Quick answer
Understand the water systems and assess whether they create a foreseeable legionella exposure risk. Appoint competent people, prevent or control identified risks, keep appropriate records and act on monitoring failures. The controls and checks must fit the system rather than a generic flushing checklist.
Why the assessment begins with the water system
Relevant risks may arise from hot and cold water systems and other equipment that can produce contaminated droplets. HSE’s legionella duties guide explains the assessment, management, control and record requirements.
A contractor’s invoice or a single sample result does not explain the whole system or demonstrate that all required controls are effective.
Understand the system, exposure and people affected
HSE’s assessment guidance addresses the system description, storage or recirculation, growth conditions, potential droplets and exposed people. Obtain competent help if the business lacks the necessary knowledge.
| Assessment item | Management question |
|---|---|
| System description | Are all relevant parts and equipment identified, including unused areas? |
| Risk and existing controls | What could permit growth and exposure, and what prevents it? |
| People and responsibility | Who could be exposed and who has authority to manage the controls? |
| Checks and action | What is monitored, against what criteria, and who responds to a failure? |
| Review | What system, occupancy or performance change requires reassessment? |
Assign responsibility and a suitable control scheme
HSE’s management guidance requires competent responsible people with appropriate authority and system knowledge. Using contractors does not remove the need to manage and check their work.
Where risk cannot be prevented, follow the written-control-scheme guidance. Specify the relevant operation, maintenance, precautions, monitoring and response arrangements. Use system-specific technical advice for temperatures, cleaning, flushing, sampling or treatment; this article is not a water-treatment instruction.
Use monitoring results to trigger action
Record relevant results, defects, actions and closure. HSE’s record guidance distinguishes current assessment/scheme records from monitoring records and their retention arrangements.
HSE’s review guidance asks whether the system description, responsibilities, contractor competence and controls remain suitable. Set a prompt response to failed control criteria. A monitoring result filed without escalation leaves the underlying risk unresolved.
A simple assessment may conclude that risk is low and existing controls are sufficient. Maintain those controls and review the assessment when relevant circumstances change; do not invent an automatic expiry date.
Example: partial closure changes water use
A hospitality business stops using part of its premises for a period. The responsible person checks the effect on the water system and infrequently used outlets with suitable competent advice, rather than assuming the previous pattern of use continues.
The resulting arrangement identifies the relevant controls, checks, action owner and conditions for safe return to use. It does not apply a generic disinfection recipe.
For NI, use HSENI’s legionella resources and the correct legal framework.
Common mistakes
- No clearly responsible person.
- Unidentified system changes or unused areas.
- Sampling or flushing treated as the complete assessment.
- No action when a control check fails.
Next steps
Confirm the competent responsible person, current system description and assessment. Check that the control scheme, monitoring and escalation arrangements reflect the actual system and recent changes.
Use HSE’s legionella management guidance — System assessment and control require appropriate competence and system-specific technical guidance.