Legionella risk assessment: what it is and what the law requires

Legionella bacteria can grow in water systems and cause Legionnaires' disease when contaminated water droplets are inhaled. An assessment identifies whether a system can support bacterial growth and expose people, and what controls are needed. It must reflect the actual installation and the people who may be exposed. A short generic form cannot decide whether a complex cooling tower, hot-water system or other installation is adequately controlled.

For Great Britain, the relevant duties arise from the Health and Safety at Work etc. Act 1974, the Management of Health and Safety at Work Regulations, and, where relevant, COSHH. Northern Ireland employers should check their own legal framework. HSE's L8 Approved Code of Practice provides an authoritative route to compliance and includes guidance on risk assessment, a competent responsible person, a control scheme, review and records. HSG274 gives technical guidance for different water systems. L8 is an Approved Code of Practice, not a freestanding Act named the Legionella Law. Duty holders should distinguish legal duties from the practical methods described in guidance.

Who is responsible?

The employer or person in control of premises should identify the relevant duty holder and appoint a competent person to manage the controls. HSE commonly calls this person the responsible person. The appointment does not transfer away the duty holder's legal responsibility. The person needs enough knowledge of the system, authority to act, and access to competent support where the system exceeds their expertise. A landlord, managing agent and occupier should agree who controls each part of a shared system and who receives records and alerts.

Competence matters because water systems differ. Temperature, storage, circulation, stagnation, outlets and exposure routes affect the risk. A competent assessor should inspect or understand the actual installation, not simply copy a previous building's control scheme.

What should the assessment examine?

Start with a system description and schematic where needed. Identify stored and distributed water, temperatures, little-used outlets, dead legs, aerosol-generating equipment, maintenance arrangements, and the people who may be exposed. Review the age and condition of plant and the history of remedial work or test results. Consider whether others, such as residents, patients, visitors or contractors, may be more vulnerable. A healthcare setting may need more specific arrangements than a small office.

Then assess whether the risk can be avoided or controlled. If the system is complex or not well understood, obtain competent specialist input. The assessment should specify the controls, who performs them, what records will show they were done, and what action follows a result outside the expected range. Do not assume that routine sampling alone controls a poorly designed or maintained system.

An example of an assessment question

Consider a building with a hot-water system that serves a rarely used changing room. The duty holder should identify where water is stored, heated and circulated; whether the outlet is used often enough to avoid prolonged stagnation; who may inhale droplets when it is used; and whether the current control scheme includes the outlet. The response may involve changes to use, design, monitoring or maintenance. The correct measure depends on the actual system and HSG274 guidance. A one-off sample from the outlet cannot answer every question about system control.

In a larger building, the same assessment must consider the interfaces between landlord, tenant and maintenance contractor. Who has access to the plant schematic? Who receives out-of-range readings? Who authorizes remedial work and checks it was completed? The record should identify these responsibilities. A contract that says the maintenance provider handles legionella does not by itself resolve who controls the risk under health and safety law.

Control scheme and monitoring

Where the assessment shows a risk that cannot be eliminated, L8 expects a written scheme for controlling it. The scheme should describe the system, the control measures, checks and frequency appropriate to that system, limits or targets, corrective actions, and responsibilities. HSG274 provides technical detail for particular systems. A reader should not use a temperature number lifted from a generic article as a substitute for the full guidance and the actual water system.

Monitoring records need to connect a result to an outlet, date, method, responsible person and action. An out-of-range result should trigger investigation and corrective work, not merely a red cell in a spreadsheet. Review the risk assessment and scheme after significant system changes, loss of control, relevant sampling results, a case of illness, or when existing arrangements may no longer be effective. Set a planned review as well, but respond to changes when they happen.

What records should show

A usable scheme should let another competent person understand the system, the risks identified, the intended controls and what actually happened. Keep the current assessment, system description, control scheme, monitoring and maintenance logs, exceptions, corrective actions and reviews together or connected by clear references. HSE's L8 discusses records and retention in detail. Do not replace those provisions with a generic statement that every record is kept for the same number of years. Different records and circumstances can require different treatment.

Where readings or other checks fall outside the scheme's limits, record the investigation, action taken and evidence of return to control. A record showing repeated exceptions without action is evidence of a problem, not evidence that monitoring succeeded. Keep staff who use or maintain the system informed of the controls relevant to them.

Keeping the boundaries clear

The existing healthcare positive-sample response guide addresses a specific incident in a specialist setting. This guide owns the general assessment and control-process question. The legionella compliance software page is the commercial route for reviewing record-management options. No claim is made here that Complys takes water samples, measures temperatures automatically, interprets microbiological results, or certifies a water system safe.

General information, not legal advice. Verify current requirements against the primary sources linked above.