How Your HVAC System Might Increase Risks of Legionnaires’ Disease

Air conditioning plays an important role in keeping homes, workplaces and care settings comfortable — especially during Australia’s hot summers. However, air conditioning systems are also environments where moisture, dust, bacteria and mould can build up over time if not properly maintained.

But while air conditioning itself doesn’t cause Legionnaires’ disease, poorly maintained systems can create conditions where Legionella bacteria may grow.

Understanding how and where Legionella can grow inside air conditioning systems is a key step in prevention. This article explains the conditions that allow the bacteria to multiply, which parts of HVAC systems are most vulnerable, and why regular hygiene and maintenance matter — especially during warmer months.

What is Legionella and how does it spread?

Legionella is a type of bacteria that occurs naturally in the environment. It is commonly found in freshwater sources such as rivers, lakes, creeks and soil, usually at low levels that do not pose a risk to human health. Problems arise when the bacteria enter man-made water systems and are able to multiply to higher levels.

When conditions allow Legionella to grow unchecked, it can cause Legionnaires’ disease, a serious form of pneumonia, or a milder flu-like illness known as Pontiac fever. Infection does not occur from swallowing water or from contact with other people — it occurs through inhalation.

How Legionella spreads

Legionella spreads when people breathe in tiny airborne droplets of water (aerosols) that contain the bacteria. These droplets are so small they are invisible to the naked eye and can be inhaled deep into the lungs without being noticed.

Common sources of aerosolised water can include:

• Cooling towers and components of air-conditioning systems
  

• Showers, taps and plumbing systems
  

• Spas, fountains or other water features
  

• Any system that sprays, mists or circulates water

Once inhaled, the bacteria can reach the lungs and cause infection, particularly in people who are more vulnerable due to age, smoking history, chronic lung disease or weakened immune systems. It’s important to note that Legionella is not spread from person to person.

Why air conditioning systems can create ideal conditions

Air conditioning systems don’t “create” Legionella on their own — but certain parts of HVAC systems can accidentally provide the conditions that help the bacteria multiply if moisture, temperature and hygiene aren’t well managed. Understanding these conditions makes it much easier to see where risk can build up and why maintenance matters.

1) Moisture is unavoidable in many HVAC systems

Most air conditioners remove humidity from the air as part of normal operation. That moisture condenses on cooling surfaces and is meant to drain away safely. If drainage isn’t working properly — or if water is allowed to sit in trays, pipes or internal components — it creates an environment where bacteria can persist and multiply.

Examples of how moisture can build up:

• Condensation collecting in drip trays or drain pans
  

• Blocked or slow drains causing water to pool
  

• Hidden damp spots around coils or inside ducting
  

• Areas that stay wet because airflow is poor

2) Warm temperatures can support bacterial growth

Legionella bacteria grow best in warm water, and many HVAC-related components can sit within temperature ranges that support growth — particularly during Australian summers and heatwaves. Even if the air coming out of the unit feels cool, internal parts of a system can still experience warm conditions, especially where water sits and warms over time.

Heatwaves can intensify this effect by:

• Increasing overall ambient temperatures in plant rooms, roofs and enclosed service spaces
  

• Increasing system runtime (more hours operating = more moisture cycling through the system)
  

• Warming stored or stagnant water in components that aren’t flushing regularly

3) Stagnant or slow-moving water is a major risk factor

Legionella risk increases most when water is not moving, because stagnation allows bacteria to multiply and concentrate. In HVAC systems, stagnant water can occur during shutdowns, low-use periods, or when systems are only run intermittently.

Common scenarios include:

• Office buildings, schools or accommodation facilities that close for holidays
  

• Systems that sit idle over weekends or between seasons
  

• Units with blocked drains or areas where water collects and doesn’t clear

When systems restart after a period of stagnation, any built-up contamination may be disturbed — which is why routine checks and scheduled maintenance before peak season are so important.

