Each year, thousands of people worldwide are diagnosed with Legionnaires’ disease. In the U.S., waterborne pathogens, particularly Legionella and other microbes that thrive in biofilms, are now responsible for an estimated 118,000 hospitalizations and over 6,600 deaths annually, costing roughly $3.33 billion in direct healthcare expenses.
One recent outbreak shows how serious this can be. In August 2025, New York City reported 114 confirmed cases of Legionnaires’ disease and seven deaths, all traced back to Legionella found in several local cooling towers.
If you’re a facility manager or building owner, you should actively manage the risks linked to cooling towers. Neglecting maintenance or water treatment causes outbreaks of Legionnaires’ disease, which can result in illness or death for both building occupants and people living nearby.
Let’s break down what causes these outbreaks, what makes them more likely, and practical ways to keep your facility safe.
Understanding Legionella and Legionnaires’ Disease
What is Legionnaires’ Disease?
Legionnaires’ disease is a serious form of pneumonia caused by infection due to Legionella pneumophila bacteria. Common symptoms include cough, shortness of breath, fever, muscle aches, and headaches.
Hospitalization may be required in serious cases, and the disease can be fatal, particularly for older adults, smokers, and people with weakened immune systems.
How Cooling Towers Spread Legionella Bacteria
Cooling towers create perfect conditions for Legionella to grow and spread. They circulate large volumes of water and generate fine mists and aerosols. If Legionella-contaminated water becomes airborne, in particular from neglected towers, it can put people both inside the building and in the nearby community at risk.
Key Factors That Promote Legionella Growth in Cooling Towers
Sediment and Biofilm Accumulation
Biofilm is a slimy layer that sticks to wet surfaces inside cooling towers. It shelters Legionella, making disinfection less effective. Sediment and debris also block treatment chemicals from reaching bacteria. When biofilm isn’t cleaned out regularly, bacteria can multiply unnoticed and spread through the system, creating a serious health risk.
Water Temperature and Legionella Survival
Legionella grows best in water temperatures between about 68 and 122 degrees Fahrenheit. Many cooling towers work within this temperature range, which makes it easy for the bacteria to multiply if the system isn’t managed carefully.
Cooler water will slow their growth, while temperatures above 140°F (60°C) can kill them, but most cooling tower equipment isn’t designed to run that hot.
Stagnant Water and Water Age
When water isn’t used or circulated regularly, Legionella can quickly multiply. Unused pipes or parts of the system that sit idle give bacteria a chance to settle and spread. Running water through the whole system on a regular schedule keeps it fresh and helps stop Legionella from growing.
Disinfectant Residual Levels
Keeping disinfectant levels steady and effective is important. If these levels dip or fluctuate, bacteria can come back. Using real-time monitoring tools and keeping good records helps find problems early, before Legionella has a chance to multiply.
Cooling Tower Design and Legionella Control
Open vs. Closed-Circuit Cooling Towers
Open-circuit towers let water come into contact with the air, which means they’re more likely to collect biofilm, debris, and bacteria. In contrast, closed-circuit towers move water through a sealed loop, so there’s less exposure but still some risk. Each type of tower needs its own specific plan for water treatment and maintenance.
Design Recommendations for Legionella Prevention
Smart tower design uses drift eliminators to decrease airborne droplets to help keep contaminated mist from leaving the site. Good-quality filters trap sediment and organic matter, while using surfaces and building materials that don’t rust or grow biofilm, making it tougher for bacteria to multiply.
It also helps to have separate pipes for fresh and discharge water, sloped basins for better drainage, and easy-to-reach areas for cleaning and inspections.
Operation and Maintenance Strategies for Cooling Tower Legionella Control
Routine Cooling Tower Disinfection
Disinfection works best when the system is free of buildup. Oxidizing chemicals like chlorine or bromine are most effective when paired with regular cleaning that removes biofilm and sediment. Some facilities also add treatments like chlorine dioxide or UV light for added protection.
Preventive Maintenance Practices
Preventive maintenance goes beyond disinfection. It includes deep cleaning, descaling, and checking for rust or corrosion. Depending on system use, some facilities clean towers monthly, while others do it quarterly. Taking notes and photos during each job makes it easy to prove the work was completed.
