Short answer. Legionella bacteria grow in poorly maintained water systems and cause a serious pneumonia when inhaled in fine water droplets. The person in control of a building's water system, the duty holder, must assess the risk, control temperatures, and keep a written scheme in place under COSHH 2002 and the Health and Safety at Work Act 1974.
How legionella grows
Legionella bacteria exist naturally in water sources at low levels. The problem starts when conditions let them multiply. The growth range for legionella is roughly 20°C to 45°C, which happens to be the temperature range of most poorly maintained hot and cold water systems.
Add stagnation (water sitting still in pipes that aren't used regularly), biofilm (the slimy deposits that build up inside older pipework), and scale or sediment, and the system becomes an effective breeding ground. The bacteria become dangerous once they are dispersed as fine droplets, through showers, cooling towers, spa pools, decorative fountains or any system that creates an aerosol.
The bacteria cannot be seen, smelled or tasted in water. Inhaling contaminated droplets can cause Legionnaires' disease, a serious and sometimes fatal pneumonia. The overall case fatality rate is around 10%, and it is highest among people who are older, smokers, or have an existing lung condition or weakened immune system.
Who is legally responsible
Legionella is treated in law as a biological agent under the Control of Substances Hazardous to Health Regulations 2002 (COSHH), alongside general duties under the Management of Health and Safety at Work Regulations 1999 and the Health and Safety at Work Act 1974. HSE's Approved Code of Practice L8 sets out how those duties should be met, with the practical detail on temperatures, flushing and monitoring in HSG274 Part 2: The control of legionella bacteria in hot and cold water systems.
The duty holder, the employer, building owner, landlord or managing agent with control of the premises and its water systems, must:
- Appoint a competent person to manage water safety
- Carry out a legionella risk assessment
- Implement a written scheme of control
- Maintain temperature controls: hot water stored at 60°C or above, reaching 50°C at the outlet within one minute of running, and cold water below 20°C
- Monitor and record temperatures and other control measures regularly
- Review the risk assessment periodically and after any change to the system or its use
This is a legal duty, not a formality. Failing to meet it can lead to enforcement action, prosecution and heavy fines, and in the most serious cases to corporate manslaughter charges where a failure leads to death.
The buildings most at risk
Some water systems carry a higher risk than others. A building carries a higher risk if it has any of the following features.
Complex water systems. Multi-storey buildings, long pipe runs, multiple risers and extensive hot water distribution all give legionella more opportunity to establish itself somewhere in the system.
Infrequently used outlets. Showers in hotel rooms that aren't occupied every night, rarely used basins in meeting rooms, and seasonal facilities all allow water to stagnate.
Dead legs. Sections of pipework that no longer serve an active outlet but still hold water. These are stagnation hotspots and a common finding on assessment.
Water sitting between 20°C and 45°C. Calorifiers running below 60°C, cold water tanks in warm plant rooms, and mixed-temperature distribution all sit inside the growth range.
Vulnerable occupants. Care homes, hospitals, and buildings serving elderly or immunocompromised people face the most serious consequences if control fails.
What a legionella risk assessment covers
A proper legionella risk assessment is a systematic review of the whole water system, not a glance at a boiler. The assessor examines cold water storage, hot water generation and distribution, temperatures throughout the system, the condition of pipework, the presence of dead legs, outlet usage patterns, any water treatment regime, and the management arrangements in place.
The output is a report identifying the risks specific to that building, a prioritised action plan, and a written scheme of control setting out what needs monitoring, how often, and by whom.
Common problems on assessment
Recurring issues on legionella assessments include temperatures sitting 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, no flushing regime for infrequently used outlets, temperature checks that are done but not acted on when readings fall outside safe parameters, and no written scheme of control in place at all.
Bringing a system under control
Legionella risk is manageable. Temperature control, a regular flushing regime, proper maintenance, removal of dead legs and a documented monitoring programme bring most water systems under control without shutting the building down.
What it requires is a proper risk assessment carried out by someone who knows what they are looking for, followed by implementation of the recommendations. York Green's Legionella risk assessment service covers exactly that: the survey, the written scheme, and the ongoing monitoring plan.
For the full technical detail, see HSE's Legionnaires' disease guidance and the Approved Code of Practice L8.