In one line
If you're responsible for premises, you have a legal duty to assess and control the risk of legionella in your water systems. That means a risk assessment, a written control scheme, routine monitoring (like temperature checks and flushing), and records that prove it's being done.
Legionella bacteria live in water systems and can cause Legionnaires' disease, a serious and sometimes fatal form of pneumonia, when contaminated water droplets are inhaled — from showers, taps, cooling towers, spa pools and similar. Because the bacteria thrive in the water systems almost every building has, the duty to control them is broad.
Who has a duty, and under what law
Duties fall on employers, the self-employed and those in control of premises. They flow from the Health and Safety at Work etc. Act 1974 and the Control of Substances Hazardous to Health Regulations (COSHH), with practical guidance in the HSE's Approved Code of Practice L8 ("Legionnaires' disease: the control of legionella bacteria in water systems") and the technical guidance HSG274. Landlords have duties too, for the water systems in properties they let.
When is legionella a risk?
The conditions that let legionella multiply are worth knowing, because your controls target them directly:
- Water temperature between roughly 20°C and 45°C — the growth range. Hot water stored below ~60°C and cold water above ~20°C is a red flag.
- Stagnation — little-used outlets, dead legs in pipework, infrequently used showers.
- Nutrients — scale, rust, sludge and biofilm feeding the bacteria.
- Aerosols — anything that creates a fine spray people can breathe in.
The four jobs of legionella control
1. Assess the risk
A competent legionella risk assessment identifies your water systems, where the risks are, and who's exposed. It must be reviewed regularly and whenever the system or its use changes.
2. Manage it
Appoint a responsible person to take day-to-day charge, and put a written control scheme in place describing the system and exactly how the risks will be controlled.
3. Control and monitor
Carry out the routine tasks the scheme sets — temperature monitoring, flushing of little-used outlets, showerhead cleaning and descaling, tank inspections, and keeping hot water hot and cold water cold. These are recurring, scheduled jobs, not one-offs.
4. Keep records
Log the assessment, the scheme, and every monitoring task and result. If someone falls ill or an inspector calls, these records are the evidence that you managed the risk.
Legionella control is almost entirely about consistency — the same temperature checks and flushes, done on time, recorded every time. It's a scheduling and evidence problem more than a technical one.
Where it goes wrong
The common failures are mundane: a monitoring task missed on a busy week, a little-used outlet nobody flushed, a temperature reading that drifted out of range and wasn't acted on, or records that exist on clipboards nobody can produce. Each is a gap between "we have a scheme" and "we followed it".
Making it stick
The reliable approach is to treat each water asset and control task as scheduled, evidenced work: register the systems and outlets, set the monitoring tasks to recur with reminders, capture each reading against the asset, and flag any out-of-range result as work to action. That turns a scheme on paper into a control regime you can prove — the same discipline behind preventive maintenance and other safety duties.
This article is general guidance, not legal advice. Legionella duties depend on your premises and water systems — follow the HSE's ACOP L8 and HSG274 and consult a competent water hygiene adviser.