A Legionella bacterium, the fundamental agent of Legionnaires' disease, is a water-based organism that causes infection when inhaled in an aerosol form. Although normally associated with cooling towers and evaporative condensers, it also can be found in showers and has been known to affect workers engaged in the manufacture of plastic parts using injection molding equipment.

Legionnaires' disease

Legionnaires' disease is a severe, progressive form of pneumonia, which is fatal in up to 15 percent of cases. The cause was identified as a rod-shaped bacteria later named Legionella Pneumophila (LP). Affecting between 10,000 and 20,000 people in the U.S. every year, LP thrives in wet areas where conditions are favorable for growth.

Prevention

LP may be able to colonize certain types of water fittings, pipe work and materials used in the construction of water systems. Legionella is transmitted exclusively by inhalation of contaminated water droplets. There is no evidence of obtaining the disease from ingestion or contaminated water or absorption through the skin. High-risk areas are therefore primarily associated with showers, sprays, spray taps, taps with high water pressure and refrigeration and air conditioning cooling towers.

Scale

Scale is a major problem in both hot and cold water systems. Dripping taps can deposit scale in and around the tap and, with high ambient room temperatures, provide an ideal growth medium for LP. In hot systems, scale can trap Legionella and biofilms. Water softeners can reduce scale, but there is a growing concern over the increase to sometimes-high levels of sodium in the water.

Air conditioning and refrigeration water systems

Many air conditioning and refrigeration plant systems are water-cooled. The heat generated by cooling coils is removed by water, which is passed through a water-cooling tower. These are recirculating systems, which operate at temperatures ideal for bacterial and algal growth and have plentiful supplies of nutrients. They have been highlighted as a major possible source of Legionnaires' disease mainly because of the large number of people that can be affected.

Many agents are used to control these factors, including scale and corrosion inhibitors, dispersants and biocides. Water softeners are sometimes used for soft water, which can cause a problem with foaming.

Biofilms are a major problem in cooling towers. Biofilms and scale can reduce the efficiency of cooling systems to the point where the system no longer functions with regard to heat transfer. Health and safety officers recommend the periodic chlorination and descaling of cooling towers.

However, chlorine is not always compatible with other treatment chemicals like corrosion inhibitors, it is not effective in alkaline water and it can cause corrosion. Some biocides are effective against LP if used in sufficient concentration. Strains of LP and other bacteria may become resistant to particular biocides, hence dual or alternating biocides are used.

Legionella can be a harmful bacteria and Legionnaires' disease can cause a dangerous situation. The need to prevent and control this bacteria is very important.