Legionella is a type of bacteria that occurs naturally in freshwater environments — rivers, lakes, and soil. In small quantities, it's harmless. But when Legionella finds its way into building water systems and encounters the right conditions, it can multiply rapidly to dangerous levels.
The danger comes when contaminated water is dispersed as fine droplets — aerosols — that people can breathe in. Showers, taps, cooling towers, spa pools, and any system that creates a spray or mist can be a transmission route. When someone inhales water droplets containing high concentrations of Legionella bacteria, they can develop Legionnaires' disease — a serious form of pneumonia that can be fatal, particularly in older people, smokers, and those with weakened immune systems.
Legionella multiplies most rapidly in water temperatures between 20°C and 45°C — the so-called 'danger zone'. Below 20°C, the bacteria are dormant. Above 60°C, they're killed. Between those two temperatures, particularly around 35-40°C, they proliferate quickly. Other factors that help Legionella colonise a water system include stagnation (water sitting still in unused pipes or dead legs), biofilm (the slimy layer that forms on the inside of pipes), and sediment, scale, or rust that provides nutrients and harbourage.
Under the Health and Safety at Work Act 1974 and the HSE's Approved Code of Practice L8, every employer, landlord, or building owner who controls a water system has a duty to assess and manage the risk from Legionella. This isn't optional. The responsible person — known as the 'duty holder' — must ensure a suitable and sufficient risk assessment is carried out, control measures are implemented, and an ongoing monitoring programme is in place.
At a minimum, you need a current Legionella risk assessment, a written scheme of control, regular temperature monitoring at sentinel points, periodic water sampling, and documented records of all checks and actions. Depending on your system, you may also need annual tank inspections, TMV servicing, and remedial plumbing works to address identified risks.
The HSE investigates Legionella outbreaks and can take enforcement action against duty holders who haven't met their obligations. Penalties range from improvement notices to criminal prosecution. In serious cases — particularly where someone has died — duty holders have been sent to prison. Even without an outbreak, if an HSE inspector finds that you don't have a current risk assessment or monitoring records, you could face enforcement action.
Legionella is a manageable risk. With a competent risk assessment, sensible control measures, and regular monitoring, the vast majority of buildings can maintain safe water systems without excessive cost or disruption. The key is to take it seriously, act on your risk assessment recommendations, and keep your records up to date.
Not sure if your building is compliant? Book a free compliance review with Same Water.