Legionella risk assessment: what employers must check in workplace water systems

Start with the dutyholder, not the water-treatment contract

Employers and people in control of premises must assess and control foreseeable exposure to Legionella. The legal framework includes the Health and Safety at Work etc. Act 1974 and the Control of Substances Hazardous to Health Regulations 2002. HSE's Approved Code of Practice L8 explains how dutyholders can comply; it is not a certificate or a substitute for a site-specific assessment.

Appoint a competent person, often called the responsible person, with enough authority, knowledge and resources to manage the precautions. External consultants can assess or monitor the system, but outsourcing the work does not transfer the dutyholder's responsibility. Define who receives results, challenges missed visits, authorises remedial work and confirms that a failed control has recovered.

Where a landlord, tenant, managing agent or contractor shares control, record the boundaries. The assessment should identify who controls incoming water, storage, distribution, leased areas and common facilities rather than assuming the maintenance contract answers every question.

HSE: Legionnaires' disease — what dutyholders must do ↗ · HSE: Approved Code of Practice L8 ↗

Map the real system and the people who may be exposed

The assessor needs to understand how water enters, is stored, heated, circulated and discharged. Record tanks, calorifiers, showers, spray taps, thermostatic mixing valves, expansion vessels, redundant pipework and outlets that receive little use. Include temporary shutdowns, seasonal rooms and areas awaiting refurbishment; stagnation can develop while the rest of the building operates normally.

Legionella can multiply where water is between 20°C and 45°C, nutrients such as scale, rust or sludge are present, and water stagnates or recirculates. Exposure normally requires contaminated droplets or aerosols from outlets, showers, atomisers or similar equipment. A risk assessment should therefore connect conditions in the system with a credible route to the people affected.

Consider susceptibility as well as system condition. Older people, smokers, people with chronic respiratory or kidney disease and those with weakened immune systems can face greater consequences. Care homes and healthcare premises need more rigorous, setting-specific arrangements and competent interpretation of HSG274 Part 2. This is about the people and consequences, not a loose building label.

HSE: hot and cold water systems ↗ · Related guide: making risk assessments work in practice →

Decide whether the risk can be removed or needs a control scheme

Remove water systems or features that are not needed where reasonably practicable. Redundant dead ends, over-sized storage and unused showers add complexity without operational value. Where a risk remains, prepare a written control scheme that states what must be done, by whom, at what frequency and what action follows an unacceptable result.

A simple mains-fed system in a small, regularly occupied office may be assessed as low risk. That conclusion still needs evidence: no unnecessary storage, no aerosol-producing equipment of concern, regular outlet use, suitable temperatures and no unusually susceptible population. A generic 'small premises — low risk' statement is not enough.

Cooling towers, evaporative condensers, spa pools and other complex systems require different and often specialist controls. Do not stretch a basic hot-and-cold-water checklist to cover them. Define the system scope and obtain competent technical support where the equipment, treatment regime or monitoring results exceed the responsible person's expertise.

HSE: HSG274 Legionella technical guidance ↗

Use temperature benchmarks as controls, not as a certificate

Where temperature is the primary control, HSE says cold water should be maintained below 20°C where possible. Hot water should be stored at least at 60°C and distributed so it reaches 50°C within one minute at outlets; HSE gives 55°C for healthcare premises. For cold sentinel outlets, its practical guidance uses below 20°C after two minutes.

These are HSE control benchmarks, not universal statutory pass marks. The scheme must specify representative and sentinel outlets, test frequency, measuring method and corrective action. A reading taken immediately after a quiet weekend may reveal something that a fully flushed Monday-afternoon reading hides.

Control scalding without undermining Legionella control. Where vulnerable people can access baths or showers, assess their ability to recognise and respond to hot water and use suitable thermostatic mixing where required. Keep pipework downstream of mixing valves short, monitor the hot supply to the valve and maintain the valve itself. A cool blended outlet reading does not prove that the hot distribution system is working.

HSE: practical hot and cold water checks ↗

Control stagnation and low-use outlets

Low use is a management issue, not just a plumbing detail. Identify every outlet that may sit unused because a team moved, a room closed, a tenant left or hybrid working reduced occupation. HSE guidance recommends a regular flushing programme for infrequently used outlets, typically weekly, carried out in a way that avoids releasing aerosols.

Flushing is guidance applied through the assessment, not a statutory seven-day rule for every tap. It should have a named owner, safe method, complete outlet list and missed-task escalation. If weekly flushing becomes a permanent workaround for numerous redundant outlets, removal or system alteration may be the better control.

Keep storage tanks covered, insulated and clean; inspect for sludge, scale and corrosion; and clean and descale showerheads at the assessment-led frequency. Planned building shutdowns should trigger a safe mothballing, recommissioning and flushing plan before people return—not an assumption that turning the taps on solves the risk.

Monitor the scheme and respond to failures

Monitoring should show whether the control scheme is operating, not merely produce readings. Check trends, missed tasks, abnormal temperatures, loss of circulation and recurring failures. Set action thresholds and timescales in advance, then record investigation, temporary precautions, remedial work and confirmation that control has been restored.

Routine Legionella sampling is not normally required for enclosed hot and cold water systems. HSE says testing may be needed where there is doubt about the control regime or recommended temperatures or disinfectant concentrations are not consistently achieved. Where sampling is appropriate, use competent methods and an appropriate accredited laboratory, and interpret results in the context of the assessment.

A clear sample does not repair stagnation, and a temperature log is not a microbiological result. Sampling, where justified, supplements system control. It does not replace design, maintenance, flushing, temperature or treatment arrangements.

HSE: testing and monitoring water systems for Legionella ↗ · Related guide: COSHH assessments and safety data sheets →

Keep usable records and review when the risk changes

If the organisation has five or more employees, record the significant findings, people particularly at risk and steps taken to control exposure. Records should identify the assessor and responsible people, the assessment findings, control scheme, system operating status and the results and dates of checks. HSE says current records and the control scheme should be retained while current and for at least two years afterwards; monitoring records should be retained for at least five years.

Review periodically and whenever there is reason to suspect the assessment is no longer valid. Triggers include a system extension, refurbishment, different occupancy, increased vacancy, new susceptible users, a control failure, adverse sampling, contractor change, suspected illness or new technical information. Update the control scheme and responsibilities, not only the assessment date.

HSE: monitoring and keeping Legionella records ↗ · HSE: reviewing Legionella controls ↗

Ten-point Legionella control checklist

Test the arrangements with one failed reading. Can the person reviewing it identify the outlet, understand why it matters, apply temporary precautions, authorise corrective work and confirm recovery? If the record ends at the number, the control system is incomplete.

• Name the dutyholder, competent assessor, responsible person and deputy.
• Map storage, circulation, outlets, mixing valves and redundant pipework.
• Identify aerosol routes, stagnation and susceptible occupants.
• Remove unnecessary water features and dead ends where practicable.
• Set temperature or alternative treatment controls for the actual system.
• List sentinel, representative and infrequently used outlets.
• Define monitoring frequencies, methods, limits and missed-task escalation.
• Balance Legionella control with a specific assessment of scalding risk.
• Record failed controls, remedial actions and proof of recovery.
• Review after system, occupancy, use, contractor or control changes.