The wall-mounted air conditioner in your office almost certainly cannot give you Legionnaires’ disease. It holds no meaningful reservoir of warm water and it does not spray a breathable mist into the room. That is the honest answer to the question most people are actually asking.
The reason the worry exists is that “air conditioning” is a single phrase covering two completely different machines. One is the quiet box on the wall. The other is the large cooling tower or evaporative condenser sitting on a roof or in a plant yard — and that one is a real, well-documented Legionella source [2].
So the useful question is not “is air conditioning dangerous?” It is “which kind of air conditioning, and is there standing warm water that gets turned into aerosol?”
How Legionnaires’ disease actually reaches a person
Legionella makes people ill when contaminated water is broken into a fine aerosol of tiny droplets, and someone breathes those droplets deep into the lungs [1]. No aerosol, no realistic route. The bug also needs warmth to multiply, broadly in the range commonly cited for tepid stored water rather than properly hot or genuinely cold systems — treat the exact figure as the one your risk assessment sets [4].
You do not catch it from drinking water, and it does not pass from person to person [11]. That second point matters here: if a colleague had Legionnaires’ disease, the air con did not “give it to the office” the way a cold spreads.
Hold those two requirements in mind — warm water plus aerosol — and the split-unit-versus-cooling-tower question answers itself.
The split unit: why the myth does not hold
A domestic or office split system (and its bigger cousin, a VRF/VRV system) cools air by passing it over a refrigerant coil. There is refrigerant gas in sealed pipework, not a tank of warm water. The only water involved is condensate — the moisture that drips off the cold coil — which is, by definition, cold, and which drains away rather than being sprayed into the room.
Recirculating air is not the same as recirculating water. A split unit blows room air across a cold surface and pushes it back out. Even if dust and mould build up on a neglected filter (a genuine indoor-air-quality nuisance), that is not the warm-water-plus-aerosol combination Legionella needs.
In my view this is the single most over-worried risk in any normal office. The energy is better spent on the systems that genuinely matter — the showers, the calorifier, the rarely-used taps feeding a dead leg.
Myth vs reality
| The worry | What is actually true |
|---|---|
| ”The office air con could give us Legionnaires’.” | A wall-mounted split or VRF unit has no warm-water reservoir and produces no waterborne aerosol. Not a realistic source. |
| ”Air conditioning spreads Legionella round the room like a cold.” | The disease does not pass person to person, and a split unit recirculates air, not contaminated water [11]. |
| ”All air con is the same risk." | "Air conditioning” covers split units and rooftop cooling towers/evaporative condensers — utterly different machines with utterly different risk. |
| ”The rooftop unit is just a big air conditioner, so it’s fine too.” | Cooling towers and evaporative condensers hold warm recirculating water and deliberately throw off mist (drift). They are among the highest-risk systems known [2]. |
| ”If there’s no cooling tower, there’s nothing to think about.” | Air handling units with humidifiers or pooled condensate can, if poorly maintained, become a minor aerosol concern worth checking [4]. |
The cooling tower: where the real risk lives
A cooling tower exists to dump heat by evaporating water. It runs a large volume of warm water round and round, exposes it to air, and unavoidably releases some of it as fine droplets — drift — into the open air [14]. That is warm water plus aerosol, by design, often at roof level where the mist can travel.
This is why cooling towers and evaporative condensers sit at the top of the regulator’s risk hierarchy and why, unlike a split unit, they carry a legal duty to notify the local authority of their existence [9]. If your building has one, it should already be on a formal control scheme with treatment, monitoring and cleaning — that is a different article, and we have one Cooling towers and evaporative condensers: high-risk systems.
The pragmatic call: if someone in your building is worried about “the air con”, the first thing to establish is whether there is a cooling tower or evaporative condenser anywhere on site. That single fact changes the answer completely.
Why the myth persists
The confusion is linguistic. The plant on the roof is often labelled “cooling”, it is part of the building’s climate system, and to a non-specialist it looks like air conditioning — because in a broad sense it is part of it. Outbreaks traced to cooling towers get reported as “linked to air conditioning”, which is technically defensible and badly misleading. The public hears “air con causes Legionnaires’” and reasonably applies it to the box on their own wall.
The fix is to stop saying “air conditioning” and name the actual machine. For more on how this and similar misconceptions take hold, see Legionella myths and facts: science vs misconceptions; for the underlying spread mechanism, How Legionella spreads through water systems.
The one honest caveat for air systems
Not every air system is automatically irrelevant. Large air handling units that use evaporative or spray humidification, or that let condensate pool in a tray, can in principle create a fine mist over standing water [4]. The risk is far lower than a cooling tower and depends entirely on design and upkeep, but it deserves a line in your risk assessment rather than a blanket “air con is fine”.
This is general guidance, not a verdict on your building. Whether a specific unit, AHU or rooftop plant matters on your site is a judgement for a competent person doing a site-specific Legionella risk assessment under ACoP L8 and HSG274 — they can see your actual kit; an article cannot.
What to do today
Walk the building, or pull the asset list, and answer one question: is there a cooling tower or evaporative condenser anywhere on the premises? If yes, confirm it is notified to the local authority and on a live control scheme. If no, you can reassure the worried office without dismissing them — and redirect attention to the showers and stored hot water, where Legionella actually lives.
If that walk-round currently lives in your head or on a dog-eared spreadsheet, it is worth capturing the answer properly: a digital logbook lets you record which plant exists, who checks it and when, so the next person who asks “can the air con make us ill?” gets an evidenced answer instead of a guess.
FAQ
Can a portable or wall-mounted air conditioner spread Legionnaires’ disease?
In normal use, no. These units cool air over a sealed refrigerant coil and produce only cold condensate, which drains away rather than being aerosolised. They lack the warm-water reservoir and the breathable mist that Legionella infection requires [1].
Was an outbreak “linked to air conditioning” caused by office units?
Almost always, reports of Legionnaires’ “linked to air conditioning” mean a cooling tower or evaporative condenser serving the building’s cooling, not the indoor units in occupied rooms. The aerosol comes from the warm-water plant outside, not the box on the wall [2].
Do I need to test my office air con for Legionella?
A standard split or VRF system is not a water system in the Legionella sense, so routine Legionella sampling of it is not the priority. Your risk assessment should instead focus testing and monitoring on hot and cold water services and on any cooling towers, evaporative condensers or humidified air handling units [5].
Sources
[1] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [2] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [4] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [5] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [9] HSE, “Other duties: RIDDOR and notification of cooling towers or evaporative condensers”. https://www.hse.gov.uk/legionnaires/what-you-must-do/duties.htm [11] NHS, “Legionnaires’ disease”. https://www.nhs.uk/conditions/legionnaires-disease/ [14] CDC, “Controlling Legionella”. https://www.cdc.gov/control-legionella/index.html