Legionella and Air Conditioning: Where the Real Risk Sits

Air conditioning can spread legionella when it involves water that is stored or sprayed, as in cooling towers, evaporative condensers and humidifiers. A standard split or VRF unit cools with refrigerant rather than water, so its main water point is the condensate tray, which needs keeping clean.

You have probably had the email. A hot week in July, somebody shares a story about a legionella outbreak, and the question lands on your desk: is our air conditioning safe? This post is the answer with the sources attached.

At a glance

Question	Short answer
Is air conditioning a legionella source?	The NHS lists air conditioning systems and humidifiers among the things you can catch it from. HSE's examples of where outbreaks come from are wet systems: cooling towers, evaporative condensers, hot and cold water systems and spa pools.
How do people catch Legionnaires' disease?	By inhaling small droplets of water (aerosols) containing the bacteria. Not person to person.
At what temperature does legionella thrive?	Risk rises where water in all or part of a system sits between 20°C and 45°C.
Which plant carries the real risk?	Cooling towers, evaporative condensers, humidifiers and air washers, plus hot and cold water systems and spa pools.
Who is legally responsible?	Employers and those in control of premises, who must assess and manage the risk and appoint a competent "responsible person".
Do cooling towers have to be registered?	Yes. The Notification of Cooling Towers and Evaporative Condensers Regulations 1992 require written notification to the local authority.

Source: What is Legionnaires' disease, HSE and Legionnaires' disease, NHS

How legionella actually spreads

Legionella lives in water, and HSE puts the mechanism in one sentence: "People contract Legionnaires' disease by inhaling small droplets of water (aerosols), suspended in the air." Not from another person, and not from cold air: you catch it by breathing in contaminated water turned into a mist fine enough to reach deep into the lungs.

That is why the risk follows conditions rather than building types. Risk rises where "the water temperature in all or some parts of the system may be between 20-45 °C", where water is stored or recirculated, and where something turns it into spray. The NHS describes the disease as "usually caught in places like hotels, hospitals or offices where the bacteria have got into the water supply", which is why this question lands on facilities managers and landlords.

The consequences of getting it wrong are not theoretical. HSE's account of the worst UK outbreak records that "in August 2002, 7 members of the public died and 180 people suffered ill health" at a council-owned arts and leisure facility in Barrow-in-Furness (The legionella outbreak in Barrow, HSE). HSE published the investigation report to "allow dutyholders with similar responsibilities for controlling legionella to benefit".

Where a split or VRF system fits in

A standard split or VRF system, whether a wall unit in a meeting room or a VRF installation across a whole floor, cools with refrigerant in sealed copper pipework. Room air passes over a cold coil and gives up its heat to the refrigerant. Unlike a cooling tower, it does not hold a body of warm recirculating water or throw a plume of droplets into the air.

HSE says outbreaks come from legionella growing in purpose-built water systems, giving as examples "cooling towers, evaporative condensers, hot and cold water systems and spa pools". The NHS casts the net wider and lists "air conditioning systems" and "humidifiers" among the things you can catch it from. Put the two together and the useful question is not whether something is called air conditioning, but whether water is stored, recirculated or sprayed inside it.

There is one wet component inside every split system: the condensate tray. Air crossing the cold coil drops its moisture, the water collects in the tray and should run away down a drain line. When the drain blocks, water stands in the tray, and stagnant water, biofilm, musty smells off the coil and stained ceiling tiles follow. It is also the side of air conditioning and indoor air quality people actually notice.

Where the real risk sits

HSE is direct about the wet end of cooling plant: "If you have a cooling tower or evaporative condenser, you should put in place appropriate measures to prevent or control the risk of legionella" (Evaporative cooling systems, HSE). Both devices reject heat by evaporating water into moving air: warm recirculating water, nutrients, and a plume of droplets drifting off the roof.

