Cooling towers are an essential part of many air conditioning and industrial cooling systems. However, if not properly designed, maintained and monitored, they can also become a serious public health risk. One of the most significant hazards associated with cooling towers is Legionella, a bacterium that can cause Legionnaires’ disease.

Understanding how and why this risk occurs — and how it is managed — is critical for building owners, facility managers and anyone responsible for maintaining cooling tower systems.

What is Legionella?

Legionella is a naturally occurring bacterium found in water. It can multiply rapidly in man-made water systems under the right conditions. Infection occurs when contaminated water droplets (aerosols) are inhaled into the lungs — not through drinking water, and not through person-to-person contact.

Legionnaires’ disease can present as flu-like symptoms initially, but in severe cases may lead to pneumonia, intensive care admission, and death. Outbreaks are often difficult to trace and diagnose, which makes prevention and control essential.

Why cooling towers are high-risk systems

Cooling towers operate by dispersing warm water over internal surfaces while air is drawn through the system. This process efficiently removes heat — but it also creates ideal conditions for Legionella growth:

• Warm water temperatures between 20°C and 42°C
• Constant moisture
• Large surface areas where biofilm can form
• Aerosol generation
• Open exposure to the surrounding environment

If Legionella is present, contaminated aerosols can be released into the air and carried significant distances beyond the site boundary.

Temperature matters

Legionella behaviour is strongly influenced by temperature:

• Below 20°C: Bacteria remain dormant
• 20–42°C: Bacteria multiply rapidly
• Above 50°C: Bacteria begin to die
• Above 60–70°C: Bacteria will not survive

Unfortunately, cooling towers typically operate squarely within the temperature range that encourages growth, making active management essential rather than optional.

Common sources of contamination

While cooling towers are a key focus, Legionella can also be found in:

• Warm water systems and showers
• Decorative fountains
• Humidifiers
• Spa pools and hydrotherapy pools
• Ice machines
• Evaporative air coolers

This reinforces the need for a system-wide approach to water hygiene, not isolated fixes.

What increases the risk?

Risk assessments consistently identify several critical contributors to Legionella proliferation:

Stagnant water

Systems (or parts of systems) that sit idle for extended periods allow bacteria to multiply. Dead legs, poor circulation and infrequent use are common culprits.

Nutrient growth

Environmental contamination such as dust, soil and organic matter can enter cooling towers, feeding bacterial growth — particularly where corrosion control or biodispersants are inadequate.

Poor water quality

Inadequate treatment programs, lack of automated biocide dosing, or inconsistent monitoring significantly increase risk.

System deficiencies

Design flaws, ageing infrastructure, poor physical condition, inefficient drift eliminators and inadequate aerosol control all contribute to elevated exposure risk.

Location and access

Cooling towers located near hospitals, aged care facilities, business districts or high-density public areas pose a higher potential exposure risk and therefore require stricter controls.

Risk management is not guesswork

Australian and New Zealand Standard AS/NZS 3666.3 outlines a structured, evidence-based approach to cooling tower risk management. Victoria and NSW also provide specific guidance on assessing the risks of legionella proliferation in cooling towers. This includes asking critical questions about:

• How the system operates
• How often it is used
• How it is maintained
• Who may be exposed

Risk is then classified from A (highest) to D (lowest) (VIC) or Low/Medium/High (NSW) based on factors such as stagnation, nutrient exposure, water quality, system condition and proximity to people.

The objective is clear:
 reduce the overall risk classification wherever possible — ideally moving systems from higher to lower risk.

Monitoring and response thresholds

Routine testing plays a vital role in determining if a tower is under control, and specific responses to testing results are required by AS366 and state/territory legislation. 

Heterotrophic Colony Count (HCC)

HCC is a general indicator of microbial activity:

• <100,000 cfu/mL: System under control
• ≥100,000 cfu/mL: Immediate review and corrective action required

Legionella testing

Specific Legionella results trigger defined responses:

• Not detected (<10 cfu/mL): Maintain program
• Detected <1,000 cfu/mL: Immediate disinfection and review
• Detected ≥1,000 cfu/mL: Immediate decontamination and urgent remedial action

These thresholds ensure responses are proportionate, timely and effective.

Matching risk with operational effort

Operational requirements scale with risk classification:

• Higher-risk systems require more frequent inspections, servicing and testing
• Lower-risk systems still require consistent monitoring, but at reduced frequency

Across all risk categories, six-monthly cleaning is the absolute minimum expectation — with more frequent cleaning required where environmental contamination is likely or the tower is high risk.

The role of Risk Management Plans (RMPs)

LCooling tower owners should prepare and implement a Risk Management Plan.

An effective RMP includes:

• A comprehensive risk assessment
• Defined control strategies
• Operational and monitoring programs
• Clear communication pathways
• Ongoing review and improvement

Annual audits and inspections ensure these plans remain effective and up to date.

Can the risk be eliminated entirely?

A fundamental principle of risk management is to first ask:
 Can the risk be removed altogether?

In some cases, this may mean decommissioning a cooling tower and replacing it with an alternative system. Where this isn’t feasible, the focus shifts to risk reduction through best practice design, operation and maintenance.

QED’s approach

At QED Environmental Services, we work with building owners, facility managers and asset operators to:

• Assess cooling tower risks accurately
• Develop compliant, practical Risk Management Plans
• Implement effective monitoring and treatment programs
• Support audits, inspections and regulatory compliance

Our approach is grounded in Australian standards, state legislation and real-world operational experience — ensuring risks are managed proactively, not reactively.

Final takeaway

Cooling towers don’t have to be dangerous — but they do require diligence, expertise and ongoing oversight. Legionella risk is manageable when systems are properly designed, monitored and maintained.

The cost of inaction, however, can be severe — for public health, regulatory compliance and organisational reputation.

If you’re responsible for a cooling tower system, understanding and managing this risk isn’t just best practice — it’s essential.