Mar 14, 2025, 3:40:30 PM

Cooling Towers and Legionella: What Facility Teams Need to Know

14:09

updated 09/25/2026

Cooling towers are critical components in many commercial, institutional, industrial, and mission-critical cooling systems. Because they use recirculating water and evaporation to reject heat, they can also create conditions that support Legionella growth and aerosolization when water systems are not properly designed, operated, and maintained. [1][2]

For facility managers, engineers, maintenance leaders, and operations teams, Legionella control is not a single cleaning task or test. Current public-health and industry guidance emphasizes a systematic approach built around water management, routine maintenance, monitoring, corrective action, and documentation. [1][3]

Understanding the Risk

Legionella bacteria occur naturally in freshwater environments, but they can become a health concern when they grow in building water systems and are dispersed in small water droplets that people can inhale. Cooling towers, evaporative condensers, and fluid coolers are recognized potential sources because their evaporative processes can aerosolize water. [2]

Legionella growth is encouraged by conditions such as warm water, stagnation, biofilm, scale, sediment, and the presence of other microorganisms. OSHA identifies 68°F to 122°F (20°C to 50°C) as a temperature range in which Legionella can multiply, with optimal growth occurring within a narrower portion of that range. [2]

Prevent Legionella in your Cooling Towers

Focus on the Conditions That Allow Legionella to Grow

Rather than treating temperature as the only control point, facility teams should evaluate the broader conditions within the water system. CDC's current Legionella Control Toolkit highlights four key factors when assessing growth risk: sediment and biofilm, temperature, water age, and disinfectant residual. [4]

- Sediment and biofilm can provide nutrients and protected environments where microorganisms persist.
- Water temperature can influence bacterial growth.
- Water age and stagnation can allow disinfectant levels to decline and biofilm to develop.
- Disinfectant residual must be managed as part of the facility's established water treatment and control strategy.

Scale, corrosion, debris, fouled surfaces, and poor water distribution can also affect tower condition and create maintenance challenges. Regular inspection and cleaning help facility teams identify and address these conditions before they become larger reliability or water-management concerns. [2]

Build Legionella Control Around a Water Management Program

An effective Water Management Program (WMP) should be the foundation of Legionella risk management. CDC and OSHA both emphasize water management programs as a primary method for reducing Legionella growth and exposure in building water systems. [1][3]

A facility-specific WMP should identify relevant water systems and devices, establish control measures and limits, define how those controls are monitored, outline corrective actions when limits are not met, and maintain documentation that supports verification and ongoing program improvement. [3]

Cooling tower maintenance should support that broader program. Cleaning, decontamination, inspection, water treatment, mechanical maintenance, and documentation each address different conditions that can affect system cleanliness, reliability, and biological control.

Cooling Tower Legionella Control Best Practices

1. Inspect, Clean, and Disinfect the Tower

Routine inspection and cleaning are central to Legionella control. OSHA recommends scheduled cleaning and disinfection of cooling towers and additional cleaning for systems that have been out of service for extended periods. [2]

Facility teams should pay attention to cold-water and hot-water basins, fill, drift eliminators, water distribution components, louvers, and accessible internal surfaces. The goal is to control debris, sediment, biofilm, scale, and other conditions that can support biological growth or interfere with system performance.

OBR's cooling tower service offerings include inspection, cleaning and decontamination, pressure washing, debris removal, preventative maintenance, and service documentation.

2. Use a Water Treatment Program Designed for the System

Water chemistry and microbiological control should be managed through a qualified water treatment program appropriate for the tower and its operating conditions. Treatment strategies may include oxidizing or non-oxidizing biocides, corrosion and scale control, blowdown management, and monitoring of established control parameters.

OSHA notes that cooling-system water treatment requires monitoring of factors such as disinfectant levels and pH, while CDC guidance emphasizes maintaining disinfectant residual as one of the key factors in Legionella control. [2][4]

Automated treatment, monitoring, and blowdown controls can help facilities maintain established operating parameters more consistently, but the appropriate control strategy should be determined within the facility's WMP and water treatment program. [4]

3. Reduce Stagnation and Maintain Circulation

Stagnant water can promote biofilm development and create conditions favorable to Legionella growth. Facility teams should identify dead legs, low-flow areas, idle equipment, and operating practices that allow water to remain stagnant. [2]

System design, operation, and maintenance should promote appropriate circulation and minimize unnecessary water age wherever practical.

4. Plan for Shutdowns, Startups, and Extended Idle Periods

Periods of low use, seasonal shutdowns, maintenance outages, and recommissioning deserve specific attention in the WMP. OSHA recommends cleaning and disinfecting cooling systems that have been out of service for extended periods, as well as new systems where construction residue may contribute to biological growth. [2]

Facilities should establish documented procedures for shutdown, startup, recommissioning, treatment restoration, inspection, and any required cleaning or disinfection before returning equipment to normal operation.

5. Manage Drift and Consider Nearby Air Intakes

Legionella exposure risk depends not only on bacterial growth but also on how contaminated water can become aerosolized and reach people. OSHA recommends high-efficiency drift eliminators and advises locating fresh-air intakes so they do not draw mist or water vapor from cooling towers, evaporative condensers, or fluid coolers into the HVAC system. [2]

Existing facilities should include drift eliminator condition, tower discharge, nearby air intakes, windows, doors, and surrounding site conditions in routine inspection and risk assessment.