4) Biofilm and “built-up grime” can protect bacteria

Even more important than water alone is biofilm — a thin, slimy layer of microorganisms that can form on damp internal surfaces. Biofilm acts like a protective home for bacteria. Once it forms, it can:

• Help Legionella survive and multiply
  

• Make bacteria harder to remove with basic cleaning
  

• Allow bacteria to persist even if conditions change temporarily

Dust, organic material and scale can also accumulate in moist areas, feeding microbial growth and making the environment more hospitable for bacteria.

5) Aerosols can be produced in certain HVAC setups

Legionella spreads when contaminated water becomes airborne as tiny droplets. Some HVAC and cooling systems — particularly larger commercial systems — can generate aerosols through water circulation and cooling processes. That’s why high-risk systems like cooling towers are managed under strict maintenance and monitoring programs.

Where Legionella can grow inside air conditioning systems

Within air conditioning and HVAC systems, Legionella doesn’t grow everywhere — it tends to establish itself in specific areas where moisture, warmth and poor drainage come together. Knowing where these risk points are helps explain why routine inspection and targeted cleaning are so important.

1) Cooling coils

Cooling coils remove heat and moisture from the air, which means condensation forms on their surfaces during normal operation. If coils are not cleaned regularly, dust and organic debris can mix with moisture, creating an ideal surface for microbial growth. Over time, this can allow biofilm to form and protect bacteria such as Legionella.

2) Condensate drain pans and drip trays

Drain pans and drip trays are designed to collect and remove condensed water. If drains become blocked, slow or poorly sloped, water can pool and remain stagnant. These shallow pools of warm water are one of the most common places for bacterial growth inside HVAC systems.

3) Drain lines and pipes

Drain lines that are partially blocked or infrequently flushed can retain standing water. Sediment and biofilm can accumulate along the inside of pipes, allowing bacteria to persist even when the system appears to be functioning normally.

4) Cooling towers (commercial systems)

In larger commercial or industrial HVAC systems, cooling towers are a well-known risk area. These systems circulate warm water and can generate aerosols if not properly treated, monitored and maintained. Because of this, cooling towers are subject to strict maintenance and compliance requirements.

5) Ductwork with moisture build-up

While ductwork is typically dry, condensation can occur if insulation fails, humidity is high, or airflow is uneven. Damp sections of ducting can trap dust and organic material, creating pockets where microbes may grow unnoticed.

6) Filters and internal surfaces

Filters that are clogged or overdue for replacement can hold moisture and debris. While filters are designed to capture particles, they can become secondary reservoirs for bacteria if not maintained as part of a broader hygiene plan.

Supporting HVAC hygiene: What helps reduce risk

Reducing the risk of Legionella growth in air conditioning systems comes down to managing moisture, removing microbial build-up and preventing regrowth over time. While routine servicing and inspections are essential, hygiene-focused cleaning plays a key role in addressing the areas where bacteria are most likely to persist.

This is where purpose-designed HVAC hygiene solutions can support broader maintenance programs. SAN-AIR develops professional-grade cleaning and antimicrobial solutions specifically for indoor air, HVAC and surface hygiene, with a focus on reducing microbial load rather than simply masking odours.

SAN-AIR products are formulated using plant-based active ingredients combined with advanced antimicrobial technology, designed to disrupt the growth cycle of bacteria, mould and fungi. Instead of relying on harsh solvents or corrosive chemicals, their solutions work by targeting microbial survival mechanisms — helping to limit regrowth on treated surfaces while remaining suitable for use on HVAC components and air-handling systems.

Within HVAC maintenance programs, SAN-AIR solutions are typically used to:

• Remove existing microbial contamination such as mould, grease and biofilm
  

• Treat internal surfaces where moisture and warmth allow bacteria to thrive
  

• Provide longer-lasting antimicrobial protection between scheduled services

This approach complements routine mechanical servicing by addressing the hygiene side of HVAC systems.

