What Happens When Cooling Tower Maintenance Lapses
Picture a mid-sized office building facility team preparing for the upcoming Labor Day weekend. To cut costs during a severe late-August heatwave, building management temporarily reduces the frequency of automated biocide treatments in the rooftop cooling tower. They assume the existing chemical residuals will hold over the holiday break. Within two weeks, aerosolized water drift from the tower enters the building's fresh air intake. Several office workers on the upper floors develop severe pneumonia, later confirmed as Legionnaires' disease. The cooling tower water temperature had settled in the optimal growth range for Legionella bacteria, and the lack of chemical treatment allowed the biofilm to multiply unchecked.
Legionella bacteria thrive in warm water environments. Cooling towers reject building heat by evaporating water, which naturally places the remaining basin water in a temperature zone that accelerates bacterial growth during late summer. When the water is not properly treated with biocides, the bacteria multiply within the protective slime, known as biofilm, on the tower's surfaces. The mechanical action of the tower's fans creates a fine mist or aerosol. If this contaminated mist drifts into the building's HVAC air intakes or falls on people walking below, the bacteria are inhaled directly into the lungs, causing severe respiratory illness. Crews often underestimate this hazard because the bacteria are invisible, and the resulting illness is easily mistaken for common seasonal pneumonia until it is too late.
Key Components
1. Water Temperature and Stagnation
- Legionella growth accelerates rapidly when building water systems remain in warm temperature bands.
- Dead legs—sections of piping with low or no flow—create stagnation where temperatures can easily enter this dangerous range.
- Late summer heat waves mean cooling towers operate constantly, keeping basin water consistently warm and primed for bacterial growth.
- Routine flushing of system dead legs is necessary to prevent the stagnation that allows bacteria to multiply.
2. Biofilm and Scale Amplification
- Biofilm is a slimy layer of microorganisms that forms on wet surfaces and physically protects Legionella from chemical treatments.
- Scale and corrosion in pipes provide nutrients and hiding places for the bacteria to multiply, creating a microscopic fortress.
- Standard chlorine treatments often cannot penetrate thick biofilm, requiring mechanical cleaning of the cooling tower basin to physically remove the hazard.
- Routine inspections must physically check for scale, rust, and slime buildup in the cooling tower and associated piping before it becomes unmanageable.
3. Aerosol Generation and Drift
- Cooling towers are designed to move massive amounts of air through water, inherently creating aerosols that can carry bacteria over long distances.
- Drift eliminators are physical engineering controls—baffles designed to catch water droplets before they exit the tower exhaust.
- If drift eliminators are damaged, missing, or improperly installed, contaminated mist escapes into the surrounding air and can travel on wind currents.
- Building fresh air intakes located too close to cooling tower exhausts can draw this mist directly into the office environment, exposing occupants who never even set foot on the roof.
Building Your Safety Mindset
- Question the Treatment Schedule
- Never assume the automated chemical dosing system is working perfectly without physical verification of the chemical reservoirs. - Recognize that extreme summer heat requires more frequent testing of biocide levels to maintain safe water chemistry. - Understand that a lapse over a long weekend—like the upcoming Labor Day break—is enough time for a dangerous bacterial bloom to occur.
- Look for the Physical Warning Signs
- Notice unusual odors coming from the HVAC vents, which can indicate biological growth somewhere in the system. - Report visible algae, slime, or heavy scale around rooftop cooling towers, decorative water features, or drain pans. - Check the condition of drift eliminators during routine roof inspections; they should be intact, clean, and properly seated.
- Prioritize System Design and Controls
- Use the hierarchy of controls: elimination and engineering are far more effective than administrative checks. - Ensure fresh air intakes are positioned upwind or adequately distanced from cooling tower exhausts to prevent drift ingestion. - Support the implementation of a comprehensive water management program to systematically monitor and treat building water systems.
Discussion Points
- Where are our building's fresh air intakes located in relation to the cooling tower, and what is the first sign we are going off-plan with our water treatment?
- Thinking about the hierarchy of controls (elimination -> substitution -> engineering -> administrative -> PPE), what is the highest-order control we can use TODAY to prevent Legionella exposure?
- If you notice a lapse in our water treatment logs or see visible slime on the cooling tower, how do we ensure everyone feels comfortable speaking up without fear of retaliation?
Action Steps
- Verify biocide dosing system chemical levels Owner: Facility Manager Due: Today 10:00 AM
- Inspect drift eliminators for physical damage Owner: Maintenance Technician Due: Today 12:00 PM
- Flush dead legs in the piping network Owner: HVAC Technician Due: Before Labor Day
Every worker has the authority to stop work if asked to mechanically clean a contaminated cooling tower basin without verified respiratory protection—with zero retaliation.
The Facility Manager will report back at tomorrow's huddle on the status of the biocide reservoirs and drift eliminators.
Do we have the proper biocide levels and intact drift eliminators in place RIGHT NOW?
What are we doing differently today to manage aerosolized water hazards?