Legionella prevention in buildings: what actually prevents growth, and what only looks like it

“Legionella prevention” is a question shared between people asking about a disease and people asking about buildings. This page answers the building question only. If you are asking because of a person’s health, the right person to ask is a doctor.

Prevention is about conditions

Since small numbers of legionella can arrive with ordinary supply water, as explained in how legionella gets into building water, prevention in a building does not mean keeping the organism out. It means keeping the building from giving it what it needs to multiply: warm water held still, biofilm, sediment and decayed disinfectant.

Keep water moving

Stagnation is the condition most within an owner’s control. That means removing dead legs where possible, flushing outlets that are rarely used on a written routine, and paying attention to periods of low occupancy, when a whole building can become a set of rarely used outlets.

A flushing routine only works if it is specific. “Flush unused outlets” on a wall chart is not a routine. A list of the actual outlets, a stated frequency, a named person and a line in the log each time it is done is a routine. Guest rooms left empty, showers in a changing room nobody uses in winter and an eyewash station tested only on paper are the usual places where the list and the building disagree.

Keep temperatures where the program says

Hot water hot enough, cold water cold enough, measured at defined points. The limits are the program team’s decision for that building, within local code and with scalding in mind. Storage vessels deserve particular attention, for reasons set out in legionella in water heaters and storage tanks.

Maintain the equipment that makes spray

Cooling towers, hot tubs, decorative fountains, humidifiers and showers are the devices that put water into the air. Each has its own maintenance routine, its own treatment where relevant, and its own record. A cooling tower in particular has a treatment program that has to agree with the building program, which is a frequent gap.

The practical step is an inventory. Walk the building and list every device that can release droplets, including the ones nobody thinks of as plumbing: a lobby water feature, a humidifier in an air handling unit, a misting system on a terrace. A device missing from the inventory is a device with no routine and no record.

Plan for disruptions

Construction, repairs to the water system, a main break nearby and shutdowns all disturb building water. Prevention includes deciding in advance what will be done around those events, not working it out afterwards.

In practice that means a short written step for each kind of event: who is told when a contractor plans to isolate part of the system, what is flushed or checked before the section returns to service, and who signs that it was done. A building that reopens a wing after a renovation without that step has skipped the moment when prevention matters most.

What only looks like prevention

A test result filed once a year. A binder bought from a consultant and never opened. A filter fitted to one shower in a building of four hundred. A chemical dosed without anyone checking residual at the far end. Each of these produces paper or a feeling of having done something. None of them, alone, controls the conditions above.

Sampling has a real place, as evidence that the program is working. It is not prevention in itself, and a program that relies on it instead of on control has the order backwards, as described in what ASHRAE 188 asks of a building.

Who does it

Prevention is done by the people who run the building: engineers, maintenance staff, the person who walks the empty floors. Contractors carry out parts of it. The team that owns the program decides what is done and checks that it was. That is why the first element of every program is a named team, not a vendor.

The records are what turn that work into prevention someone else can see. A dated reading with initials, a flushing log that matches the outlet list, a note of what was done when a reading was out of range: these let a new manager, an insurer or an inspector understand what the building does without relying on anyone’s memory.