6. Use Legionella Testing as Part of a Broader Control Strategy

Routine Legionella testing can provide useful information, but testing should be considered within the context of the facility's WMP rather than as a substitute for water management and maintenance. CDC's 2026 routine-testing guidance notes that testing can help establish baseline measurements, validate a WMP, evaluate potential growth and transmission sources, and confirm whether remedial treatment was successful. [5]

CDC also notes that routine testing may be particularly beneficial for facilities serving people at increased risk, facilities that have difficulty maintaining control limits, and facilities with a history of associated Legionnaires' disease cases. [5]

Sampling plans, laboratory methods, interpretation, and corrective actions should be established before testing so the facility knows how results will be used.

7. Keep Detailed Records

Documentation is a core part of effective water management. Records may include inspection findings, cleaning and disinfection activities, water-treatment data, monitoring results, testing results when applicable, corrective actions, equipment changes, and maintenance history.

Good documentation gives facility teams greater visibility into system condition, supports maintenance and capital planning, and helps demonstrate that established water-management procedures are being followed.

8. Train the People Responsible for the Program

The people responsible for operating, monitoring, and maintaining the system should understand their roles within the WMP. Training should address the facility's control measures, monitoring responsibilities, documentation procedures, escalation process, and corrective actions.

Clear responsibilities help turn the WMP from a written document into an active operating process.

Industry Standards and Guidance to Know

ANSI/ASHRAE Standard 188-2021 establishes minimum Legionellosis risk-management requirements for building water systems. ASHRAE states that the standard is relevant to the design, construction, installation, commissioning, operation, maintenance, and service of centralized building water systems and components. [6]
ASHRAE Guideline 12 provides additional guidance for managing Legionellosis risk in building water systems and is referenced by CDC's Legionella Control Toolkit. [5]
ANSI/ASHRAE Standard 514-2023 addresses broader building water-system risk management, including physical, chemical, and microbial hazards. Standard 188 remains the Legionellosis-focused standard. [9]
CDC's Legionella Control Toolkit provides practical guidance for developing and strengthening Water Management Programs and includes modules specific to cooling towers and routine testing. [4][5]

Standards and guidance evolve. Facility teams should verify the current edition and determine which requirements apply to their specific building, system, and jurisdiction.

Know Your State and Local Requirements

Cooling tower requirements are not uniform across the United States. Some jurisdictions have specific rules governing registration, maintenance plans, testing, cleaning, disinfection, record keeping, and reporting.

New York State is one example. Owners of cooling towers in New York are required to register towers, prepare and implement a Maintenance Program and Plan, perform required Legionella testing, clean and disinfect towers, and meet applicable reporting requirements. [7] New York's regulations also establish specific sampling intervals for operating cooling towers. [8]

Facilities should determine the requirements that apply in each jurisdiction where they operate rather than assuming a single national procedure covers every location.

What Facility Teams Should Look for During Cooling Tower Inspections

A thorough cooling tower inspection can support both reliability planning and water-management efforts. Depending on the tower and scope, facility teams should evaluate:

- Sediment, debris, biofilm, algae, scale, and visible fouling
- Fill condition and water distribution
- Drift eliminator condition
- Basins, casing, structural components, and corrosion
- Fans, motors, belts, gearboxes, driveshafts, bearings, and other mechanical components
- Leaks, low-flow areas, or conditions that may contribute to stagnation
- Access and safety concerns that could interfere with routine maintenance
- Maintenance history, treatment records, and unresolved recommendations

The objective is not simply to identify contamination. It is to understand tower condition, identify developing problems, and give the facility a practical plan for maintenance, cleaning, repairs, and future capital work.

How OBR Supports Cooling Tower Maintenance and Decontamination

A well-maintained cooling tower is an important part of an effective Legionella risk-management strategy. OBR Cooling Towers supports facility teams with professional cooling tower inspections, cleaning and decontamination, preventative maintenance, mechanical service, and detailed service documentation.

OBR's service process can include pre- and post-decontamination, debris removal, pressure washing, mechanical inspection and maintenance, distribution-system inspection, general tower inspection, and reporting. These services can support the maintenance portion of a facility's broader Water Management Program.

Water-treatment program design, Legionella sampling strategy, laboratory analysis, regulatory interpretation, and WMP oversight should be performed by the appropriately qualified parties responsible for those functions.

The Bottom Line

Legionella risk reduction in cooling towers requires more than an occasional cleaning or a single test result. Effective control combines a facility-specific Water Management Program with proper operation, water treatment, inspection, cleaning and disinfection, monitoring, documentation, and timely corrective action.

For facility managers, maintenance leaders, and engineers, the practical goal is straightforward: understand the tower's condition, control the conditions that support biological growth, maintain the equipment consistently, and document the work.

Need support with the cooling tower maintenance portion of your water-management strategy? Contact OBR Cooling Towers to discuss inspection, cleaning and decontamination, and preventative maintenance options for your facility.