What happens if legionella is found in my water system?
Updated 12 September 2026
Short answer
Finding legionella at a low count is usually a sign to tighten controls, not a crisis. A count above the action threshold triggers a specific response: disinfection, investigation of the cause, and repeat sampling. You do not have to notify anyone unless a case of Legionnaires' disease has been linked to the premises, in which case RIDDOR applies.
The response to a positive sample depends on the count, the system it came from, and whether anyone has been ill. See our detailed guide at What to do if legionella is detected for the step-by-step action thresholds.
Most important first steps
When you get a result above the action threshold:
- Do not panic, but do not delay either. The system has been as it is for some time before the sample was taken.
- Isolate the affected outlet or sub-system if the count is high or the users are vulnerable.
- Engage your water hygiene contractor or another competent person to plan the disinfection and investigation.
- Consider whether the users of the affected outlet need to be informed, particularly in healthcare or care home settings.
Do I have to notify HSE?
Not automatically. A positive sample on its own is not a RIDDOR-reportable event. The duty to notify under RIDDOR arises only if a case of Legionnaires' disease is linked to the premises, in which case the duty holder must report within ten days under Schedule 2.
If HSE is already investigating, or if the premises are subject to CQC inspection, the relevant body should be kept informed of the result and the action being taken.
Will the building have to close?
Almost never. Disinfection can usually be carried out with the building in normal use, sometimes zone by zone. Temporary closure may be necessary for pools, spa pools, and any system where the count is very high or multiple cases are linked, but that is rare.
Documenting the outcome
The full record should include the original positive sample result, the investigation, the action taken, the post-action sample results, and any changes made to the control scheme to prevent recurrence. This becomes part of the legionella records for the premises and should be retained for at least five years.
Legal basis
HSG274 Part 2 Table 2.3 on action levels. RIDDOR 2013 regulation 5 and Schedule 2 on reportable diseases. HSE guidance on investigating Legionnaires' disease outbreaks.
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Free legionella risk assessment template
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