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UV- C 254 nm  
Micro-organisms are microscopically small. They are everywhere and have an extremely high metabolism. For example, in hot and humid weather the number of moulds and bacteria increases exponentially.  
Micro-organisms produce mucus substances that are deposited on surfaces and can contaminate food and cause microbial decay of the products.  
The cell nucleus of micro-organisms contains thymine, a chemical element of the DNA / RNA.  
This element absorbs UV-C light at a specific wavelength of 253.7 nm and changes to such an extent (formation of thymine dimers) that the cell is no longer capable of multiplying and surviving.  
UV-C light at an optimal wavelength of 253.7nm penetrates the cell wall of the micro-organism.  
The high energy photons of the UV-C light are absorbed by the cell proteins and DNA / RNA. 
UV-C damages the protein structure causing metabolic disruption.  
DNA is chemically altered so organisms can no longer replicate and are unable to metabolise and replicate.  
This process has a lethal effect on all microorganisms such as bacteria, fungi, moulds, yeasts and viruses.  
Germicidal UV has been used safely and effectively in industry, healthcare, institutions & commercial / domestic premises for more than 100 years.
UV-C Lamps  
The Hot Cathode Germicidal UV-C Lamps are similar in their operation to a standard fluorescent lamps.  
The Hot Cathode lamp operates from a ballast or transformer the best has a pre-heat for the electrodes in order to start the lamp.  
The electrodes, located at the ends of the lamp, are tungsten filaments coated with emission material and, under normal operation, govern the life of the lamp. In view of the fact that the life of the electrodes is shortened by frequent starts, the lamp life is rated according to the number of times the lamp is started.  
Operation at refrigerator temperatures will need Quartz sleeve protection against the lower operating temperatures.  
Starting of the Hot Cathode lamps at low temperature is sometimes unreliable and may require special equipment.  
With life duration of 12,000 hours and almost linear performance degradation the high output UV-C lamps are setting the standards for the development of high performance UV-C technology
Microbial Testing  
Surface sampling of Cooling Coils, Condensate Drain Trays using contact plates to indicate fungal and bacteria CFU’s.  
Approximately 1-2 weeks (depending on the state of the coil and the material that the coil is made of) per coil row to destroy the mold growing on and in the coil.  
So on a 4-row coil allow 4 to 8 weeks.  
The cleaning is slower if the AHU is not producing condensate. 
Approximately 1-2 days to remove the mold, bacteria and virus traveling in the air that is exiting the AHU. 
The purpose of the coil is to provide thermal comfort in the building by removing condensate and heat from the air.  
As soon as the coil produces condensate, mould starts growing. Initially, it is invisible. 
The mould provides biological glue that allows the dirt passing through the filters to stick onto the coil fins and form a biofilm. 
If the coil has bio-film on the fins, the fin’s ability to transfer heat is reduced, causing the chiller to work harder and therefore increasing energy costs.
Please note that many variables (air flow, humidity, distance of microorganism to the UV light, irradiation time) take place in a real world environment that make actual calculating of the UV dosage very difficult. However, it is proven that UV light will kill any DNA-based microorganism given enough UV dosage. UV breaks down DNA on a cumulative basis. Therefore, as air circulates through the ductwork of an HVAC system containing an UV light, the UV light continuously disinfects the air. If a microorganism is not effectively deactivated on the first pass through the ductwork, the UV light will continue to break its DNA down on subsequent passes. Microorganisms do not sit in a static environment in HVAC systems except on coils which can be exposed to UV light also. Microorganisms multiply rapidly if not controlled. The UV light helps to reduce airborne microorganisms from the indoor environment. 
The following are incident energies of germicidal ultraviolet radiation at 253.7 nanometers wavelength necessary to inhibit colony formation in microorganisms (90%) and for 2-log reduction (99%):
UV Tests 
Tests conducted by Light Sources Inc - Orange, CT and verified by American Ultraviolet Company - Lebanon, IN revealed that American-Lights® produces 800 microwatts @ 1 foot with 534 feet per minute air flow @ 55º F. To compute time needed to sterilize germs in the following chart at 1 foot distance from the light, divide the dosage required by 800. Example: for 90% kill factor of Bacillus subtilis spores:11,600 divided by 800 = 14.5 seconds.