Legionnaires disease arises as a result of an infection by legionella bacteria that are common in natural and artificial water systems, from rivers and lakes to storage tanks and distribution pipes. In buildings, legionella are normally associated with larger artificial water systems, such as those found in hotels, factories and hospitals. However, the bacteria can also proliferate in the smaller water systems found in any building or residential accommodation. Legionella can also be found in spa and whirlpool baths, humidifiers and permanently wet fire-fighting systems. Like most bacteria, legionella can survive low temperatures, while temperatures over 60°C will kill them. Most growth occurs in the temperature range between 20°C and 45°C (the critical range), being especially rapid near the normal body heat temperature of 37°C.

Legionnaires disease is a potentially fatal form of pneumonia, and can affect anyone. Those most at risk include elderly people, patients suffering from respiratory or kidney disease, and anyone with a compromised immune system. The disease is acquired by way of water droplets or vapour released by a contaminated source and passing straight into the lungs. The disease is unlikely to be transmitted by drinking the water or by person-to-person contact.

The law

The legal framework for the control of the risk from legionella is detailed in the HSE publication ‘Approved Code of Practice: The Control of Legionella Bacteria in Water Systems’. This approved code is intended to give practical advice on the requirements of the Health and Safety at Work Act and the Control of Substances Hazardous to Health Regulations. An approved code of practice and the guidance it includes can be regarded as an extension of the law, and it would be foolish to ignore it. Until recently, the approved code generally applied only to the workplace and there was a threshold which excluded hot and cold water systems with a volume of less than 300 litres. The approved code has recently been amended by the removal of this 300 litre threshold and the extension of its requirements to providers of residential accommodation. An HSE leaflet describes providers of residential accommodation as including local authorities, housing associations, hostels, landlords in the private sector, managing agents, hoteliers, guest house and camping site owners, etc.

Risk assessment

Basically, the approved code requires that whoever is responsible for a water system carries out a risk assessment, and then establishes a management regime to control the risks identified by the assessment. A risk assessment is a structured and logical inspection of all the component parts, starting at the incoming supply and working through the system, listing the findings, identifying shortcomings and summarising any action needed. In most cases, the supply is likely to be from the mains, therefore responsibility for the quality of the incoming water rests with the utility company. If the water comes from a natural source, such as a well, spring or borehole, then steps need to be taken to make sure that the water is not contaminated and safe to drink. Storage systems, both hot and cold, need to be documented, and their construction, condition and temperatures recorded. Examination of pipework will show whether insulation needs to be improved, or where long runs or ‘dead legs’ need to be removed or altered. Risk assessments should be reviewed or amended when parts of the system are altered or other changes take place. Regular audits, say at annual intervals, are useful in making sure that water quality management controls remain appropriate and effective.

Water systems - cold water storage tanks

In the water systems in most buildings, primary colonisation by bacteria can take place when cold water storage tanks are contaminated from an exterior source, such as birds, insects and organic substances falling into the water. All cold water storage tanks should therefore be fitted with robust close-fitting lids, and insect screens should be fitted to overflow pipes.

In order to prevent heat take-up, particularly in the summer, cold water storage tanks should be insulated so that cold water remains below the 20°C threshold of the critical temperature range. Cold water storage tanks with little through-put will tend to stagnate and encourage the growth of all types of bacteria. Once a system has become contaminated, the only answer may be to clean and chlorinate. The best management system is to ensure that the water temperature does not enter the critical range.

Water systems - cold water pipes

Like cold water storage tanks, cold water pipes can take up heat from their surroundings. This may not just be a seasonal phenomenon in premises that are centrally heated. The best management practice is to insulate cold water pipes to reduce heat take up so that temperatures do not enter the critical range. In addition, stagnation can occur in long or little-used pipe runs and organic materials in jointing compounds can provide the nourishment for bacteriological growth. Back-siphoning, for instance from an outside tap during a loss of water pressure, can also introduce organic material into a cold water system. Where all parts of the cold water system in a building are directly from the mains, there is little risk of legionella establishing itself.

Water systems - hot water storage tanks

As with other parts of a domestic water system, the best way to manage hot water storage tanks is to make sure that temperatures remain outside the critical range, i.e. over 60°C. This should be achieved by setting temperature controls high enough to achieve this, but not so high as to introduce a risk from scalding. Hot water storage tanks should be insulated to reduce heat loss, particularly during the winter. Water in hot water tanks tends to stratify, that is where convection in the body of water allows cooler water to settle at the lower end of the tank. This may result in water temperatures in a part of a hot water tank entering the critical range. Larger hot water systems that are little used are more vulnerable to this effect than smaller or more frequently used systems. Where hot water is provided by a combination boiler drawing its supply directly from the mains, there is a reduced risk of legionella in the hot water system.

Water systems - hot water pipes

Hot water pipes should be insulated to reduce heat loss, therefore reducing the risk of water temperatures entering the critical range. Even if they are properly insulated, long or infrequently used pipe-runs will lose heat over time, allowing the temperature to drop below 45°C. This is difficult to manage, and building occupiers should be made aware of the likely consequences.

Water systems - hot and cold water outlets

Transmission of legionella bacteria is generally by way of water mist or fine droplets. It is therefore prudent to use taps and outlets that do not incorporate a spray head. Showers, by their nature, need a spray head to achieve their effect. Ideally, shower heads should be cleaned and disinfected on a regular basis. Outlets in hard water areas are vulnerable to the formation of scale, which can provide a foothold for bacteriological growth. Excessive scale should be removed.

Summary

A documented management system should be established for all parts of cold and hot water systems so that:

• Stored cold or hot water volume is reduced to the minimum needed.

• Systems are enclosed to prevent the ingress of organic material.

• Component parts are insulated so that temperatures remain outside the critical range.

• Only metal or inorganic substances are used for making joints or fittings.

• System designs and alterations comply with current best practice.

• Contractors are competent and aware of the risks associated with legionella.

• Vulnerable parts of the system are regularly cleaned.

• Building occupiers are made aware of the risks.

Karl Lawrence is an independent health and safety consultant specialising in buildings and building services, operating throughout the UK. Tel: 01803 855439.

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