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Legionella in Air Conditioning Systems: Can Legionella Spread Through AC?

October 26th, 2023

Last updated: December 18th, 2023

Air conditioning systems have become a common feature in modern life, providing heating and cooling in homes, offices, and public spaces.

You may have heard horror stories about HVAC systems carrying Legionella, a bacterium responsible for Legionnaires’ disease, a potentially fatal respiratory illness. In this blog post, we will explore whether Legionella can be carried and grow in air conditioning systems and discuss preventive measures to mitigate this threat.

Can Air Conditioning Systems Spread Legionella?

The short answer is yes. Legionella can indeed be carried in air conditioning systems. These systems provide an ideal environment for the growth and transmission of the bacterium under the right conditions.

Here’s how it can happen:

Water as a Vector: Some air conditioning systems, especially cooling towers and evaporative coolers, use water for temperature regulation. If this water is contaminated with Legionella, the bacteria can then spread through the air, posing a risk to anyone in the vicinity.

Legionella thrives at temperatures between 77°F (25°C) and 113°F (45°C), which are often the typical operating temperatures of air conditioning systems.

If any water at all within the system becomes stagnant or is not properly maintained, it can encourage Legionella growth. Stagnation allows biofilms to form, providing an excellent breeding ground for the bacterium.

Can Legionella Grow in Air Conditioning Systems?

Yes, Legionella can grow in air conditioning systems, particularly in parts where water is involved. Legionella thrives in biofilms, slimy layers of microorganisms that can form on surfaces, including pipes, tanks, and cooling coils. These provide a protective environment for the bacterium to multiply.

The good news is that air conditioning systems are rarely the root cause, just a way for legionella to potentially spread. So with some measures in place it is possible to prevent legionella.

Preventing Legionella in Air Conditioning Systems

  • Regular Maintenance: The key to preventing Legionella growth is maintaining air conditioning systems properly. This includes regular cleaning and disinfection of water reservoirs, coils, and any areas where water stagnates.
  • Water Treatment: The use of biocides or chlorine to treat the water in cooling systems can inhibit the growth of Legionella and other harmful microorganisms.
  • Risk Assessment: Employ a qualified professional to conduct a risk assessment of your air conditioning system to identify potential sources of Legionella and develop a prevention and maintenance plan.
  • Adequate Ventilation: Proper ventilation can help disperse any potential aerosols carrying Legionella, reducing the risk of exposure.
  • Legionella Testing: Regular testing for Legionella in air conditioning systems can help detect contamination early and take appropriate action.


Legionella is a hidden threat that can lurk in air conditioning systems. While the risk is present, it can be effectively managed with regular maintenance, temperature control, water treatment, risk assessment, and testing. By implementing these preventive measures, you can enjoy the benefits of air conditioning without worrying about the potential health hazards associated with Legionella. Staying informed and proactive is the key to ensuring the safety and comfort of your indoor environment.

We can provide you with regular help and planned maintenance for your water systems, including water sampling, giving you peace of mind and legal compliance. We can also help with Legionella awareness and staff training across the north of England.

Working across Yorkshire, we cover North YorkshireWest YorkshireEast Yorkshire and South Yorkshire and offer a fast, efficient response time to all breakdowns and repairs. Contact us today for all plannedreactive maintenance or emergency call out. Call 01423 226578 or get in touch to see how Robinsons Facilities Services can help.

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