Legionella can kill - but so can electricity!
Whilst tests have shown that the Legionella Pneumophila bacteria is either killed or rendered unable to reproduce after exposure to ultrasonic transducers, there have been a few cases where ultrasonic humidifiers have been implicated in Legionella outbreaks.
This does not mean however that humidifiers with ultrasonic transducers are fundamentally unsafe, merely that they should be properly built, installed and in particular, properly maintained. Everybody knows that electricity will kill you if it is not used in safe and correctly maintained equipment and the same is true for all equipment that uses water.
In those few cases that may be found when searching the internet for humidifier related Legionella , it would appear that in each case there had been poor maintenance of the equipment.
When installing any cold water humidifier system, it is essential that in addition to correct installation, the end user should be made aware of the need to have the equipment regularly maintained.
The supplier can also help by designing the installation and equipment in a way that will inhibit the development of Legionella bacteria colonies. Typical strategies include:-
Use demineralised water without exposure to the air. Note that a reverse osmosis unit will "filter out" any bacteria.
Provide ultra violet sterilization to any stored water.
Provide automatic drain down equipment that will drain the humidifier at least once a day.
Provide automatic drain down equipment that will drain the humidifier if it has not been used for 12 hours.
Fit humidifiers in positions that prevent people breathing in the mist.
Design and position humidifiers so that they can be easily maintained and cleaned.
A comprehensive guide to the safe design, installation and operation of cold water humidifiers may be found at