What is Legionella?
Legionella is a saprophyte bacterium, that is, it obtains the substances it requires from decomposing organic materials. The ideal temperature for the multiplication of the bacterium is between 25 and 42°C, with a pH of the water between 5 and 7; its survival time is estimated at around 12 months.
The presence of Legionella bacteria in water is not synonymous with a deadly epidemic. In fact, there are more than 30 types of Legionella, around 20 of which are pathogens to humans.
Legionella Pneumophila is the variety most responsible for diseases. There are two consequences caused by contagion with this bacterium: the first is Pontiac Fever, which is often not diagnosed due to the fact that the symptoms are similar to those of the flu; the second, much more serious and virulent, is Legionnaire's disease, a serious form of pneumonia with a mortality rate of up to 15%.

Legionnaire's disease: the infection from Legionella may cause two distinct clinical conditions: Pontiac fever and Legionnaire's Disease.
Pontiac Fever, after an incubation period of 24-48 hours, this appears in acute form without affecting the lungs, similar to the flu, and passes in 2-5 days. The symptoms are: general discomfort, muscle pain and headache, followed quickly by fever, at times with coughing and sore throat. It may be accompanied by diarrhoea, nausea and slight neurological symptoms such as dizziness or photophobia.
Legionnaire's disease, after an incubation period that varies from 2 to 10 days (on average 5-6 days), this disease becomes apparent with moderate or serious effects on the lungs.
The first symptoms are normally discomfort, headache, fever and slight coughing, which are accentuated by the appearance of the respiratory symptoms.

Where does it live? What are the sources of contamination?
The Legionella bacterium preferably lives in water, with a temperature between 5°C and 60°C.
It nests in the pipes of instant water heaters, in hot water tanks and in water pipes. The colonies of Legionella bacteria measure around 1 mm in size, and may be transported over large distances by a water aerosol in which the droplets have a diameter of over 2-3 mm. This is why appliances and devices that create water aerosols (such as showers, whirlpools and atomisers in general) may represent the vehicle for the spread of the bacteria, naturally only if the water that they atomise contains it. Air-conditioning systems fitted with humidifiers and evaporative coolers may also be sources for the spread of the bacteria.
The most important source for Legionella contamination is in any case outdoor air-conditioning systems, as the bacteria are very often transported by the fresh air entering the environment.
This occurs in a rather high number of cases, if a cooling tower is located near fresh air intake (up to 200 metres away); these towers have been identified as the most dangerous potential source of contamination (they contain recirculated water at a temperature of between 25 and 40°C, with the frequent presence of algae and bio-organisms).

What measures can be taken to prevent the risk of the Legionella bacteria spreading?
The measures to minimise the risk of spreading bacteria through atomisation appliances are:

1. the use of a bacteriologically pure supply;
2. any residual drops of water that have not been evaporated are eliminated using mist separators;
3. all forms of stagnant water are eliminated from the appliance;
4. no recirculated water is atomised.

In addition, to make atomising humidifiers safer, the following steps are necessary

• all the atomised water derives from the mains drinking water, which is bacteriologically pure;
• the compressed air used to transport the droplets derives from a compression cycle in which the temperature reached is such as to eliminate any form of bacterial life.
• the atomised water that is not evaporated is in no way reused or recirculated for subsequent atomisation, but rather eliminated;
• at the end of each humidification phase, the system runs an empty cycle through the spray nozzles, by blowing compressed air only, so as to completely dry all the parts affected by the air-water mix and therefore exclude the conditions for the survival of any germs that may be accidentally present.

Finally, it is worth noting that the only humidifiers universally recognised as preventing the spread of bacteria are steam humidifiers, as the steam produced does not contain bacteria.