Humidifiers are an integral part of building water systems and as such should comply with the regulations and abide with the guidance described in the Approved Code of Practice and Guidance: Legionnaires’ disease (L8).

When correctly designed, installed and maintained, humidifiers present a relatively low risk. However, they inject aerosols (fine droplets of water) into the atmosphere that can be inhaled by occupants, therefore it is important to ensure aerosols produced are not contaminated with legionella bacteria. Humidifiers must always be carefully considered when appraising the building’s risk assessment.

Humidifiers are complex appliances and require specialist attention, expert advice from the manufacturer or supplier is always recommended.

For guideline purposes only, risk assessments associated with humidifiers should always consider the following:

• Is there a risk of the water becoming stagnant?

Stagnant water has been shown to provide suitable conditions for the rapid proliferation of legionella. Consideration should not only be given to the water in the humidifier but also to any water tanks and pipework supplying it. Oversized tanks will result in reduced water turnover and pipework supplying humidifiers must never fall out of use for extended periods of time, especially in the summer. Systems should be designed to avoid stagnation, flush regularly, and purge all water from the system when not in use for a predetermined time and then isolate the water supply.

• Is the temperature of the water or immediate environment around associated pipework above 20°C?

Legionella bacteria multiply most successfully in water temperatures between 20-45°C. In warm buildings and in the summer, cold water temperatures raising above 20°C in tanks and pipes must be avoided otherwise the risks will increase from humidifiers operating on cold water. Many cold water humidifiers incorporate a sterilisation system such as UV or copper/silver ionisation.

Steam humidifiers boil water which destroys legionella. However, steam humidifiers may present a risk if they are not used for any length of time. Although aerosols are produced through hygienic steam, a unit could potentially infect a maintenance engineer when shut down. For this reason all steam humidifiers should be drained when not in use for periods more than 72 hours.

• Is there a source of nutrients in the water?

Dirty water, scale in pipes, slime or biofilm have been shown to provide conditions suitable for legionella multiplication. Always avoid rubber, leather and non-approved jointing compounds which have been shown to support microbial growth.

Water softeners and reverse osmosis (RO) can reduce scaling and help improve water flow by exchanging minerals or removing them altogether. However, once water is treated by RO it must be kept in a clean environment as any residual purification chemical, such as chlorine, will also have been removed. Further exposure of treated water to UV light is often practiced to kill waterborne bacteria.

• Are aerosols produced?

Legionella must become airborne to enter the lungs of occupants – this may occur from a spray or a splash in a dirty reservoir during maintenance. If aerosols are produced, are occupants present? Direct air atomising humidifiers and ultrasonic units working directly in the air where occupants are present, is most relevant to this concern, however, if these devices are well-designed, maintained and properly risk assessed, the system will be significantly safer.

The elderly and infirm are most at risk from legionella infections and as such care should exercised when selecting suitable humidifiers. Always consider the above points when specifying and operating humidifiers, however, it is important to remember that each application is different and a system which is safe in one environment might create a hazard in another. If in doubt always get expert advice.

 

Article by:
Rik Prowen
Operations Director
JS Humidifiers plc