What to do if legionella is detected
Updated 7 September 2026
Short answer
A positive sample is not the same as an outbreak. The action depends on the count and the type of system. Below 100 cfu/litre, review your controls. Between 100 and 1,000, resample and investigate. Above 1,000 in a domestic water system or above any detectable level in a cooling tower, take immediate action including disinfection and cessation of use where necessary.
A positive legionella sample is a signal, not an emergency by itself. Almost every water system produces an occasional positive at low count, usually because conditions have drifted at one outlet. The right response depends on what the count is, where it came from, and who is exposed.
For domestic hot and cold water systems (HSG274 Part 2)
The action thresholds in HSG274 Part 2 Table 2.3 are:
- Below 100 cfu/litre: system is under control. Review the results, check that the sample was taken correctly, continue monitoring. No urgent action.
- Between 100 and 1,000 cfu/litre: review the control scheme. Take a repeat sample. Check temperatures at the outlet. Investigate whether there is a local factor (a little-used outlet, a recently refitted section) that explains the result. If repeat samples also show counts in this band, take action to regain control.
- Above 1,000 cfu/litre: take immediate remedial action. Usual steps are to take the affected outlet out of use, thermally disinfect the local branch, sample again after the disinfection has had time to work, and investigate the root cause. Consider whether the building occupants need to be informed.
For cooling towers (HSG274 Part 1)
Cooling towers are treated more strictly because aerosols can travel hundreds of metres downwind. Any legionella detection at 1,000 cfu/litre or above requires immediate action: shut down the tower, disinfect, and investigate before restart. Lower detections trigger review of the treatment regime and repeat sampling.
For healthcare premises (HTM 04-01)
Healthcare premises operate to stricter action thresholds because of the vulnerability of patients. Any positive detection of Legionella pneumophila serogroup 1 in a clinical setting is treated as requiring immediate investigation, regardless of the count.
If someone has been ill
If a case of Legionnaires' disease has been linked epidemiologically to your premises, the health protection team (UKHSA in England, Public Health Wales, Public Health Scotland) will lead the investigation. You are required to cooperate, provide records, and allow sampling. HSE will also be notified under RIDDOR.
The wider lesson
A positive sample is usually a sign that the control scheme needs adjustment, not that the whole system needs to be stripped out. Review the assessment, look at whether monitoring and flushing are actually being done, and focus on the specific outlet or sub-system where the positive came from.
Legal basis
HSG274 Part 2 Table 2.3 on action levels for domestic water systems. HSG274 Part 1 Table 1.3 on cooling tower action levels. HTM 04-01 Part B for healthcare premises. RIDDOR 2013 Schedule 2 on reporting of Legionnaires' disease cases.
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Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
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