Phoenix Journal · Ductwork
Legionnaires' disease is preventable, yet it thrives in the warm, still water and neglected air plant of ordinary commercial buildings. Here is how one site let it happen - and the plain steps that stop it.
Case postmortem
A quiet building over a long summer break looks like a saving. Six weeks later, a cluster of chest infections traces back to a lukewarm storage tank and an air handling unit nobody had opened in two years.
Here is the scenario, drawn from the kind of failure that plays out in commercial buildings every year. A busy site - offices above, a production kitchen and canteen below - winds down for a summer shutdown. Occupancy drops to a skeleton crew. To trim costs, someone turns the heating down and lets the hot water plant idle. The cold water tanks sit still. The rooftop cooling plant keeps ticking over on a timer, and the air handling units that feed the canteen and kitchen extract keep running with nobody watching the drip trays.
Nothing looks wrong. The taps still run, the air still moves, the building is warm. But in the background, water that should be moving has gone still, and water that should be hot has drifted into the danger zone. Legionella bacteria multiply fastest between 20°C and 45°C, and by the time the building fills back up, several systems have been sitting squarely in that range for weeks. When the crews come back, they flush the taps, fire up the showers in the changing rooms and switch the ventilation back to full. Aerosols carry the bacteria into the air people breathe. Within a fortnight, occupational health starts getting calls.
None of this was bad luck. Every failure had a name and, under UK law, an owner. Legionnaires' disease is a preventable pneumonia, and the duty to control it sits with the person in charge of the premises - the dutyholder - under the Health and Safety at Work etc Act 1974 and the Control of Substances Hazardous to Health Regulations 2002. The Health and Safety Executive's Approved Code of Practice, ACOP L8, and its technical guidance HSG274 spell out exactly what that duty looks like in practice. Fall short of an ACOP and a court can treat that as evidence you broke the law unless you can show you did something equally effective instead.
Walk back through the shutdown and the gaps line up:
The bitter part is scale. This is not a rare event. In 2024 there were more than 460 cases of Legionnaires' disease reported in residents of England and Wales, the great majority caused by Legionella pneumophila. Most cases hit adults over 60, and the illness can be fatal. One idle building can put its own staff and everyone downwind of its cooling plant at risk.
Getting a building back under control is not exotic work. It is a written plan, a competent person and a habit of checking. If you manage a site with hot and cold water services, cooling plant or air handling units, this is the order that closes the gaps:
There is a quiet early warning most managers miss. Systems full of stagnant water, running calorifiers that never cool down and cooling plant left on a timer all show up on the meter as well as in the risk assessment. If your consumption has crept up with no obvious cause, it is worth reading it as a hygiene signal as much as a cost one - the same idle, over-full, poorly controlled water that raises a bill is the water Legionella likes best. If the numbers look off, our note on why a water bill runs higher than it should walks through where hidden usage hides.
The lesson from every postmortem is the same. Legionella does not need a dramatic failure. It needs warmth, stillness and a route into the air - and a building that stopped paying attention. Keep the water moving and correctly hot or cold, keep the wet parts of your air plant clean and draining, keep the records, and the risk stays where it belongs: managed and boring.
Questions
The dutyholder - usually the employer or the person in control of the premises - carries the legal duty under the Health and Safety at Work etc Act 1974 and the COSHH Regulations 2002. In practice they should appoint a named, competent responsible person to manage day-to-day control. The HSE's ACOP L8 and HSG274 guidance set out what that control must cover, and a court can treat a failure to follow the ACOP as evidence you broke the law.
Legionella multiplies fastest between 20°C and 45°C, so the aim is to keep water out of that range. Hot water should be stored at 60°C or above and reach the tap at 50°C or above within one minute of running. Cold water should be kept below 20°C. Sentinel outlets should be checked monthly and the readings recorded.
Yes, where the system holds water. Air handling units with cooling coils produce condensate that collects in drip trays, and humidifiers and evaporative cooling plant hold water too. If those wet parts are dirty or fail to drain, the airflow can aerosolise contaminated moisture into occupied space. HSG274 Part 3 treats these as genuine risk systems, which is why cooling coils, trays and humidifiers need regular inspection, cleaning and confirmed free drainage.
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