the right water treatment program for my facility’s cooling tower?

Consult with a reputable water treatment supplier. A water treatment expert will: 

  • assess your open recirculating cooling system and specific needs 
  • identify key water quality issues 
  • have knowledge of local regulations 
  • recommend a water treatment program 
  • evaluate and optimize your program for cost-effectiveness, efficiency, and protection 
  • advise on and assist with regular monitoring requirements 

Your Guardian Chemicals water treatment expert will design a cooling tower water treatment program that’s right for you. All our programs reduce the potential for scale and corrosion, and prevent biofouling and bacterial growth—including Legionella. Best of all, you reduce energy use, conserve water, reduce your overall costs—and operate your tower efficiently at the lowest cost to operate. 

What are the consequences of not adequately treating cooling towers for Legionella?

Not adequately treating cooling towers for Legionella can have serious consequences, including: 

  1. Health Risks: The primary concern is the potential outbreak of Legionnaires’ disease, a severe form of pneumonia, and Pontiac fever, a milder illness. Both are caused by inhaling water droplets contaminated with Legionella bacteria.
  2. Legal and Financial Repercussions: Facilities found negligent in maintaining their cooling towers can face legal action, hefty fines, and compensation claims from affected individuals. This can also lead to increased insurance premiums.
  3. Operational Disruptions: An outbreak or detection of Legionella may require shutting down the cooling tower for thorough cleaning and disinfection, leading to operational downtime and lost productivity.
  4. Reputation Damage: Public health incidents linked to a facility can significantly damage its reputation, leading to lost business and a decrease in customer trust.
  5. Increased Maintenance Costs: Failure to prevent Legionella growth can lead to more severe contamination, requiring more extensive and expensive remediation efforts to resolve the issue. 

Proactively managing and treating cooling towers to control Legionella growth is essential to avoid adverse outcomes, ensure the health and safety of the community, compliance with regulations, and uninterrupted facility operation. 

How does temperature affect Legionella growth in cooling towers, and what temperature should be maintained?

Temperature significantly affects Legionella growth in cooling towers. Legionella bacteria thrive in warm water, particularly in temperatures ranging from 25°C to 45°C (77°F to 113°F). To inhibit Legionella growth, operate your cooling tower at the lowest possible water temperature. Due to the operational nature of cooling towers, achieving lower temperatures may not always be practical. Therefore, alongside temperature management, regular maintenance, water treatment, and monitoring are crucial to minimize the risk of Legionella proliferation.

Originally published June 5, 2023. Revised and updated with additional content and FAQ March 20, 2024.

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