Since its identification in 1977, the ''Legionnaire's disease'' has been the subject of many investigations, reports and recommendations concerning its methods of transmission and appropriate controls. Considered mysterious at the first reported case, the infection remains, to this day, a source of tragic incidents despite comprehension. The CDC (Center for Disease Control and Prevention) estimates that the disease affects 10 000 to 15 000 people annually in the United States, while the OSHA (Occupational Safety & Health Administration) advances the number of 25 000 cases, responsible for over 4000 deaths. For other organizations, this number could exceed 100 000 per year (AWT, 2003).
The control of the bacteria responsible for this infection is now included in the legislation in many countries, throughout the world, as well as the Quebec province. It will most probably be included in the nearby future for the rest of Canada and the US as well.
What is Legionellosis?
Legionellosis is a respiratory infection caused by Legionella. In more than 90% of the cases, the species most frequently responsible for the disease is Legionella pneumophila, with the serogroup 1 being implicated in about 85% of infections. Legionella occurs in two clinical forms: Legionnaires' disease and Pontiac fever. Pontiac fever is a milder form of legionellosis. It heals spontaneously after 3-6 days and is rarely diagnosed as an infection caused by Legionella.
Legionnaires' disease is a severe form of pneumonia that causes death in about 15% of cases. It is usually associated with high fever, dry cough, muscle aches, digestive disorders and mental confusion. The incubation period usually ranges from 2 to 10 days.
This disease mostly affects people who have predisposing factors such as advanced age, chronic respiratory diseases, diabetes, immunosuppressive diseases, immunosuppressive treatments, smoking or alcoholism.
When conditions are favorable, Legionella can grow in many artificial environments, including domestic water systems and cooling circuits. However, the unique presence of legionella in water is not a sufficient condition to cause the disease. Infection occurs by inhalation of the spray (micro droplets smaller than or equal to 5 mm, predominant size for penetration into the alveoli) of contaminated water. This way, the bacteria are carried to the lungs via aerosol.
Three factors must be met to cause infection :
- Water contaminated with Legionella;
- Contaminated water dispersion through spray;
- Inhalation of droplets.
Which facilities are at risk?
Contamination occurs at facilities which promote the growth of Legionella and disperse it through form of a spray, such as:
- Showers of domestic hot water networks;
- Cooling towers;
- Air conditioning systems;
- Decorative fountains;
- Hot tubs, spas, jacuzzis;
- Humidifiers, foggers;
- Medical equipment producing a spray.
Institutions highly affected by the risk of infection obviously include health centers and other places welcoming people at risk (hospitals, residential centers for the elderly). Industrial buildings and services with a cooling tower, apartment buildings and other sites open to the public such as hotels, campings, sporting halls, swimming pools, etc. are also likely to be the possible source of a legionellosis outbreak.
Although several types of facilities have been identified as potential sources of infection, we discuss only the risk of development of Legionella in cooling towers.
How can cooling towers become a source of infection
Cooling towers have the function to reject heat in the atmosphere by using the evaporation of water. To facilitate this, the systems include components allowing the diffusion of water in a current of cold air. In the evaporation process, there is still a risk of formation of droplets which can be formed through the air flow. Unlike evaporated water, the droplets have the same composition as the cooling system’s water and are therefore likely to carry bacteria. Even if cooling towers are usually equipped with a droplet separator, the formation of water mist is often unavoidable and can be the source of infection.
What promotes the proliferation of legionella in cooling towers?
First, you have to consider the water temperature which is usually between 30 and 35°C, and temperatures of 25 to 40°C correspond to the range favorable to the development of legionella. Secondly, these systems are excellent dust collectors that can accumulate a lot of air fragments. These contaminants are nutrient sources for bacteria. In addition, the accumulation of debris in the form of sludge can produce favorable sites for their proliferation.
Corrosion, scaling and fouling are aggravating factors for these systems as they provide nutrients, promote the growth of bacteria in general and produce favorable locations to the proliferation of micro-organisms. Other factors will also have the effect of amplifying the risk of development of legionella. For example the stagnation of water (dead legs, areas of low turbulence, intermittent operation...), the water quality (concentration of nutrients, presence of certain minerals, suspended solids...) as well as the condition of the surface which are in contact with water (roughness, unevenness, scale deposits, corrosion residues, type and age of materials...).
How to control the risk?
Of course, the design and location of the cooling tower and the overall network are important factors to control the risk of infection. The best possible conditions will limit the formation of water mist and its possible introduction into the air vents. Regular monitoring of equipment conditions and quality control of water are obviously essential.
Managing the risk of infection by Legionella is therefore based on proper operations maintenance of the installation, a relevant water treatment program, rigorous and regular monitoring of the water quality, increased vigilance during sensitive periods and the implementation of systematic efficiency measures of anticorrosive, antiscaling and antimicrobiological treatments. Therefore, performance evaluations by instrumentation and analytical techniques are important elements in the overall risk management.