You can’t see it, smell it, or detect it without testing, yet Legionella can multiply rapidly in the background of a cooling tower system.
Legionella bacteria are a well-known threat in building water systems, especially where warm water and airflow intersect. Cooling towers, which remove heat through evaporative cooling, can unintentionally create the perfect setting for bacterial growth if they are not properly maintained.
When Legionella bacteria in cooling tower systems become aerosolized, contaminated droplets can enter the surrounding air and pose serious health risks. When you know how Legionella forms in cooling tower water, it becomes a lot simpler to prevent problems and protect the people who use or move through the space every day.
A Brief Origin of Legionnaires’ Disease
The bacteria gained national attention in 1976 during an American Legion convention at a hotel in Philadelphia. More than 200 attendees became ill, and 29 people died before public health experts identified the cause. Investigators traced the outbreak to the hotel’s cooling tower, where contaminated water droplets were pulled into the building’s air system.
This event led to the discovery of Legionella pneumophila, the development of modern disease control guidelines, and ongoing awareness about the role of cooling tower systems in preventing future outbreaks.
Understanding Legionella and Its Behavior
Legionella is a naturally occurring waterborne bacterium found in freshwater sources, but it becomes dangerous when it multiplies inside man-made water systems. The bacteria thrive in warm, stagnant water, typically between 77°F and 113°F, and attach to surfaces where biofilm and organic matter are present.
Cooling towers, hot water tanks, and other large water systems provide these conditions when routine monitoring is overlooked. Because Legionella is not visible to the naked eye, regular testing and water quality checks are required to identify risks in cooling tower water before bacteria levels increase.
Health Risks Associated With Legionella Exposure
When Legionella becomes airborne through cooling tower mist, people nearby can breathe in the bacteria and become ill. Some cases are severe, while others are mild, but both can disrupt operations and trigger public-health concerns.
Legionnaires’ Disease
Exposure to contaminated aerosols can lead to Legionnaires’ disease, a severe form of pneumonia linked to cooling towers worldwide.
When people inhale water droplets containing the bacteria, it can infect the lungs and cause symptoms such as coughing, fever, shortness of breath, chest pain, and gastrointestinal issues. In severe cases, hospitalization is required, and complications can develop quickly without medical treatment.
Because outbreaks tied to Legionnaires’ disease cooling tower events often affect multiple people at once, facilities have a responsibility to reduce risk through proper maintenance and testing.
Pontiac Fever
A milder condition known as Pontiac fever can also occur after exposure. While it does not lead to pneumonia and usually resolves without treatment, it still results in flu-like symptoms and can interrupt business operations, trigger public-health investigations, and raise compliance concerns.
Even non-lethal outbreaks damage trust, impact productivity, and may lead to shutdowns until the source is controlled.
Why Cooling Tower Conditions Encourage Bacterial Amplification
Because cooling towers combine heat, moisture, and airflow, they can unintentionally support Legionella if maintenance slips. Once bacteria take hold, the system can amplify and distribute it beyond the immediate area.
Warm Water Temperatures
Cooling towers operate by circulating warm water to remove heat from HVAC or industrial processes. During normal operation, temperatures frequently fall within the ideal range for legionella growth, especially in periods of reduced demand or seasonal changes. Without proper treatment, bacteria can multiply rapidly.
Biofilm and Sediment Accumulation
Sediment, organic matter, and scale create a protective environment for cooling tower Legionella to thrive. Biofilm shields bacteria from disinfectants, making it harder to eliminate once established.
Poor water quality, infrequent cleaning, or inconsistent chemical treatment allows Legionella in cooling tower water to increase and spread through the system.
Aerosolized Water Droplets
Cooling towers release fine mist as part of the heat-rejection process. If contaminated droplets escape through damaged or outdated drift eliminators, bacteria can enter surrounding air and be carried toward nearby buildings or air intakes.
This is how community or facility-wide outbreaks have historically occurred, even when only one tower system is affected.
Documented Legionella Outbreaks and Safety Concerns
Cooling towers related to legionella disease cases have been reported in major cities around the world, affecting hospitals, hotels, manufacturing facilities, and commercial buildings. In each instance, contaminated water systems contributed to illness, operational shutdowns, and regulatory action.
Over the summer of 2025, more than 80 cases of Legionnaires’ disease were reported in Central Harlem due to outbreaks traced to cooling towers at 11 buildings in the area.
In our own state, the New Jersey Department of Health (NJDOH) receives approximately 250–350 reports of Legionnaires’ disease each year, many of which can be attributed to cooling towers.
Beyond health impacts, organizations face costly remediation, emergency testing requirements, and disruptions that affect both productivity and public confidence. These risks highlight why proactive management matters as part of routine operations.
Best Practices for Controlling Legionella Growth
Keeping Legionella under control requires consistent oversight of water quality, system conditions, and mechanical performance. A combination of routine testing, reliable treatment, and structured maintenance helps reduce areas where bacteria can grow and prevents small issues from turning into larger problems.
By taking care of the system regularly, facility teams can support safer operation and minimize the chances of Legionella developing or spreading through the cooling tower.
Let’s look at three core practices that make the biggest difference.
(1) Routine Water Testing and Monitoring
Regular Legionella testing services help identify bacterial activity early and support compliance with industry standards. A schedule that includes culture sampling, risk assessments, and record-keeping ensures facilities stay ahead of potential problems and maintain safe operating conditions.
(2) Proper Water Treatment & Disinfection
Consistent water treatment using biocides, filtration, and disinfection cycles helps control bacteria before they spread through the system. Treatment plans should be tailored to water quality, temperature, and system load. Regular cleaning of basins and removal of sediment reduces hiding places for bacteria and strengthens long-term control.
(3) Preventive Maintenance Plans
A structured cooling tower maintenance program includes inspection schedules, drift eliminator replacement, mechanical system checks, and proper shutdown/start-up procedures. Keeping equipment clean, repaired, and well-documented significantly reduces the likelihood of Legionella bacteria in cooling tower environments and supports smooth, compliant operation.
Protect Your Facility from Legionella Risks
Legionella bacteria can grow quickly in warm, untreated water and spread through the air in fine droplets, which is why cooling towers require ongoing attention. Routine testing, proper treatment, and consistent maintenance are the most reliable ways to lower the risk of Legionnaires’ disease and protect the people who live, work, or visit near your facility.
At Pinnacle CTS, our team has extensive experience working with cooling tower systems across a wide range of industries. We provide inspections, Legionella testing, water treatment support, and maintenance programs that help keep tower performance stable and reduce bacterial growth throughout the year.
Contact our team to discuss testing or cooling tower services that fit your system’s needs. We’re here to keep your facility safe. If you’re ready for cooling tower water treatment that feels like a true partnership, reach out to schedule a system assessment or water testing. We’re here to keep your tower running the way it should all year long.