Establishing a Water Management Program
A well-structured water management program, like those recommended by the CDC and ASHRAE Standard 188, is very important. Your plan should spell out every procedure, clearly list who’s responsible, include regular monitoring, and have backup plans in case something goes wrong.
Many places around the world now require these programs, so it’s not just a good idea; it can also be the law. For example, NYC Local Law 77 and New Jersey Bill S2188/A1970 require cooling tower registration, routine testing, and documented maintenance to lower Legionella risks.
Testing for Legionella in Cooling Towers
When and How to Test for Legionella
Most experts suggest testing for Legionella at least every three months. Local rules may require more frequent testing, particularly after repairs or when the system has been shut down. Professional Legionella testing services collect water samples from different areas around the system and send them to accredited labs for analysis.
If your building is high-risk, like a hospital or a big apartment complex, you might need to test even more frequently.
Role of Testing During Remediation
After a problem is found, run tests again after cleaning or disinfecting the cooling tower to make sure the issue is resolved. These test results show when it’s safe to start using the tower again or if more cleaning or checks are needed.
Emergency Protocols for Legionella in Cooling Towers
Steps for Suspected Outbreak or Illness
If there is a suspected case of illness related to a cooling tower, take immediate action. The first step is to increase disinfectant concentration and inform local health authorities. The system should be isolated if necessary, and the investigation should begin at once.
Offline Cleaning and Emergency Disinfection
For outbreaks or proven contamination, offline (out-of-service) cleaning is required. Start by draining the system, then use a high dose of disinfectant and flush everything out. Make sure every surface gets physically cleaned before refilling, disinfecting again, and taking samples to confirm the system is safe.
When Outside Help May Be Needed
Sometimes, you’ll need to call in certified water treatment professionals or engineering companies if you keep having recurring issues. Regulatory investigations and major repairs typically require reputable cooling tower services to make sure you meet all compliance and safety standards.
Best Practices for Cooling Tower Legionella Control
Consistent Monitoring and Documentation
Keeping chemistry, temperature, and disinfectant levels in check helps you find problems before they become a hazard. It’s smart to keep records, on paper or on a computer, so you can show what’s been done if there’s ever an inspection, an audit, or an insurance claim.
Training Staff in Legionella Prevention
Staff need hands-on training for running the system, managing chemicals safely, and pointing out warning signs. Meeting up once or twice a year for a quick refresher keeps everyone sharp and helps your team be ready if there’s ever a problem.
Partnering With Experts in Cooling Tower Disinfection
When you work with certified specialists regularly, your system gets inspected by people who know the latest rules and best ways to control bacteria. Their expertise helps protect your facility, lower health risks, and keep you compliant with water treatment and safety standards.
FAQs on Legionella in Cooling Towers
What causes Legionella in cooling towers?
Legionella thrives in water systems that are warm, poorly disinfected, and full of sediment or biofilm. Lack of regular cleaning, improper temperature control, and stagnant water increase the risk.
How often should you test cooling towers for Legionella?
Quarterly testing is recommended for most facilities, but local laws require monthly or event-driven (after repairs) sampling. High-risk sites need more frequent testing.
What is the most effective method of cooling tower disinfection?
Most systems depend on continuous chlorination or bromination to control bacteria. Many facility teams add routine shock treatments and physical cleanings. Using a mix of these methods helps keep harmful bacteria in check.
Can Legionella pneumophila be eliminated from cooling towers?
It’s very difficult to be completely free of Legionella, but a good water management plan can keep the levels low enough to stop anyone from getting sick. The best approach is to monitor regularly and act quickly when something changes.
What should be done during a Legionnaires’ disease outbreak?
Isolate and disinfect the system immediately, notify relevant health authorities, conduct thorough cleaning, and consult experts for further remediation and verification before bringing the tower back into service.
Protecting Against Legionella in Cooling Towers
Managing Legionella in cooling towers is about keeping people safe and protecting your facility’s reputation. Consistent disinfection, preventive maintenance, and regular Legionella testing are the foundation of an effective control strategy.
Staff training, well-designed systems, and expert support make sure these efforts are sustainable, practical, and compliant with global standards.
Protect your facility, your team, and your reputation. Contact Pinnacle CTS today for certified Legionella testing, disinfection, and complete water management support. Call 732-570-9392 or get in touch online to schedule your appointment.
