Legionella travels through the air inside tiny water droplets you breathe in. Most of those droplets fall to the ground within a few metres of where they were made. The fine ones can stay airborne and drift much further on the wind — in some cooling-tower outbreaks, cases have been linked to a source hundreds of metres away [1][3]. There is no single safe distance.
That is the short answer. The longer one matters if you are working out whether your building could be exposed by something nearby, or whether your own plant could put other people at risk.
Why this is an air question, not a water question
You do not catch Legionnaires’ disease by drinking contaminated water. You catch it by breathing contaminated water — specifically, by inhaling an aerosol of fine droplets small enough to carry the bacteria deep into your lungs [1]. (Occasionally it happens by aspiration, where contaminated water goes down the wrong way while drinking.) The route into the body is the airway, so the question of reach is really a question about how water droplets behave once they leave a tap, a shower, a spray nozzle or a cooling tower.
Anything that turns water into a mist is a potential source: cooling towers and evaporative condensers, showers, spray taps, hot tubs, decorative fountains, vehicle washes, misting units. The systems most associated with spreading the disease beyond their immediate surroundings are the ones built to throw water into the air in large volume — cooling towers above all [2].
Droplet size decides almost everything
When water is broken into a spray, it does not produce one uniform droplet. It produces a spectrum, from coarse drops you can feel to a fine mist you cannot see. Two things then happen at once, and they pull in opposite directions.
Large droplets are heavy. Gravity wins quickly, and they fall out of the air close to the source — often within a metre or two. They also carry more water, but they are too big to be inhaled past your upper airway, so even if you are standing in them they tend not to reach the part of the lung where infection starts.
Fine droplets are the dangerous ones. They are light enough to stay suspended, light enough to be carried on air currents, and small enough to be breathed all the way into the deep lung — this is what is meant by the respirable fraction. It is commonly described as droplets of roughly a few microns or smaller, though the cut-off is a guide rather than a bright line. The paradox of airborne reach is that the droplets capable of travelling furthest are precisely the ones capable of causing disease.
The airborne pathway, step by step
If you wanted to sketch how Legionella gets from a water system into someone’s lungs across a distance, it would have five stages — picture it left to right:
- Source. A piece of plant holds water in the range where Legionella can multiply and is designed (or has come) to aerosolise it: a cooling tower fan throwing drift off the pack, a shower head, a spray nozzle. Draw this as the box on the left.
- Aerosol generation. At the source, water shatters into the full droplet spectrum — coarse drops and fine mist together. Draw a cone of spray leaving the box.
- Fallout and sorting. Within the first few metres, the coarse drops drop out and hit the ground or nearby surfaces. The fine respirable droplets stay aloft. Draw the big drops curving down early; the fine ones continuing on.
- Drift and dilution. The remaining fine cloud — a plume — is carried by the wind. It moves in the direction the wind is blowing, spreads sideways, and dilutes as it goes: the further from the source, the fewer bacteria per breath. Draw an arrow downwind, widening and fading.
- Inhalation. Anyone in the path of that thinning plume — at an open window, an air intake, a doorway, a pavement — can breathe it in. Draw a person downwind. Whether they are infected depends on how much they inhale and how susceptible they are.
The useful insight from that diagram is that reach is governed by step 4, and step 4 has no fixed end point. It depends on how much fine aerosol the source produces, the wind speed and direction, and what is downwind. That is why “how far” has no tidy number.
So how far, in practice?
For a domestic shower or a tap, the realistic exposure is to the person standing right at the outlet — the relevant distance is centimetres, not metres. The home shower question and split-unit air conditioning concern are both essentially point-of-use exposures, not plume events.
Cooling towers are the genuine long-range case. They run fans, move large volumes of water, and sit on roofs where the plume is released high and travels. Outbreak investigations have linked clusters of cases to cooling towers some distance from where the people lived or worked — sometimes a few hundred metres, and in some large incidents considerably further. When public health teams investigate a community cluster, they draw a geographic zone around the cases and look for aerosol-generating sources inside it, then refine it; that search radius is set case by case from the wind and the local geography, not from a fixed rule [3]. The lesson is not a magic number. It is that a poorly controlled cooling tower can put people at risk who never went near the building — exactly what the cooling-tower outbreak case study shows, and why the wider mechanics of how Legionella spreads start with controlling the source.
What this means if you run a building
Chasing a “safe distance” is the wrong instinct. Two practical questions are better. First, do you operate anything that aerosolises water — and is it under proper control, so the plume that leaves it is not carrying amplified bacteria? Second, is there an aerosol-generating source near your air intakes, opening windows or busy pedestrian areas, including one on a neighbour’s roof? Siting matters: a cooling tower discharging close to a fresh-air intake is a design problem distance alone will not solve.
The defence is at the source, not in the air. You cannot control the wind, the droplet spectrum or who is downwind. You can control whether the water leaving your plant has been kept out of the growth range and properly treated.
A caveat worth stating plainly
This explains the physics of airborne reach to help you reason about exposure and siting; it is not a dispersion model and not a substitute for assessment. Real plume behaviour depends on the specific source, weather and surroundings, and any judgement about distance, source-tracing or whether a particular building is affected belongs to a competent risk assessment and, in an outbreak, to the public health investigation [3][4]. The droplet-size and distance figures here are general guidance, not fixed limits.
What to do next
Walk your roof and your boundary. Note every piece of equipment that turns water into mist — yours and any you can see on adjacent buildings — and mark where each one sits relative to your air intakes, windows and the spots where people gather. If a cooling tower or evaporative condenser is in that picture, its control regime is the single most important thing protecting people you will never meet, and it is worth confirming it is genuinely being maintained rather than assumed.
FAQ
Can you catch Legionnaires’ disease from a cooling tower without going inside the building?
Yes. That is exactly why cooling towers are treated so seriously. They release a fine aerosol that the wind can carry well beyond the building, so people passing by or working downwind can be exposed without ever entering [2][3]. It is the reason cooling towers must be notified to the local authority and kept under a proper control regime.
Is there a safe distance from a Legionella source?
No fixed one. Coarse droplets fall out within a few metres, but the fine respirable droplets that actually cause infection can drift much further on the wind, and how far depends on the source, the weather and the local layout [1]. Control is achieved at the source, not by assuming a buffer distance is enough.
What size droplet carries Legionella into the lungs?
The droplets that matter are the fine, respirable ones — small enough to stay airborne and be breathed into the deep lung, commonly described as around a few microns or smaller [1]. Larger droplets are too heavy to travel far and too big to be inhaled past the upper airway, so the smallest droplets are both the furthest-travelling and the most dangerous.
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 [3] UKHSA, “Investigation of Legionnaires’ disease: cases, clusters and outbreaks”. https://www.gov.uk/government/publications/investigation-of-legionnaires-disease-cases-clusters-and-outbreaks [4] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm