Legionella are bacteria in the natural environment: water and soil. They are usually harmless in the environment to humans. However, under conditions conducive to their proliferation (especially in industrial buildings or facilities), they can cause diseases of the respiratory system.

They are frequently found:
• in hot water systems because they prefer temperatures between 25 and 45°C;
• in any natural or artificial aquatic environment: cold water systems, tanks of cooling towers, air conditioning systems, humidifiers, ponds and fountains, jacuzzi, misters, etc.

Bacteria are released into the air through sprays (micro-droplets), in the showers, swimming pools, fountains, etc.

Legionella inhaled in large quantities when they are suspended in the air, combined with water droplets, cause two types of diseases of the respiratory system:

 first benign, Pontiac fever (95% of cases): the infection is characterised by flu-like illness with fever, chills, muscle aches, headache, dizziness and sometimes diarrhea. Rarely diagnosed as such, this form cures spontaneously within 2 to 5 days;

 the other serious, legionellosis (5% of cases): it results in a lung infection, often severe, associated with digestive disorders and mental confusion. It causes death in 10-30% of cases, but can be cured by taking antibiotics at the onset of symptoms. It affects men more than women, smokers, immunocompromised individuals, and the number of cases increases with age.

To avoid the presence of legionella in air, take care to limit their spread in water:

• while designing the domestic hot water systems, avoid water stagnation zones (elbow, dead ends, etc.)
• keep the water temperature ˂ 25°C or > 45°C (take care of burn risks)
• Insulate the cold water pipes to prevent heating of water
• fight against scaling and corrosion of valve components
• maintain the tanks and distribution channels (regular draining, de-scaling and disinfection by heat or chlorine shock)
• remove areas of stagnation