Industry News28 April 2026

London Legionnaires' Outbreak, April 2026: What It Means for Assessors

UKHSA is investigating a Legionnaires' disease outbreak across NW and SW London with 21 cases under review. Here's what assessors should be checking right now.

The UK Health Security Agency (UKHSA), Local Authority environmental health teams, and the Health and Safety Executive (HSE) are continuing to investigate an outbreak of Legionnaires' disease affecting both North West and South West London.

By 23 March 2026, eight cases had been confirmed as sharing the same sequence type, with a further 13 cases under investigation to establish whether they were linked. By early April that number had risen to 21 cases under review. As of the date of this article, no single common environmental source has been identified, and the investigation is ongoing.

This article summarises what is publicly known and -- more importantly -- what the outbreak should prompt assessors to look at when they are next on site.

What is publicly known

The outbreak spans two geographically distinct areas of London (NW and SW). That is unusual.
All confirmed cases share the same sequence type, indicating a common environmental source -- but the source has not yet been identified.
An Incident Management Team has been coordinating the response across UKHSA, the relevant London local authorities, environmental health, and HSE.
Cooling towers within the affected areas have been inspected, sampled, and in some cases precautionary shock-dosed with high-strength biocide while the investigation continues.
London hospitals serving the affected areas have implemented enhanced respiratory screening for community-acquired pneumonia admissions.

The case count is reportedly atypical for the time of year, which is one of the reasons the response has been escalated.

Why cooling towers are in scope

Cooling towers and evaporative condensers are the classic high-consequence source of community-wide Legionnaires' outbreaks. They:

Operate at temperatures that fall within or close to the legionella growth range
Aerosolise water as part of normal operation
Can disperse aerosols hundreds of metres -- in some recorded outbreaks, more than a kilometre

When cases cluster geographically without a shared building or workplace, cooling towers are the first thing investigators look at. A single poorly-controlled tower can affect anyone within the dispersal plume, regardless of where they live or work.

Cooling towers in England, Wales, and Scotland must be notified to the local authority under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992. The HSE maintains a national register. During outbreak investigations, that register is the starting point for site visits and sampling.

What assessors should be doing now

If you are responsible for, or assessing, sites that fall within the affected areas -- or any site with a cooling tower or evaporative condenser anywhere in the UK -- the outbreak is a useful prompt to verify a small number of high-leverage controls:

1. Confirm the cooling tower is actually on the HSE register.

This sounds basic but is regularly missed. Notification is the duty of the person in control of the premises. If a tower is operating without notification, that is itself an HSE-actionable issue separate to any legionella finding.

2. Review the most recent risk assessment.

HSG274 Part 1 requires that the risk assessment for a cooling tower or evaporative condenser is reviewed at least every two years, and immediately if anything significant changes (system modification, change of use, change of treatment regime, sustained out-of-spec results). Check the date. Check what triggered the last review.

3. Verify the written scheme of control is being implemented.

It is one thing to have a written scheme. It is another to be able to evidence that the monitoring, dosing, dipslide, and microbiological sampling regimes are actually being carried out at the stated frequency. Ask for the records. Sample-check three months at random.

4. Check microbiological sampling frequency and recent results.

HSG274 Part 1 expects routine legionella sampling of cooling tower water at a minimum of quarterly, more frequently if results are out of spec or the system is identified as higher risk. Look at the last four results. Look at how out-of-spec results were actioned.

5. Look at the basics that actually fail.

Drift eliminator condition, presence of biofilm or scale, water treatment dosing pump function, and the integrity of the water make-up are the conditions that show up repeatedly when an outbreak source is finally identified. None of them are exotic. They are the things that get missed when monitoring becomes a paperwork exercise.

6. Consider whether the dispersal plume could affect vulnerable populations.

Hospitals, care homes, and other healthcare premises within the dispersal radius of an evaporative source warrant specific consideration. HSG274 Part 1 includes guidance on plume modelling. If you are aware of a tower whose plume crosses a healthcare premises and you cannot evidence robust control, escalate.

The fact that an outbreak source has not yet been found does not mean cooling towers are the source in this specific case. It means the investigation has not yet identified one. Other community sources -- hot tubs, decorative fountains, and other aerosol-generating systems -- are also examined in outbreak investigations of this kind. The point for assessors is that any aerosol-generating water system that has been allowed to drift out of control can produce a community-wide event.

A note on hot and cold water systems in the affected areas

While cooling towers are the typical primary suspect for geographically dispersed outbreaks, the outbreak is also a useful reminder that hot and cold water systems in larger occupied buildings -- offices, hotels, residential blocks -- need their basics in order:

Stored hot water at or above 60°C
Hot water reaching 50°C at sentinel outlets within one minute
Cold water below 20°C at sentinel outlets within two minutes
Monthly sentinel temperature monitoring documented and acted on
Dead legs identified, removed where practical, flushed where not (we have a separate article on dead legs)
Infrequently used outlets flushed at a documented frequency

These are not novel requirements. They are the day-one expectations of HSG274 Part 2 and ACOP L8. Outbreaks rarely emerge from systems where these are routinely in place. They almost always emerge from systems where one or more have been allowed to slide.

What the outbreak does not change

Nothing about HSG274, ACOP L8, or BS 8580-1:2019 has changed because of this outbreak. The framework was already adequate. The lesson is, as it usually is, about consistent execution: notification, regular and competent risk assessment, an actually-implemented written scheme of control, documented monitoring, and timely action on out-of-spec results.

If you cover sites with cooling towers, this week is a good week to verify each of those is in order on every one of them. If you cover hot and cold water systems only, it is still a good week to walk through your sentinel temperature records and confirm dead-leg actions from last year's reports were actually done.