Legionella and aircon: which equipment carries the risk

The organism moves on water, and it reaches a person as a droplet small enough to breathe in. That one fact decides which machines belong in the conversation. The question is never whether something is called air conditioning. It is whether the thing holds water and puts that water into the air.

Health authorities describe the route in those terms. WHO says the most common form of spread is breathing in fine droplets from water that carries the bacteria. It adds that no direct spread between people has been reported. Singapore's Communicable Diseases Agency lists the man-made sources, and each entry on it is a water system: cooling towers, water storage tanks, hot water systems, spas, evaporative condensers and fountains.

The UK Health and Safety Executive sets out what turns stored water into a risk, and its conditions are worth knowing because they work as a test. Water has to be stored or recirculated. It has to sit in a temperature band that suits growth. Breathable droplets have to be created and dispersed. Last, there has to be material to live on, such as rust, sludge, scale, organic matter and biofilm.

Those conditions matter as a set, and dispersal is the one that decides whether anything can reach a person at all. Water that never leaves its container as a breathable droplet has no path out, whatever else is true of it. That is the part the headline version drops. It is also why the same two words can cover a machine that carries a legal duty and a machine that carries none.

This page stops at the equipment. Legionnaires' disease is a serious illness, and what it does to a person, how it is recognised and how it is treated belong to MOH and to a doctor. Legionellosis is notifiable under the Infectious Diseases Act, so the surveillance side sits with the health authorities too. Nothing on this page stands in for either.

A cooling tower throws a building's heat away by evaporating water into moving air. Warm water comes back from the plant and gets spread over a large wetted surface. A fan pulls outside air through it. Part of the water evaporates, and that is what carries the heat off. The rest falls into a basin and goes round again.

The aerosol is a by-product of that design rather than a fault in it. Air moving quickly through falling water picks up fine droplets and takes them out with it. Equipment is fitted to limit how much escapes, and water treatment decides what is in whatever does. A machine built to blow air through water will always put some of it into the air.

Evaporative condensers work on the same principle and sit in the same family. The Communicable Diseases Agency names them alongside cooling towers. Anything that rejects heat by wetting a surface and blowing air across it belongs to this group, whatever the building calls it.

NEA regulates that equipment for the same reason, and treats a cooling tower as an aerosol-generating system. It says the aerosol may carry Legionella bacteria where the tower water is not properly maintained. It also says those droplets can reach the public.

Registration falls on the owner or occupier of the premises, and so does water-quality testing. Both duties sit under the Environmental Public Health Act. What registration involves, and what gets filed once it is done, is a topic in its own right.

The point that belongs here is narrower. A hygiene regime backed by law exists in Singapore. It attaches to water equipment, and it does not attach to the box on a bedroom wall.

Where the equipment stands decides who should be reading on. NEA describes cooling towers as serving many commercial buildings and factories, for comfort cooling or for industrial process cooling. They sit on roofs and in plant areas, usually on a different contract from anything inside the occupied space.

A split system rejects its heat to air, and no water is involved in doing it. The outdoor unit is a coil with a fan behind it. Hot refrigerant runs through the coil, outside air is blown across the fins, and the heat leaves with that air. There is no basin, no circulating water, and nothing built to spray.

The water a split does produce runs the other way. Cooling a room takes moisture out of the air, that moisture condenses onto the cold indoor coil, and it collects in a pan and leaves through a drain pipe. It is made cold at the point it forms. It is passing through on its way out of the flat, not being held and sent round again.

Dispersal is the step the route needs, and it is the one this arrangement does not perform. Air is drawn across the face of the coil and pushed back into the room. The pan sits below the coil, catching what runs off it. Nothing there is built to turn held water into a breathable mist, and that is the whole distance between this machine and a tower.

Putting it that way is deliberate. What can be described honestly is the mechanism: what the route requires, and which of those requirements a given machine meets. A claim that a household unit is incapable of anything, or a claim that one has caused a case, both reach past what an equipment description can support.

Water inside a home is a separate question from the aircon inside it. The HSE risk list covers hot and cold water systems in domestic premises as well as workplaces, and a household's stored and heated water is looked after by whoever maintains the building's water services. That question belongs to that trade rather than this one.

A drain pan holding water is a genuine finding, and it deserves attention on its own account. It says the outlet is not keeping up, and it is where fouling, odour and eventually a wet wall begin. None of that needs this page to justify it.

What it is evidence of is a hygiene problem inside the unit. Biofilm across a coil and sludge in a pan account for smell, weakened airflow and a drain that has slowed, and those are what a service visit reads and treats. Separate pages already deal with each of them.

What it is not is a way out into the room. Water resting in a pan sits below the air the unit delivers rather than travelling in it. Clearing the pan is worth doing for the reasons above, and it settles a drainage fault rather than the question this page exists to answer.

Push back when a household unit gets quoted for Legionella treatment or Legionella disinfection. Ask which water system is being treated, and where the spray that would carry anything out of it is meant to come from. On a split system the second question has no answer, and the vocabulary has been borrowed from equipment the premises does not have.

One appliance in a home does hold water and does put it into the air, and it is not the aircon. WHO's list of aerosol sources linked with transmission includes humidifiers, alongside cooling towers, hot and cold water systems and whirlpool spas. A humidifier is a tank with a mist output, and the cleaning instructions its maker supplies exist for a reason.

Duty follows the equipment, and on a Singapore building it follows the registered equipment. NEA puts the registration and testing burden on whoever owns or occupies the premises the system runs on. That names a party. It does not spread a loose responsibility across everyone who benefits from the cooling.

First establish whether the premises has any such equipment at all. A good number of commercial buildings here run air-cooled plant with no tower anywhere on the roof. Others buy chilled water from a landlord, in which case the tower belongs to the landlord rather than to the business occupying a floor of it.

This is where a facilities manager can quietly get it wrong. Reading air conditioning as one category leads to a plan built around the visible units. Filters, coils and the boxes in the ceiling get scheduled, because those are what people complain about. The plant that holds the legal duty tends to sit on another contract, out of sight, and easy to assume someone else has covered.

The opposite error costs less but still costs. A manager who treats every mention of the illness as a building-wide alarm ends up buying hygiene work on terminal units, which is ordinary servicing sold under the wrong heading. Cleaning terminal units is worth doing. It is not a control for this.

Where an occupier's responsibility ends in a chilled-water building is set out on its own page, and it repays reading first. The short version here is simple. If heat leaves the premises through water, a named party holds duties attached to that water. If it leaves through air, that duty is not in the picture.

The record is the artifact worth asking for. NEA's regime runs on registration and on submitted water-quality results, so a building either holds that paperwork or it does not. Asking to see it is a fair question from a tenant, and a faster answer than any inspection of the units inside the space.