The problem is exacerbated during the summer holidays when water movement is even lower and temperatures are higher. Fortunately, it is the over 40's that are most susceptible to Legionella, so the risk to children is diminished. However, the threat to teachers, staff and visitors is a serious concern to facility managers. Public and private buildings

The threat of Legionella is not confined to educational establishments; many factories employ processes that result in extended water storage and many older buildings, especially hospitals, contain water distribution systems with dead-ends and areas of poor flow that create conditions in which bacteria can thrive. Cooling towers represent an area of particular concern.

Spread of infection

Before looking at treatment options, it is firstly important to be aware of the process by which infection is spread.

If a water droplet contains a single bacterial cell, the droplet will rapidly evaporate to a particle size diameter or droplet nucleus of about 1μm. A particle of this size can remain suspended in air for prolonged periods of time and travel over considerable distances.

These particles are dry and contain no free moisture and when air is inhaled into lungs, about 50 % of the particles, of approximately 1μm, will be retained in the lungs.

Water systems that are able to produce aerosols represent the highest levels of risk.  Aerosols can be generated very easily when the water surface is broken - for example, by falling water droplets, splashing, or by bubbles breaking at the surface.

Once introduced to artificial water systems, Legionella can thrive in warm water (30 - 35°C). Urban environmental isolates are commonly detected in waters collected from air conditioning cooling towers, evaporative condensers, taps, showers, hot water tanks, fountains, spa pools, machine tools, humidifiers, car washes and on the inside surfaces of shower heads.

They have also been shown to be present on flexible seals and metal surfaces within plumbing systems used in domestic potable water supplies.

Legionella pneumophila can survive for prolonged periods of time in tap water but do not grow or multiply unless supported by other organisms and for this reason Legionella infection is often associated with other forms of contamination.

Legionellae, if present within protozoa, can multiply and may be protected from normal concentrations of chlorine and/or other disinfectants commonly used to treat potable water supplies. Hence legionellae may be present in potable water that might appear to retain an adequate level of disinfection.

Treatment

As discussed, normal chlorination may not provide adequate disinfection. High temperature treatment may therefore be considered. Whilst Legionella pneumophila has been isolated from water at temperatures ranging between 7 - 58°C, the bacteria only multiply actively between 20 to 45°C.
A minimum temperature of 60°C is required to kill Legionella bacteria in hot water systems however, this would be difficult to achieve in most thermostatically controlled systems.

GB Environmental has designed a UV disinfection system, which works by exposing all micro-organisms that pass through the water system to UV light at wavelengths that disrupt their DNA chain, rendering them harmless.

The system is fitted onto existing water tanks alongside a small pump that continuously re-circulates the water.

The entire volume of water contained within the tank is re-circulated in under 20 minutes, which is less time than the re-growth rate for most micro-organisms. Fitted onto a small skid unit, the system can easily be fitted by a competent plumber and since the system runs from a 13amp single phase supply, complex electrical connections are avoided.

Laboratory testing

Kevin Woolnough, a Legionella specialist at analytical services company Eurofins, has seen a growing number of samples being tested positive for Legionella bacteria in his company's UK laboratories and is leading calls for measures to halt the rise in outbreaks of Legionnaires' disease.

He believes that whilst most people have heard of Legionnaires' disease, few know the measures that would reduce the likelihood of outbreaks.

In order for susceptible individuals (the elderly, smokers, patients recovering from surgery or taking immunosuppressive drugs, and people with lung disorders) to contract the disease, there must be a chain of infection.
In the case of Legionella the links in the chain are; a source of Legionella, a reservoir or place for it to grow, a means of transmission, and a susceptible host.

The source could be any aquatic environment and the growth reservoir could be almost anywhere within water transmission/plumbing systems where it can form bio-films and multiply amongst dirt and scale.

Prevention and control

Effective prevention and control measures must break the chain of infection and Woolnough says: "Laboratory testing is key to the successful appraisal of risk levels and subsequent control strategy."

A good strategy to combat Legionella would include the following measures:

• 1. Legionella risk assessment of water systems; laboratory analysis of samples from likely points such as cooling towers, hot & cold-water distribution systems including taps, showers, spas, jacuzzis etc.
• 2. Remedial action plan for high-risk areas, such as engineering solutions (e.g. removing dead legs & little-used outlets), a program for cleaning and disinfection of water systems.
• 3. Maintenance schedule to provide good water conditions, cleaning, heat treatment, biocide dosage etc.
• 4. Monitoring scheme to demonstrate the effect of the maintenance program, with regular analytical evaluations.
• 5. Record of actions and analytical results to demonstrate success in reducing the risk of Legionellosis.

The HSE's ‘Approved Code of Practice and Guidance for the control of legionella bacteria in water systems' (commonly referred to as L8) emphasises the need for risk assessments and proper maintenance of water systems. L8 advises that laboratory testing for Legionella is required when it is known that control measures (such as temperature) are compromised, and in special cases where vulnerable people are at risk (such as in hospitals or when there is a suspected outbreak of Legionnaires' disease). The frequency of testing is determined by the risk assessment. However, Woolnough recommends: "Control measures should be verified by laboratory analysis of water samples, rather than as a result of adequate maintenance. Even well maintained systems are occasionally culture positive for Legionella; detection can sometimes highlight a problem in the system that had not previously been recognised. The frequency of subsequent testing can then be based on the verified success or otherwise of the maintenance schedule."

In summary, effective testing forms the basis of a good risk analysis and an effective ongoing maintenance schedule, whilst UV treatment offers the ability to kill all potentially harmful organisms.