Legionella Sampling & Monitoring of Cooling Water

All legionella monitoring programmes for cooling water systems should include routine sampling and testing for the presence of bacteria, both general (aerobic) bacterial species and legionella bacteria.

Since the detection of legionella bacteria requires specialist laboratory techniques, routine monitoring for aerobic bacteria can be used as an indication of whether microbiological control is being achieved.

Recommended Frequency of Sampling

In addition to the regular sampling for aerobic bacteria in cooling water, a routine monitoring scheme should also include periodic sampling for the presence of legionella bacteria.

This should be undertaken at least quarterly, unless sampling is necessary for other reasons, such as to help identify possible sources of the bacteria during outbreaks of Legionnaires’ disease.

More frequent sampling should be carried out when commissioning a system and establishing a treatment programme.

Sampling should be carried out, on a monthly basis, until it can be shown that the system is under control.

Legionella Positive Sample

If a legionella positive sample is found as a result of routine sampling, more frequent samples may be required as part of the review of the system/risk assessment, to help establish when the system is back under control.

In the UK the Health & Safety Executive recommend that the sampling method should be in accordance with ISO 11731:1998 and the biocide neutralised where possible.

Samples should be taken as near to the heat source as possible.

They should be tested by a UKAS accredited laboratory that takes part in the Public Health Laboratory Service Water Microbiology External Quality Assessment Scheme for the isolation of egionella from water.

The laboratory should also apply a minimum theoretical mathematical detection limit of less than, or equal to, 100 legionella bacteria per litre of sample.

Interpretation of Results

Legionella bacteria are commonly found in almost all natural water sources, so sampling of water systems and services may often yield positive results and the interpretation of any results of sampling should be carried out by experienced microbiologists.

Failure to detect Legionella bacteria should not lead to the relaxation of control measures and monitoring.

Neither should monitoring for the presence of legionella bacteria in a cooling system be used as a substitute in any way for vigilance with control strategies and those measures identified in the risk assessment.

Specialist Laboratory Services

Legionella Control International offer a comprehensive range of UKAS accredited legionella testing, sampling and microbiological laboratory analysis services capable of undertaking an extensive range of analyses including those for legionella, E.coli O157, Pseudomonas, Norwalk virus and others.

Our professional lab services can also be used for the testing and microbiological analysis of samples to assist in the identification of food poisoning bacteria including Salmonella, Listeria, E.coli O157, Campylobacter and Clostridium perfringens.

Expert Legionella Assistance

For further information and expert assistance regarding legionella risk management and compliance issues please call +44 (0) 161 877 0586 to speak to one of our specialist advisors or email your questions to This email address is being protected from spambots. You need JavaScript enabled to view it..

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Legionella Control International
Unit B Badex Building
Westbrook Park
Manchester M17 1AY
United Kingdom

Manchester: +44 (0) 161 877 05 86
London: +44 (0) 203 637 47 48
Email: info@legionellacontrol.com

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