Cooling Towers and Legionella: The Heatwave Risk

Most Legionella attention in commercial buildings goes to the hot and cold water systems — the taps, showers, tanks and calorifiers covered by Part 2 of the HSE's technical guidance. That focus is right for most buildings, because most buildings only have those systems. But the systems behind the UK's largest and best-known Legionnaires' outbreaks — Barrow-in-Furness in 2002 and Stafford in 1985 among them — are a different kind, governed by a different part of the guidance, and their risk does not sit flat across the year. It rises with the temperature. In a heatwave, the highest-consequence Legionella system in the building is working at its hardest.

That system is the cooling tower, and its close relatives the evaporative condenser and the increasingly common adiabatic or hybrid cooler. They appear on offices, data centres, hospitals, industrial sites, large retail and any building with significant cooling load, and they reject heat by evaporating water into a moving air stream. That process does two things at once: it generates a large volume of fine, breathable aerosol, and it runs the water through exactly the temperature band — roughly 20 to 45°C — in which Legionella multiplies fastest. When that aerosol carries the bacteria, it can drift well beyond the building and infect people who never went near it. At Barrow, the source was a council arts-centre cooling system venting over a town-centre alleyway; seven people died and around 180 fell ill. This is why cooling towers are treated as the highest-risk Legionella system there is, and why they have their own dedicated part of the guidance, HSG274 Part 1.

The heatwave connection is not incidental. A cooling tower's whole job is to reject more heat when the weather is hotter, so in a prolonged hot spell it runs harder, moves more water and more air, and throws more aerosol — while sitting permanently in the bacterial growth range it cannot escape through temperature. Unlike a hot or cold water system, where holding the water hot or cold is the primary control, a cooling tower cannot be kept out of the danger band by temperature at all. Its water is always warm. Control depends entirely on the things that are easiest to let slip in a busy summer: the biocide and water treatment regime, the condition of the drift eliminators that are supposed to limit aerosol escape, the cleaning and disinfection schedule, and the monitoring that catches a problem before it becomes an outbreak.

Two duties sit on any operator with this kind of plant, and both are routinely missed.

The notification duty most operators do not know they carry. Under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992, anyone with control of non-domestic premises must notify the local authority, in writing, of any cooling tower or evaporative condenser on the site. It is a one-off notification absent changes, it is free, and failing to do it is a criminal offence. The gap appears at building handovers and acquisitions: a tower changes hands, the new operator assumes it was notified, and nobody checks. When the local authority's register and the plant on the roof do not match, the operator is in breach without having done anything visibly wrong. Anyone taking on a building with evaporative cooling should confirm the notification exists and reflects the current plant, exactly as they would confirm the gas and Legionella records. It is worth noting that this registration regime exists precisely because of the cooling-tower outbreaks at Stafford and elsewhere — it was the inquiries into those tragedies that prompted it.

The risk assessment and control scheme specific to evaporative cooling. A Part 2 hot and cold water risk assessment does not cover a cooling tower. Evaporative cooling requires its own assessment under HSG274 Part 1, its own written scheme of control, and a competent responsible person overseeing a treatment and monitoring regime that is genuinely specialist — which is why most organisations outsource the tower's day-to-day management to a water treatment contractor. That outsourcing does not transfer the duty. The dutyholder still owns the risk, still needs to see the records, and still has to know that the regime is being delivered rather than assumed. A contractor visiting monthly with a clipboard is not evidence of control; the readings, the biocide levels, the dip slides and the corrective actions are.

The newer wrinkle is the adiabatic or hybrid cooler. These run dry for much of the year and switch to evaporative mode when the weather is hot — which means they spend the heatwave doing exactly the thing that generates risk, having spent the cooler months looking low-maintenance. HSG274 Part 1 addresses them specifically, and they can be notifiable and require assessment in the same way as a conventional tower. A building that installed adiabatic coolers as the low-water, low-fuss option may be carrying an evaporative Legionella risk that nobody has assessed, precisely because the units are quiet for most of the year and only wake up in the heat.

For FM directors, estate managers and operators of buildings with cooling plant, the practical position is that a heatwave is the moment the highest-consequence Legionella system in the building is least forgiving. The questions worth asking now, not in October, are whether every cooling tower, evaporative condenser and adiabatic cooler on the estate is notified to the local authority, whether each has a current HSG274 Part 1 risk assessment and written scheme of control distinct from the building's hot and cold water regime, and whether the treatment and monitoring records for this summer actually exist and show control. The hot and cold water systems are the ones everyone remembers. The cooling tower is the one that makes the headlines.

Pleasant Plumbers' commercial team carries out Legionella risk assessments and water-safety oversight across London portfolios, including evaporative cooling systems under HSG274 Part 1. To review cooling tower and water hygiene risk across your estate, call or WhatsApp 0800 046 1000, or email [email protected].

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