Step 1: Targeted coil & machine cleaning (critical first step)

Before general degreasing or antimicrobial protection, cooling coils and machine components need targeted cleaning. These areas are constantly exposed to condensation, dust and airflow — making them one of the highest-risk zones for microbial build-up inside HVAC systems.

SAN-AIR Hygiene HVAC-R Coil & Machine Cleaner Concentrate is designed specifically for this stage.

It focuses on removing dirt, grease, biofilm and organic residue that can trap moisture and allow bacteria, mould and fungi to establish themselves.

Why this step matters:

• Dirty coils reduce airflow and system efficiency
  

• Biofilm on coils provides a protective layer for microbes
  

• Moisture held on contaminated surfaces creates ideal growth conditions

How it’s typically used:

• Dilute as directed and apply directly to coils and machine components
  

• Spray onto surfaces or apply with a suitable applicator
  

• Allow to penetrate to loosen embedded debris and microbial residue
  

• Rinse or wipe away, leaving coils clean and ready for further treatment

This step is especially important for:

• Cooling coils
  

• Evaporator and condenser units
  

• Internal machine surfaces exposed to condensation

Cleaning coils first ensures subsequent cleaning and antimicrobial steps are far more effective.

Step 2: Deep cleaning to remove broader microbial build-up

Once coils and machine components are clean, broader internal HVAC surfaces should be addressed.

SAN-AIR SAS Heavy Duty Microbial Degreaser Cleaner is used at this stage to remove grease, grime, mould and existing microbial contamination from areas such as drip trays, drain pans, housings and internal surfaces.

It is a non-corrosive formulation suitable for use on HVAC-R systems and air handlers, helping extend the life of equipment while improving hygiene. As it comes into contact with microbes and mould on surfaces, it works by stopping bacteria from reproducing and preventing mould from receiving oxygen, limiting further growth and spread

How it’s typically used:

• Apply by spray bottle or clean cloth to internal HVAC surfaces

• Heavily soiled areas may be lightly scrubbed to loosen debris

• Leave to sit for several minutes to penetrate biofilm and residue

• Rinse or wipe clean before further treatment

Step 3: Applying antimicrobial surface protection

Once all surfaces are clean and dry, a protective antimicrobial layer can be applied to help reduce the chance of bacteria, mould and fungi re-establishing between maintenance cycles.

SAN-AIR HVAC-R Anti-Microbial Surface Sealer Concentrate is formulated to leave a long-lasting antimicrobial barrier on HVAC components while remaining non-corrosive to system materials.

It is designed to kill 99.99% of bacteria, mould, fungus and spores, while remaining non-corrosive to HVAC materials

Where it can be applied:

• Air-conditioning ducting and return air boxes

• Refrigeration units

• Internal HVAC-R components after cleaning

Application approach:

• Surfaces must be cleaned first using a microbial cleaner

• The product is diluted with water and applied via spray, brush, roller or mop

• Allow to dry to form a protective antimicrobial barrier

• Reapplication is generally recommended annually or as part of scheduled maintenance

Prevention starts with proper HVAC hygiene

Legionnaires’ disease doesn’t start with air conditioning itself — it starts when warmth, moisture and poor maintenance create the right conditions for bacteria to grow. As we’ve seen, HVAC systems can unintentionally provide these conditions if coils, drain pans and internal surfaces aren’t cleaned and protected properly.

That said, the risks can be significantly reduced with regular maintenance and a structured hygiene approach. Thorough cleaning to remove biofilm and debris, followed by antimicrobial surface protection, helps limit the environments where Legionella and other microbes can thrive.

This layered approach becomes even more important during Australia’s hotter months, when systems run harder, moisture levels rise and microbial growth accelerates. Whether in homes, workplaces, healthcare settings or aged care facilities, proactive HVAC hygiene supports safer air, better system performance and greater peace of mind.

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