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Can air conditioning spread legionella?

Updated 23 October 2026 · SEO Dons Editorial

Can an air conditioner spread legionella?

Legionnaires' disease is caught by breathing in small droplets of water that contain the bacteria, so the risk sits in water systems, not in the act of cooling air. HSE says any water system, with the right conditions, could be a source. The conditions it lists are a water temperature between 20–45°C, a way of creating or spreading breathable droplets, stored or recirculating water, and nutrients such as rust, sludge, scale or biofilm.

That splits air conditioning in two. A refrigerant-based split, cassette or VRF unit cools air across a coil, and the water that forms there is condensate carried out through a drain. A system that rejects heat by spraying or recirculating water, or humidifies air with water, can meet all four conditions. What a service visit covers on the refrigerant side is on the air conditioning service page.

Why do people say air conditioning causes legionella?

The wording is loose. Lancaster City Council describes Legionnaires' disease as a lung infection you can get from inhaling droplets of water from things like air conditioning or hot tubs. Thurrock Council is more specific: most community outbreaks in the UK have been linked to installations such as cooling towers, which are found as part of air-conditioning and industrial cooling systems. Doncaster Council adds that some air conditioning systems, for example those which use evaporative cooling, may pose potential health risks if not properly maintained. The source is the wet plant, not the refrigerant circuit.

Which air conditioning plant is a legionella risk?

HSE's list of the systems most likely to create a risk covers evaporative cooling systems, hot and cold water systems and spa pools, and it names humidifiers and air washers among other systems that could be a source. Split, cassette, ducted and VRF units are not named. That does not remove the duty to assess the water systems on the premises.

PlantWater involvedWhat HSE's pages sayWhat follows for the duty holder
Split, cassette, ducted and VRF unitsCondensate that drains awayNot named as a risk systemCovered by the general risk assessment. Keep trays and drains clean and draining; CIBSE includes condensate drain pans in a weekly visual inspection
Cooling towers and evaporative condensersRecirculating water exposed to airHSG274 Part 1; local authority notificationWritten control scheme, monitoring and records. See the cooling tower guide
Dry/wet ("hybrid" or "adiabatic") coolersWater used in wet modeA wide range of risk profiles; Part 1 appliesAssess the actual design with a competent person; check whether notification applies
Humidifiers and air washers in air handling unitsWater held or sprayed in the unitNamed as other risk systems; Part 3 appliesAssess, keep clean, monitor where appropriate, keep records. See the humidification guide
Condensate trays with standing waterWater that stands instead of drainingNot named; the general conditions apply to any water systemPrevent standing water by cleaning and drainage; a competent person decides if more is needed

What do L8 and HSG274 require of the duty holder?

L8 is HSE's Approved Code of Practice, now in its fourth edition (2013). HSE says it is aimed at duty holders, including employers and those in control of premises, and covers assessing sources of risk, preparing a scheme to prevent or control risk, monitoring precautions, keeping records and appointing a manager to be responsible. HSG274 is the technical guidance beside it: Part 1 for evaporative cooling systems, Part 2 for hot and cold water and Part 3 for other risk systems. Its second edition (March 2024) was updated in Part 1 only. HSE's "what you must do" pages give the sequence:

  1. Identify and assess. The risk assessment is the duty holder's responsibility, with a competent person's help if needed.
  2. Appoint. Someone competent, often called the responsible person, takes responsibility for controlling any identified risk.
  3. Prevent or control. First ask whether the risk can be removed. HSE's example is replacing a wet cooling tower with a dry air-cooled system. If a risk cannot be prevented, a written control scheme sets out the controls.
  4. Monitor and keep records. With 5 or more employees, significant findings must be recorded. Records should be kept while current and for at least 2 years after, and monitoring records for at least 5 years.
  5. Review. Review regularly, particularly when there is reason to suspect the assessment is no longer valid.

On frequency, HSE says open systems such as cooling towers and evaporative condensers should be tested for legionella at least quarterly. For other systems, the frequency of inspection and maintenance depends on the system and the risks it presents.

Who is a competent person?

Responsibility stays with the employer or whoever controls the premises. HSE defines a competent person as someone with sufficient training and experience or knowledge and other qualities that allow them to assist properly. The responsible person could be the duty holder, one or more workers or someone from outside the business.

Hiring a contractor does not move the duty. HSE says it remains the competent person's responsibility to ensure water treatment is carried out to the required standards, and its review questions ask whether a contractor's competence has been checked, for example experience, qualifications, training and membership of a recognised trade body.

Where does air conditioning plant appear in the assessment?

  • Is there a cooling tower, evaporative condenser or dry/wet cooler? HSE says dry/wet coolers may not need notification under the cooling tower regulations but should be assessed against them, for example where water is sprayed directly onto the heat exchanger.
  • Do any air handling units have a humidifier or air washer?
  • Do condensate trays and drains stay clean and drain properly? The cleaning guide covers what is cleaned and how often.
  • Is there a dated assessment, a named responsible person and a review after any change?

Readers who want quotes covering their own plant list can send an enquiry through the site's form.

Sources

Common questions

Do I need a legionella risk assessment if the building only has split units?

The duty applies to the premises' water systems, not only the air conditioning. HSE says employers and those in control of premises must understand and manage legionella risks, and that all systems require a risk assessment, although a simple one may show the risks are low.

Is an adiabatic cooler the same as a cooling tower?

Not necessarily. HSE describes dry/wet coolers, sometimes called hybrid or adiabatic coolers, as running in dry or wet mode with a wide range of risk profiles. They may not need notification under the cooling tower regulations, but HSE says to assess them against those requirements.

How long do legionella records have to be kept?

HSE says records should be kept throughout the period for which they remain current and for at least 2 years after that. Records of monitoring inspections should be kept for at least 5 years.

Does a musty smell from an air conditioner mean legionella?

A smell is not a legionella test. HSE says water samples should be tested by a UKAS-accredited laboratory, and that monitoring depends on the system and the risk assessment. A musty smell more often points to filters, coils, trays and drains, as set out in the guide to why commercial air conditioning smells.

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