To cause infection, legionella bacteria normally need to be inhaled. The inhaled particles should be small enough to penetrate down to, and be retained in, the deepest part of the lungs (alveoli) but large enough to contain at least one bacterial cell. Particles of 1-3 microns (thousandths of a millimetre) satisfy these criteria. These particles are too small to be seen by eye and can remain suspended in air for prolonged periods of time. A suspension of such particles in air is termed an aerosol, and may not necessarily be visible or even wet.
It is a common misconception that a water spray is an aerosol and that legionellae have to be contained within a wet droplet. A mist of water droplets may constitute an aerosol if the droplets are small enough. Water evaporates from small droplets very rapidly. For particles of less than 4 microns, the evaporative process will usually take place in less than one second, and the exact rate of evaporation will depend on the prevailing temperature, relative humidity and airflow.
If a water droplet contains a single bacterial cell, the droplet will rapidly evaporate to a particle, or droplet nucleus, of about 1 micron. A particle of this size can remain suspended in air for prolonged periods of time and travel over considerable distances. These particles are dry and contain no free moisture. Only bound water is present which represents a small percentage of the total mass. When air is inhaled into lungs, about 50% of the particles, of this size will be retained in the lungs.
Any mechanical action that causes the surface of a liquid (which contains bacteria) to be broken up may cause the production of small droplets containing bacteria suspended in them. If these droplets are small enough, the water may rapidly evaporate leaving the dry droplet nuclei containing the bacteria, so forming an aerosol. Natural aerosols can be generated by rainfall, waterfalls, bubbles rising through water or via wave formation. In artificial water systems or environments running taps, showers, fountains, humidifiers, spa pools, whirlpool baths and evaporative cooling towers can all generate aerosols.
Infection is also thought to have resulted from the aspiration in certain hospital acquired cases either from drinking contaminated water, or ingesting liquid feeds or ice made with contaminated water, or using contaminated water for purposes such as irrigation or washing of wounds.
Last reviewed: 18 January 2011