Nobody thinks about their water system. It's one of those invisible utilities — it works, water comes out of the taps, and everybody gets on with their day. Which is precisely why legionella is such a dangerous problem. By the time you realise something's wrong, people are already ill.
Legionnaires' disease kills around 10-15% of otherwise healthy people who contract it. For those with underlying conditions, that figure climbs higher. And the bacteria responsible — Legionella pneumophila — thrives in exactly the kind of water systems found in offices, hotels, care homes, schools, gyms and apartment buildings across the UK.
Legionella bacteria exist naturally in water sources at low levels. The problem starts when conditions allow them to multiply. The sweet spot for legionella growth is between 20°C and 45°C — which happens to be the temperature range of most poorly maintained hot and cold water systems.
Add some stagnation (water sitting still in pipes that aren't used regularly), some biofilm (the slimy deposits that build up inside older pipework), some scale and sediment, and you've created an ideal breeding ground. The bacteria multiply in the water system and become dangerous when they're dispersed as fine droplets — through showers, cooling towers, spa pools, decorative fountains, or any system that creates an aerosol.
You can't see it. You can't smell it. You can't taste it. But inhale those contaminated water droplets and you could end up in hospital with a serious pneumonia.
Under the Health and Safety at Work Act 1974 and the specific guidance in L8 (Legionnaires' disease: The control of legionella bacteria in water systems), the duty holder is responsible for managing legionella risk. That's the employer, the building owner, the landlord, the managing agent or whoever has control of the premises and its water systems.
This isn't optional. The duty holder must:
Failure to do this isn't just a regulatory technicality. It can result in prosecution, heavy fines and — in the worst cases — corporate manslaughter charges.
Some water systems are inherently higher risk than others. If your building has any of the following features, your legionella risk is elevated:
Complex water systems — multi-storey buildings, long pipe runs, multiple risers, extensive hot water distribution
Infrequently used outlets — showers in hotel rooms that aren't occupied every night, rarely used basins in meeting rooms, seasonal facilities
Dead legs — sections of pipework that no longer serve an active outlet but still contain water. These are stagnation hotspots
Water stored between 20-45°C — calorifiers running below 60°C, cold water tanks in warm plant rooms, mixed-temperature distribution
Vulnerable occupants — care homes, hospitals, buildings serving elderly or immunocompromised people face the highest consequences if things go wrong
A proper legionella risk assessment isn't someone glancing at your boiler and handing you a certificate. It's a systematic review of your entire water system — every tank, every pipe run, every outlet, every piece of plant.
The assessor examines the cold water storage, the hot water generation and distribution, the temperatures throughout the system, the condition of the pipework, the presence of dead legs, the usage patterns of outlets, the water treatment regime (if any), and the management arrangements in place.
The output is a detailed report identifying the risks specific to your building, with a prioritised action plan and a written scheme of control that tells you exactly what needs monitoring, how often, and by whom.
After hundreds of legionella risk assessments, there are patterns. The most common issues include:
Temperatures in the danger zone — calorifiers set too low, cold water tanks in warm roof spaces, mixing valves reducing distribution temperatures below safe levels
Dead legs and unused outlets — the tap in the meeting room nobody uses, the shower in the old changing room, the basin behind the filing cabinet
No flushing regime — infrequently used outlets need regular flushing (at least weekly) to prevent stagnation, but most buildings don't have a programme in place
Poor records — temperature checks not being done, or being recorded but not acted upon when readings fall outside safe parameters
No written scheme — the legal requirement for a documented control scheme is often completely absent
The good news is that legionella risk is entirely manageable. Temperature control, regular flushing, proper maintenance, removal of dead legs and a documented monitoring programme will bring most water systems under control. It's not expensive relative to the consequences, and it doesn't require shutting down your building.
What it does require is a proper risk assessment by someone who knows what they're looking for, followed by implementation of the recommendations.
Concerned about your water system? Book a Legionella Risk Assessment →
York Green Safety Partners provides comprehensive legionella risk assessments for commercial, residential and healthcare premises across the UK. Based in Cheshire, covering the whole country.