Introduction to Legionella

Legionella is found naturally in the environment in low concentrations  and only when it enters man-made buildings does it multiply and create a health risk.

Mains water supply contains low numbers of bacteria and fungi, but it is only when these micro-organisms are allowed to multiply in water systems that problems arise.  Although current legislation maintains keeping water systems free of Legionella pneumophila bacteria, it is our intention to prevent or control the development of pathogenic micro-organisms of any kind in water systems so far as is reasonably practical, where we undertake to carry out our service.

The degree of microbial growth that can occur in building water systems, such as hot and cold-water storage tanks and distribution pipe work depends on a number of interrelated factors. These include the water temperature within the system and transfer of heat from heating and hot water pipes, the length of time of the water within the system, such as stagnant areas, the availability of nutrients, organic and inorganic substrates and the presence of sediments or corrosion products, which act as food sources for the micro-organisms.

Cooling towers are well known for problems of Bio-fouling, caused in part by the open nature of the systems, poor maintenance and chemical control, along with dust and other contaminants from the air providing nutrients for bacterial growth.

Of particular concern is the presence of Legionella pneumophila bacteria, which are capable of sustained growth in water at temperatures in the range of 20 – 46°C.  They are therefore able to survive at higher temperatures than most other aquatic micro-organisms can tolerate. This higher temperature tolerance gives Legionella pneumophila bacteria a distinct ecological advantage which makes them ideally suited for colonising building water systems, many of which run at higher than ambient temperatures.

If storage tanks, hot water system or cooling towers are not kept clean, microbial sludge, sediments and scale can accumulate and under the right conditions large concentrations of micro-organisms including L. pneumophila bacteria occur. In these circumstances pneumophila are able to survive and grow as part of the bio film, or in amoeba. In this case chemical  treatment is difficult to eradicate amoeba, hence the ability to kill the Legionella bacterium. But there are some chemicals more effective than others.  

Legionnaires ‘disease is an illness characterised mainly by pneumonia. It results from the inhalation of fine water mist contaminated with a virulent strain of Legionella bacteria and can be fatal.

Fourteen different serogroups of L. pneumophila have been described. L. pneumophila serogroup 1 is most commonly associated with cases of Legionnaires’ disease in the UK although other serogroups have been associated with outbreaks world-wide. The case fatality rate is about 12%, which is similar to that seen in most other types of pneumonia. Investigations of outbreaks of the disease in the UK have shown that it requires several stages to develop:


Firstly. Legionella bacteria must enter the water system.


Secondly. Once in the building water system, the bacteria must reproduce to reach dangerous  concentrations. Important factors critical to promote a rapid increase, one of the most important factors is temperature, this is pivotal, especially within the range 20-46°C,  and a supply of iron salts (e.g. rust), bio film and long lengths of stagnation.


Thirdly. Water containing high concentrations of Legionella bacteria must be disseminated in the form of a respirable aerosol which will be capable of being inhaled and reaching deep inside the lung.  Aerosols of this nature are readily produced by evaporative cooling towers (the drift released from the pack and drift eliminators) and domestic hot water systems from shower heads and taps situated over baths or sinks.


Finally, The final and most critical factor is that the aerosol, containing a virulent strain of the organism must be inhaled by a susceptible individual who may subsequently develop Legionellosis. Susceptible individuals will vary due to age, sex, male and female, smokers, individuals with a low immune system perhaps due to illness or treatment.