Refrigeration systems all contain two coils: evaporator coils and condenser coils. Evaporator coils
remove heat from the area to be cooled and condenser coils remove that heat from the system. Such
coils are also known as "heat exchangers", because they serve to move heat from one environment to
another. These coils can also be referred to as "tubes".
Condenser coils in refrigeration systems will typically be exposed to the outside so that heat can transfer through either the coil surface or a heat exchanger to the atmosphere. This can result in contamination of the condenser coil. Evaporator coils are typically not exposed to the outside; however, evaporator coils that are contained in an environment that is poorly maintained can become contaminated. It is also important to note that condenser coils and evaporator coils are all part of the same system, so if one component (typically the condenser coil) becomes contaminated, the whole system must be decontaminated.
If a coil is dirty or grimy, then the outside layer of dirt or grime will interfere with heat transfer, and the coil will lose some efficiency. If the coil is exposed to a biological contaminant such as mold spores or bacteria, it may result in mold or bacterial cultures growing on the exterior of the coil. Heat is a source of energy, so a coil in a dark area that is exposed to moisture can make a prime breeding ground for biological contaminants.
Furthermore, a coil with biological contaminant on the exterior increases the risk of biological contamination within the coil itself. If the contaminant is near the break in the system, then the contaminant may enter the coil tubes. A biofilm can form on the interior of the tube, and the exterior contaminant can enter and either grow or facilitate growth. For example, mold that is growing on the exterior of the coil can provide nutrients to bacteria cultures growing on the interior.
Once a coil is biologically contaminated, not only will the coil become more inefficient over time due to an inability to transfer heat, but if the coil develops a break or gap in it, then the contaminants can enter the system, and from there may enter wherever the system cools. Ultimately, the best way to prevent widespread biological contamination is to thoroughly and diligently maintain condenser coils with proper inspection and decontamination.
Legionnaire's Disease can be contracted by breathing air that contains droplets of water contaminated
with legionella bacteria. Often, these droplets of water will trace their origin to a cooling tower, since
pipes cycling in cooling towers are typically the perfect environment for legionella to grow, causing
cooling towers to facilitate the evaporation of contaminated water. Cooling towers then push out this
evaporated water and then the air will contain droplets of water contaminated with legionella.
There is no evidence to suggest Legionnaire's Disease can be spread from person to person. Cases of legionellosis are usually found to have their origin in evaporative cooling systems - if an origin is found at all. Airborne legionella can travel up to six kilometres. In outbreaks, such as the 2005 Toronto outbreak, nearby cooling towers not associated with the outbreak were also found to contain legionella.
To sum up: while Legionnaire's Disease cannot be spread from person to person, legionella can travel very far distances and therefore outbreaks can be spread over a large area of population. When outbreaks occur, they are curbed by prompt and thorough inspection and decontamination of cooling towers and facility mechanical systems.