Control of legionella: Inspection of evaporative cooling systems and investigation of outbreaks of Legionnaires' disease

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This document provides guidance on inspection, investigation and enforcement on legionella risk from evaporative cooling systems.

It also describes HSE's role in outbreaks of Legionnaires' disease.


This Operational Guidance updates and replaces Operational Circulars OC255/9. OC255/12 and LAC 46/1. 

Legionnaires' disease can be acquired occupationally but it can also be a major public health issue. This guidance sets out the key requirements for managing evaporative cooling systems in order to control the growth and spread of legionella bacteria.  It will assist inspectors in deciding whether the measures in place are adequate to control the bacteria and provide guidance on enforcement action where controls are inadequate.  It is aimed primarily at evaporative cooling systems but may be applicable to legionella risks from other systems.

The guidance also sets out HSE's role in the investigation of outbreaks and provides a guide to determine whether the investigation of single cases of Legionnaires' disease is appropriate.


Inspectors should use this guide when:


Legionnaires' disease is a pneumonia - like illness caused by inhaling an aerosol containing legionella bacteria.  It can be fatal in susceptible individuals. Legionella bacteria may also cause Pontiac and Lochgoilhead fevers which are similar, but generally milder, illnesses that are not fatal [note: legionellosis is the name for the group of diseases caused by the legionella bacteria].

Legionella bacteria are aquatic organisms commonly found in natural water sources such as rivers, lakes and reservoirs. They are generally present in low concentrations in such situations and do not give rise to illness. Outbreaks of illness can occur when the bacteria colonise water systems, proliferate and are spread by aerosol generation.  Any water system may become colonised including hot and cold water systems, spa pools and industrial sources using process water. Evaporative cooling systems have been linked to higher numbers of Legionnaires' disease cases than other types of water system (HSL report).

Evaporative cooling systems, such as cooling towers and evaporative condensers, are found in manufacturing processes which use evaporation to reduce the heat of process water. Such systems generally produce an aerosol when the water stream interfaces with airflow.  Drift eliminators are normally fitted to reduce and prevent aerosol spread. They trap most, but not all, of the aerosol, a small proportion will always escape to atmosphere even where the drift eliminators are in good condition and well-fitted. Where the quality and cleanliness of cooling water is not satisfactorily maintained, it is possible for a contaminated aerosol to be dispersed over a wide area, potentially affecting members of the public.  Certain factors contribute to the growth and spread of bacteria including 

In favourable conditions the bacteria may grow rapidly. The growth and spread of legionella bacteria in system water must therefore be effectively controlled by maintaining both plant and process water in a clean condition and reducing, so far as is reasonably practicable, the possibility of aerosol generation or spread.  This requires regular cleaning and maintenance and, in most cases, the addition of a proprietary biocide.   In some circumstances, other technologies may be used that do not rely on chemical treatment or alternatives to wet cooling systems can be considered and the risk thus eliminated.

More information on legionella bacteria and conditions favouring growth is available on the HSE 'Legionella and Legionnaires' Disease web page.

Health and safety of HSE staff

HSE inspectors should not undertake any inspection of water systems or investigate an outbreak of Legionnaires' disease unless they

Inspectors must ensure that the installation is made non - operational before a physical examination takes place. In outbreak situations, if examination of a system is required quickly and it is not possible to make the installation non-operational for process reasons, inspectors should contact a specialist and ask for advice on how to proceed. In practice, this requires the duty holder to switch off the tower fan for a period of approximately 30 minutes prior to approaching the device in order that the system has time to equilibrate and for aerosols to disperse. [Note: see also page 8 physical examination]

[Note: LA enforcement officers should familiarize themselves with their authority's own health and safety policy]

Further information on investigation during a Legionnaires' disease outbreak in contained in Appendix 2

Further References


Specialist Occupational Hygiene Inspectors (SG) should be contacted for operational support when:


Updated 2020-12-14