Management of the risks from legionella in cooling towers and evaporative condensers

Health and Safety Executive - Safety alert

Department Name:
Specialised Industries Division

Bulletin No:
SID 2-2012

Issue Date:
27 July 2012

Target Audience:
Any industry sector which uses cooling towers or evaporative condensers

Key Issues:
This safety notice draws attention to key aspects of the proper management of the risks from legionella


This safety notice draws attention to key aspects of the proper management of the risks from legionella. It is informed by a recently completed review of outbreaks in the UK over the past ten years that shows common failings in control, and a potential risk of further legionella outbreaks, such as that in Edinburgh in June 2012. This notice refers to cooling towers and evaporative condensers but the principles apply to other water systems.

Legionella safety - general approach

If not vigilantly controlled, the risks from legionella in cooling towers and evaporative condensers can become serious, and problems can build up rapidly. Fortunately, the means of control are well understood. An effective approach is set out in HSE's Approved Code of Practice L8: Legionnaires' Disease - The control of legionella bacteria in water systems

 An analysis of past outbreaks indicates it is vital to follow the practical advice in L8 on:

  1. identifying and assessing sources of risk;
  2. preparing a scheme for preventing or controlling the risk;
  3. implementing, managing and monitoring precautions;
  4. keeping records of the precautions; and
  5. appointing a senior person to be responsible for making sure that (a) - (d) happen.

Risk identification and control

90% of the outbreaks had their root causes in failures to identify risk (ie to recognise possible legionella problems) or to put in place effective schemes of control to deal with the identified risks. These failures left the plant vulnerable to a range of practical threats, for example from:

  • departures from planned maintenance and cleaning schedules (allowing plant conditions to get worse, and longer periods for problems to develop);
  • changes in the process (leading to changes in the risks, or rendering existing precautions ineffective);
  • staff/contractor changes (leading to a loss of knowledge);
  • intermittent use of plant (resulting in inconsistent control measures);
  • unusual weather conditions (eg bacteria multiplying very fast in warm weather).

The written scheme for controlling the risk should be specific to the site and system, and supported by clear working procedures.  It should be updated whenever issues which can affect the ability to control the risks change eg as in the examples above.

System monitoring

Effective and consistent monitoring of water quality is essential to maintaining control including:

chemical and biological monitoring and focused, specified visual checks that the system is working as it should be.

Routine monitoring of bacterial levels, whilst a useful tool, is no substitute for making sure the plant is kept in good condition and is cleaned regularly.

Carrying out system monitoring, then interpreting the results and identifying trends, all need specialist knowledge. For instance, it is not good practice to rely on frequent shot dosing for routine control without identifying the underlying problem of why the bacteria levels keep increasing.

Advice for senior managers

Senior managers (including the responsible person at (e) above) should seek assurance that effective controls are in place and that they are maintained. They should ensure that monitoring and auditing are carried out and the results acted on.

Lack of training and poor communication have been identified as contributory factors in outbreaks of Legionnaires' disease. It is therefore important that everyone involved is competent, trained and aware of their responsibilities. This is made more challenging by the likely division of roles between the company on site, maintenance staff, the water treatment contractor and (possibly) a separate subcontractor for cleaning and disinfection.

Roles and responsibilities must be assigned to named individuals with clear lines of communication, tracking and signing off of work. The company must have adequate oversight of contractors. Contractors should have clear responsibilities and reporting lines.

Further information

Further information and assistance is available on the HSE website

General note:

Please pass this information to a colleague who may be responsible for or use cooling towers or evaporative condensers.

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