System type	Legionella risk factor	What HSE expects
Split or VRF air conditioning (dry, direct expansion)	Low where no water is stored or sprayed. Sealed refrigerant circuit; the condensate tray is the only water in the unit.	Not among HSE's examples of outbreak sources. Keep condensate trays and drains clean as part of routine servicing, and include the system in your risk assessment if you are unsure.
Cooling tower	High. Open, warm, recirculating water with a constant drift of fine droplets.	Written scheme of control, water treatment programme, routine monitoring including for legionella, and cleaning, per HSG274 Part 1. Must be notified to the local authority.
Evaporative condenser	High, for the same reasons. A heat exchanger sprayed with recirculating water, with air drawn across it.	Same regime as a cooling tower under HSG274 Part 1. Also notifiable.
Humidifier or air washer in an air handling unit	Real if poorly managed. Water is deliberately introduced into the airstream feeding occupied rooms.	HSE lists humidifiers and air washers among "other risk systems"; stored or recirculated water between 20°C and 45°C that produces aerosols is a foreseeable risk to assess and control.
Hybrid or adiabatic (dry/wet) cooler	Variable. Runs dry most of the time but wets its coils or pads in hot weather.	HSE notes these "can have a wide range of risk profiles"; evaluate the design rather than assume it is safe.

Sources: HSG274 Legionnaires' disease: Technical guidance, HSE; Other risk systems, HSE; Dry/wet cooling systems, HSE

Two points in that table catch people out. A humidifier inside an air handling unit looks like "air conditioning" to the building, but HSE lists humidifiers and air washers among the other systems that could be a source of legionella, and says any water system is potentially a reasonably foreseeable risk, so it belongs in your legionella risk assessment. And water cooled condensers vary: some water cooled air conditioning rejects heat to a closed loop or uses mains water once and discharges it, while other designs feed an evaporative tower on the roof. The pipework decides the regime, not the badge on the indoor unit.

Our notes on hotel air conditioning cover the comfort side; in a hotel, the legionella side belongs to whoever holds the water safety plan.

What the law asks of a duty holder

HSE says duties under the Health and Safety at Work etc Act extend to risks from legionella bacteria, and its guidance is addressed to employers and anyone in control of premises, including landlords. It boils the job down to four tasks: "identify and assess sources of risk", "manage any risks", "prevent or control any risks" and "keep and maintain the correct records".

The practical handbook is the Approved Code of Practice L8, "Legionnaires' disease: The control of legionella bacteria in water systems". HSE describes it as "aimed at dutyholders, including employers, those in control of premises", and it sets out "the specific role of an appointed competent person, known as the 'responsible person'" (L8 Approved Code of Practice, HSE). The technical detail sits in HSG274, whose Part 1 covers evaporative cooling systems.

The registration duty is older and simpler than people expect. The Notification of Cooling Towers and Evaporative Condensers Regulations 1992 define a cooling tower as "a device whose main purpose is to cool water by direct contact between that water and a stream of air", and require the person in control of the premises to notify the "local authority in whose area the premises are situated", in writing, with changes reported "within one month after its occurrence" (Regulation 3, SI 1992/2225, legislation.gov.uk).

Two honest caveats. A legionella risk assessment, and any sampling or treatment regime, is a water hygiene specialism and not something we do; you want a competent water treatment provider for that. And nothing here replaces reading L8 against your own building: as of October 2026 the documents above are the current versions.

The part of an AC service that actually matters for hygiene

Once you accept that the legionella risk lives in the water systems and wet cooling plant, there is still a hygiene job on the air conditioning itself. On every service visit the condensate tray should be emptied, flushed and checked for slime, the drain line cleared and its fall confirmed, and any lifting pump tested. Coil and filters come next, because a coil matted with dust holds moisture longer and feeds the smells people complain about.

This is worth specifying rather than assuming. A contract that lists filter changes and a refrigerant check but never mentions the tray leaves it to chance. Our post on what to specify in an air conditioning maintenance contract applies directly: ask whether condensate management is itemised, and how a blocked drain gets reported between visits.

Symptoms, and why the timing matters

This section stays within what the NHS and the WHO publish; anything beyond that is a doctor's conversation, not ours.

