Legionella control, and why summer raises the stakes

In 2024 there were 472 confirmed cases of legionellosis in residents of England and Wales, 463 of them Legionnaires' disease, an incidence of 0.7 per 100,000 people and a fall of 22 per cent on the 609 cases reported in 2023 (UKHSA, Legionellosis in residents of England and Wales: 2024, published 2025). A falling number is welcome, but it is not a reason to ease off. As recently as March 2026 the UKHSA, several London local authorities and the Health and Safety Executive were investigating a cluster of cases linked to north-west and south-west London, a reminder that the bacterium is still in the water systems around us.

Why summer is the awkward season

Legionella bacteria multiply fastest in water between roughly 20 and 45 degrees. Warm weather pushes stored and incoming cold water above 20°C, and the long days slow nothing down. At the same time, summer is when buildings empty out. Flats are let go over the holidays, offices run on skeleton staff, a wing closes for refurbishment. Every tap, shower and outlet that stops being used becomes a dead leg where water sits warm and still, which is exactly the condition the control regime exists to prevent.

What the law actually asks for

Controlling legionella is not optional guidance. The duty sits under the Health and Safety at Work etc. Act 1974 and the Control of Substances Hazardous to Health Regulations 2002, and the HSE sets out how to meet it in the Approved Code of Practice L8 and the technical guidance HSG274. In practice the duty holder has to do four things.

• Assess the risk across the water systems, and review it whenever the building or its use changes.
• Appoint a responsible person with the authority and competence to manage the controls.
• Keep a written scheme that records how each risk is controlled, and by whom.
• Monitor and record the things that keep water safe: cold water held below 20°C, hot water stored at 60°C and reaching the tap above 50°C, little-used outlets flushed weekly, shower heads and hoses cleaned and descaled.

None of that is hard in isolation. What makes it fail is the same thing that undoes every compliance duty. The schedule lives in someone's head, the weekly flush gets missed during the holiday, and the temperature log has a three-week gap nobody noticed until an inspector asked.

Keeping the regime live when the building is quiet

The summer gap is a records problem before it is a plumbing problem. A control scheme only protects residents if its recurring tasks have an owner and a date, and if a missed one is visible rather than silent. That is the same discipline behind a working compliance calendar: every flush, temperature check and clean is a dated, owned action that chains to the next, so cover during leave becomes a handover rather than a guess. The readings and certificates then sit in a document register against the building they belong to, ready to show as a continuous record rather than something reconstructed after the fact. We have written before about why planned maintenance beats reactive every time, and water hygiene is the clearest case of it.

The 2024 figures show the controls work when they are kept up. The risk was never that anyone forgets legionella exists. It is that a quiet building in August lets the routine slip, and the routine is the whole of the protection.