First: read the result properly
Legionella results in hot and cold water systems are reported in colony-forming units per litre (cfu/L), and HSG274 sets clear action levels. Up to 100 cfu/L is generally acceptable in a well-managed system. 100–1,000 cfu/L means the system is colonised and requires investigation and likely resampling. Over 1,000 cfu/L demands immediate action.
Before anything else, confirm which system the sample came from, the exact count, and whether it was a single outlet or a system-wide pattern. The right response to one outlet at 300 cfu/L is very different from multiple outlets above 1,000 cfu/L.
Hours 0–4: contain and assess
Notify your responsible person and senior management immediately, and start a written log — every decision from here should be timed and recorded, because that record is what protects the duty holder later.
For high counts (over 1,000 cfu/L) or any result affecting vulnerable occupants, take immediate precautionary control of the affected outlets: restrict or stop use, put up clear signage, and where showers are involved consider removing them from service to prevent aerosol exposure.
Review your existing risk assessment and control records for the affected system. Recent temperature failures, a missed flush regime, or a known dead-leg often explain the result and point straight to the corrective action.
Hours 4–24: investigate and plan remediation
Get a competent engineer on site. A reputable provider mobilises within 24 hours of a positive result with a remedial plan — that response time is precisely what demonstrates the duty holder acted reasonably.
The engineer will identify the probable cause, take confirmatory and surrounding samples to map the extent of colonisation, and define the remedial route: typically a thermal or chemical disinfection of the affected system, plus correction of the underlying control failure.
Agree the plan in writing: what's being disinfected, when, how occupants are protected in the meantime, and when resampling will confirm clearance.
Disinfection — putting it right
Remediation usually means a system disinfection to BS8558 / PD8554680 — either chlorination (dosing the system to a high free-chlorine concentration for a defined contact period, then flushing) or a controlled thermal disinfection where chlorination isn't suitable.
Disinfection treats the symptom. It must be paired with fixing the cause — reinstating correct hot and cold temperatures, removing or remediating dead-legs, restoring the flushing regime for infrequently-used outlets, or servicing failed plant. Disinfect without fixing the cause and the colonisation comes straight back.
Resample and close out
After disinfection and a suitable settling period, resample to confirm the count has returned to acceptable levels. Don't lift precautionary controls on the affected outlets until clearance results are in hand.
Close the loop with a written corrective action report: the original result, the investigation, the remediation carried out, the clearance results, and any changes to the written scheme of control to prevent recurrence. This document is your evidence the matter was managed competently end-to-end.
Who to notify
Most single positive results in a managed system are a control matter, not a reportable incident. However, if anyone is diagnosed with Legionnaires' disease in connection with your premises, that is reportable to the HSE under RIDDOR and you should take advice immediately.
In healthcare, care and other regulated settings, internal infection-control and estates protocols will usually require notification regardless of count — follow your local policy, and when in doubt, escalate rather than sit on it.