What	Detail
Incubation period	2 to 10 days after breathing in the bacteria, for the pneumonic form (Legionnaires' disease).
Pontiac fever	The milder, non-pneumonic form. Incubation from a few hours up to 48 hours, with fever, chills, headache, malaise and muscle pain.
Main symptoms	A cough, shortness of breath, chest pain or discomfort particularly when breathing or coughing, a high temperature, and flu-like symptoms.
How serious	HSE calls it a potentially fatal form of pneumonia. The WHO puts the overall death rate usually within the range of 5 to 10 per cent.
Who is most at risk	Everyone is susceptible, but risk is higher in people over 45, smokers and heavy drinkers, and anyone with an impaired immune system.
Treatment	Hospital treatment may include antibiotics into a vein, usually for 1 to 3 weeks. Most people make a full recovery, though it can take a few weeks.

Source: Legionnaires' disease, NHS and Legionellosis fact sheet, WHO

The incubation window is the operationally important row. If several people connected to your building fall ill in the same fortnight, the exposure may have happened anywhere from 2 to 10 days earlier, and the WHO notes that up to 16 days has been recorded in some outbreaks. If anyone develops these symptoms after a suspected exposure, that is an urgent medical conversation.

When to get help with the air conditioning side

If your building only has split or VRF air conditioning, your legionella position is mostly about the water systems, and a water hygiene specialist should carry out the risk assessment. Where we come in is the air conditioning plant: condensate trays and drains cleaned and checked on every visit, coils and filters kept in order, and AHUs flagged where a humidifier section belongs in your water safety plan. Be Cool is a family-run firm trading since 2004, with F-Gas certified engineers and more than 2,000 projects completed across London and the South East, and our air conditioning maintenance visits include condensate drain checks and clearance.

Frequently asked questions

Can you get Legionnaires' disease from air conditioning?

From some systems, yes. The NHS lists air conditioning systems among possible sources, and HSE's examples of outbreak sources are wet systems such as cooling towers and evaporative condensers. A standard split or VRF unit holds no stored or sprayed water apart from its condensate tray, which is why keeping trays and drains clean matters.

Do I need a legionella risk assessment for my air conditioning?

The duty to assess covers your water systems, and wet cooling plant sits firmly inside that. A building with only dry, direct expansion AC has little for the assessment to bite on, but the duty sits with whoever controls the premises, so have it done by a competent water hygiene specialist.

Does a cooling tower have to be registered?

Yes. Under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992, the person in control of the premises must notify the local authority in writing, and report changes within one month. Evaporative condensers carry the same duty.

Are dry coolers and adiabatic condensers a legionella risk?

A purely dry cooler sits outside the usual risk profile. Hybrid or adiabatic units that wet their coils in hot weather are different: HSE says they "can have a wide range of risk profiles", so evaluate the design rather than assuming either extreme.

What are the first symptoms of Legionnaires' disease?

Per the NHS: a high temperature and flu-like symptoms, then a cough, shortness of breath, and chest pain particularly when breathing or coughing. Symptoms usually start 2 to 10 days after exposure. Anyone affected needs medical advice, urgently if they are in a higher risk group.

How often should condensate trays and drains be cleaned?

None of the HSE pages above sets a fixed interval for split system condensate trays. The sensible answer is every service visit, and immediately if you get reports of musty smells or water marks below indoor units.

The question worth asking on your next site walk

Most of the anxiety in this topic comes from one blurred word. "Air conditioning" covers a sealed box on the wall and a wet tower on the roof, and the risk sits with the wet one. Walk your plant list once and label every item: dry refrigerant system, humidifier in the airstream, evaporative heat rejection, domestic water. Each label lands in a different regime.

If that walk turns up questions about the air conditioning side, an AHU nobody has opened in years, or a maintenance contract that never mentions trays and drains, talk to us about a maintenance visit. We will tell you plainly what is ours to fix and what belongs with your water hygiene provider.

About the author: Ali Elm runs Be Cool Refrigeration & Air Conditioning, a family-run London firm that has been installing, servicing and repairing commercial refrigeration and air conditioning since 2004. His F-Gas certified team has delivered more than 2,000 projects across London and the South East.

Last updated: 12 October 2026