Legionnaires’ disease is a potentially fatal form of pneumonia which can affect any person, but statistically the risk of contracting the disease increases with factors such as age, illness or immunosuppression, smoking or sex. The disease is caught by inhalation of very fine water droplets, known as water aerosol, which have been contaminated with the bacteria called legionella pneumophila. The higher the growth level, the higher the risk of infection. Legionella bacteria, not just the legionella pneumophila strain, can also cause less serious and non-pneumonic illnesses which are self limiting and not fatal or permanently debilitating, such as Pontiac fever. Legionellosis refers to the contracting of any pulmonary disease associated with any of the 40 plus species of legionella bacteria.

The bacteria are ubiquitous in the natural environment, found in soil and water bodies such as rivers, lakes and reservoirs, usually in low numbers. It should be assumed that legionella bacteria will enter a manufactured water system as there is no requirement for supply companies to provide water that does not contain legionella bacteria. It is the responsibility of the designer, installer, owner and manager of any water system to ensure that this system does not encourage a significant increase in growth.
Conditions and factors that will allow for multiplication of legionella bacteria can be summarised as the following and can all be easily created/provided in a number of engineered water systems:

Water Temperatures between 20-45°C.
Water Stagnation.
Sources of Nutrients – Materials of construction or presence of fouling.
Presence of oxygen and light.

Legionella bacteria can survive below 20°C and these water temperatures, even below freezing, will merely aid in reducing growth rates. Above 50°C the bacteria will stop growing and start to die. Above 60°C bacteria will die within a couple of minutes and above 70°C the majority of bacteria will die within seconds.

Large outbreaks of more than 2 or three people, examples being Barrow in Furness in Cumbria in 2002, and the original known outbreaks in the US – at the convention of the American Legion (hence the name) - and also in the UK at Stafford General Hospital, both in the mid 1970’s, are attributed to open evaporative cooling systems. The systems, used to provide cooled water to refrigeration equipment and compressors for air conditioning purposes, through their design create a large amount of aerosol that can be blown several hundred metres. When these cooling towers are therefore located in areas of dense population, the risks of susceptible individuals being exposed to this aerosol increases heavily. These systems are on the whole well maintained through treatment processes and much improved awareness, backed by Local Authority and Environmental Health Office checks and legal requirements for registration of all cooling systems. The Barrow in Furness incident highlighted a break down in contractor services through poor council communication.

The majority of the statistics today (see the information below) highlight domestic and other water systems including spa systems and various water features as responsible for cases of legionellosis within the UK. There is however at present lower awareness of the dangers for these systems.

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