Preventing or Controlling the Risk from Exposure to Legionella Bacteria

Regulations

Control of Substances Hazardous to Health Regulations 1999, Regulation 7 and 9, Health and Safety at Work etc. Act 1974, Sections 2, 3 and 4.

Acop

Where the assessment shows that there is a reasonably foreseeable risk, the use of water systems, parts of water systems or systems of work that lead to exposure has to be avoided so far as is reasonably practicable.
Where this is not reasonably practicable, there should be a written scheme for controlling the risk from exposure which should be implemented and properly managed. The scheme should specify measures to be taken to ensure that it remains effective. The scheme should include:
* an up-to-date plan showing layout of the plant or system, including parts temporarily out of use (a schematic plan would suffice);
* a description of the correct and safe operation of the system;
* the precautions to be taken;
* checks to be carried out to ensure efficiency of scheme and the frequency of such checks; and
* remedial action to be taken in the event that the scheme is shown not to be effective.
The risk from exposure will normally be controlled by measures which do not allow the proliferation of legionella bacteria in the system and reduce exposure to water droplets and aerosol. Precautions should, where appropriate, including the following:
* controlling the release of water supply;
* avoidance of water temperatures and conditions that favor the proliferation of legionella bacteria and other micro-organisms;
* avoidance of water stagnation;
* avoidance of the use of materials that harbor bacteria and other micro-organisms, or provide nutrients for micro-bacterial growth;
* maintenance of the cleanliness of the system and the water in it;
* use of water treatment techniques; and action to ensure the correct and safe operation and maintenance of the water system.

Guidance

Once the risk has been identified and assessed, a written scheme should be prepared for preventing or controlling it. In particular, it should contain such information about the system as is necessary to control the risk from exposure.

The primary objective should be to avoid conditions which permit legionella bacteria to proliferate and to avoid creating a spray or aerosol. It may be possible to prevent the risk of exposure by, for example, using dry cooling plant, adiabatic cooling systems or point-of-use heaters (with minimal or no storage). Where this is impractical, the risk may be controlled by minimising the release of droplets and by ensuring water conditions which prevent the proliferation of legionella bacteria. This might include engineering controls, cleaning protocols and other control strategies. Decisions should be made about the maintenance procedures and intervals, where relevant, on equipment used for carrying out the control measures. Legionella bacteria may be present in very low numbers in many water systems but careful control will prevent them from multiplying.

In general, proliferation of legionella bacteria may be prevented by:
* avoiding water temperatures between 20C and 45C - water temperature is particularly important factor in controlling the risks;
* avoiding water stagnation which may encourage the growth of biofilm;
* avoiding of the use of materials that harbor bacteria and other micro-organisms, or provide nutrients for micro-bacterial growth;
* keeping the system clean to avoid the build-up of sediments which may harbor bacteria (and also provide a nutrient source for them);
* the use of suitable water treatment program where it is appropriate and safe to do so; and ensuring that the system operates safely and correctly and is well maintained.
The scheme should give details on how to use and carry out the various control measures and water treatment regimes including:
* the physical treatment programme - for example, the use of temperature control for hot-and-cold-water systems;
* the chemical treatment programme, including a description of the manufacturer's data and effectiveness, the concentrations and contact time required;
* health and safety information for storage, handling, use and disposal of chemicals;
* system control parameters (together with allowable tolerances); physical, chemical and biological parameters, together with measurement methods and sampling locations, test frequencies and procedures for maintaining consistency;
* remedial measures to be taken in case the control limits are exceeded, including lines of communication; and
* cleaning and disinfection procedures.
The scheme should also describe the correct operation of the water system plant including:
* commissioning and re-commissioning procedures;
* shutdown procedures;
* checks of warning systems and diagnostic systems in case of the system malfunctions;
* maintenance requirements and frequencies; and operating cycles - including when the system plant is in use or idle.
Detailed guidance on how to effectively prevent or control exposure can be found in part 2.
Review of control measures - monitoring and routine inspection
If precautions are to remain effective, the condition and performance of the system will need to be monitored. This should be the responsibility of the responsible person or, where appropriate, an external contractor or an independent third party and should involve:
* checking the performance of the system and its component parts; inspecting the accessible parts of the system for damage and signs of contamination; and monitoring to ensure that the treatment regime continues to control to the required standard.
The frequency and extent of routine monitoring will depend on the operating characteristics of the system, but should be at least weekly.

Testing of water quality is an essential part of the treatment regime, particularly in cooling towers. It may be carried out by a service provider, such as a water treatment company or consultant, or by an operator, provided they have been trained to do so and they are properly supervised. The type of tests required will depend on the nature of the system and for both cooling towers and hot and cold water systems.

The routine monitoring of general bacterial numbers (total viable count) is also appropriate as an indication of whether microbiological control is being achieved. This is generally only carried out for cooling towers, rather than hot and cold water systems. Periodic sampling and testing for the presence of legionella bacteria may also be relevant to show that adequate control is being achieved. However, reliably detecting the presence of legionella is technically difficult and requires specialist laboratory facilities. The interpretation of results is also difficult; a negative result is no guarantee that legionella bacteria are not present. Conversely, a positive result may not indicate the failure of control systems as legionella are present in almost all natural water sources.

The results of monitoring and testing should be interpreted by a suitably experienced and competent person and any remedial measures, where necessary, should be carried out promptly.

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