Cooling Tower Legionella Risk Assessment

Legionella pneumophila is a bacterium that causes a type of pneumonia known as Legionnaires’ disease. It can breed in cooling towers and become airborne, affecting nearby people. Cooling towers are commonly blamed for Legionella outbreaks. In 2025, Chardon Labs reported on four major Legionella outbreaks in Chicago, Columbus, Marshalltown, and New York City. In three of these outbreaks, cooling towers were suspected to play a role when the story was reported. To prevent outbreaks, it is essential to understand Legionella safety best practices and address associated risk factors.

Cooling Tower Risk Factors

Legionella is a naturally occurring bacterium found in bodies of water in the environment. The following best practices can mitigate their growth and spread:

• No chemical treatment- It is necessary to regularly feed biocide chemicals. Biocides kill/prevent bacterial growth. It also targets biofilm, which is an organic residue that shields and breeds bacteria. At Chardon Labs, we recommend that everyone feed dual biocides. This kills both aerobic bacteria, such as Legionella, and anaerobic bacteria. Both types are responsible for bacterial residue and biofilm.
• Stagnation – Significant periods of downtime can cause still water, which fosters bacterial growth. It is recommended to operate the cooling tower at least once every 3-7 days unless the cooling tower is shut down for the season.
• No testing- Legionella always presents a risk; in addition to control measures, it is important to test the levels of Legionella bacteria in your cooling tower. 1 CFU/mL and above of Legionella bacteria present in your water is considered “poorly controlled” according to the CDC. 10 to 100 CFU/mL or above is “uncontrolled,” and 0.9 CFU/mL is “well controlled.” Outbreaks of Legionella require initial and follow-up testing based on relevant laws.

• No reporting/role designation- It is important to define who is responsible for Legionella safety for your cooling tower, and which parts specifically they oversee. Additionally, reporting is important to ensure these practices are being done, and it is required after outbreaks.
• No cleaning- Bacteria and other debris can build up in your cooling tower. The degree to which this occurs depends on the quality of the chemical water treatment program you have. Cleaning can be achieved in two ways: chemical feeding and flushing, as well as physical cleaning. Physical cleaning involves pressure washing the fill media, basin, drift eliminator, distribution, etc. (This is also a good time to check valve operation, as dirty cooling towers will ruin valves. Remove any other debris and then apply biocide to the system. After a final flush, your system is ready to operate again.
• Temperature- Legionella grows best in temperatures of 77°F or more. This is why the hottest months of the year are when your cooling tower is at risk. Because of this, it is important to pay extra attention to your cooling tower during this.

How Cooling Towers Actually Spread Legionella to People

Powerful fans meant for the movement of water and evaporation in a cooling tower result in airborne water droplets. If these water droplets are contaminated with Legionella, people who encounter them are at risk. Cooling towers are usually on rooftops, but they can also be at any other outside part of the building. It is important to note that there are other ways people can contract Legionnaires’ Disease than through a cooling tower. Potable drinking water, water fountains, ice machines, and certain manufacturing facilities can all be sources of Legionella. According to expert Victor Yu, MD, Chief, Infectious Disease Section, VA Medical Center, Pittsburgh, PA, “Up to 70% of all buildings greater than three stories in the US may be contaminated with Legionella.”

Relevant Guidelines

There are laws in the United States regarding Legionella, such as for all government facilities nationwide. In recent years, more state and city regulations have been implemented in locations such as New York and New Jersey. Regardless of the city, if an outbreak occurs, the government’s disease control will get involved.

There are also guidelines and nationally recognized training. All Chardon Labs service managers are ASSE 12080 certified. This means they are informed about Legionella water safety and management. Nationally recognized ASSE and ASHRAE standards also call for water management plans.

Facilities At-Risk

CDC has guidance on specific fixtures and facilities most at risk for the spread of Legionella. Facilities include:

• Healthcare facilities where patients stay overnight, or houses, treat people who have chronic and acute medical problems or weakened immune systems.
• Elderly housing for people older than 65 years (like a retirement home or assisted-living facility).
• Multi-housing units and a centralized hot water system (like a hotel or high-rise apartment complex).
• Tall buildings with more than 10 stories and basements are factored into this.

Fixtures At-Risk

Fixtures that result in water that is stagnant, hot, or aersolized have a higher risk of spreading Legionella.

• Cooling towers (whether roof-top or ground level).
• Hot tubs or spas, specifically where the water is not drained between users.
• Any decorative fountains.
• Misters that are installed for the whole or large areas of the building, atomizers, air washers, or humidifiers.

Legionella Assessment Service

Chardon Labs is dedicated to Legionella control for your cooling tower and facility. If you want a free risk assessment from an ASSE 12080 service manager, please book an appointment. We offer Legionella testing, control, and remediation services.