Ultraviolet light (UV) with wavelength shorter than 300 nanometers is extremely effective in killing microorganisms. The most effective sterilizing range of UV is within the C bandwidth (UVC - 253.7nm). This range is called germicidal UV bandwidth or UVC. UVC has extremely low penetrating ability and does not penetrate past the deadcell layers of the skin. UV will cause eye irritations or burns after prolonged exposure.


Germicidal UV has been used in water disinfection systems for many years. As stated by the American Water Works Association, "... UV light disinfection process does not use chemicals. Microorganisms, including bacteria, viruses, and algae, are inactivated within seconds of UV light disinfection... UV light is effective in inactivating Cryptosporidium, while at the same time decreasing chlorinated disinfection by-products... "


The UV topics below offer detailed information on the nature of the ultraviolet light, science research, facts and conclusions for UV effectiveness. Most importantly, artificially produced UV light by germicidal UV lamps can be utilized to improve the indoor air quality and introduce a healthier environment to live, work or study.


Germicidal ultraviolet (UVC) light kills cells by damaging their DNA. The light initiates a reaction between two molecules of thymine, one of the bases that make up DNA. UV light at this wavelength (shortwave UV or UVC) causes adjacent thymine molecules on DNA to dimerize. The resulting thymine dimer is very stable. If enough of these defects accumulate on a microorganism's DNA its replication is inhibited, thereby rendering it harmless.

Ultraviolet photons harm the DNA molecules of living organisms in different ways. In one common damage event, adjacent bases bond with each other, instead of across the "ladder". This makes a bulge, and the distorted DNA molecule does not function properly.

The longer the exposure to UVC light, the more thymine dimers are formed in the DNA. If cellular processes are disrupted because of DNA damage, the cell cannot carry out its normal functions. If the damage is extensive and widespread, the cell will die.


The LIGHT SPECTRUM ranges from the infrared at wavelengths longer than visible light to the ultraviolet at wavelengths shorter than visible light. Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region.

Please note that many variables take place in a real-world environment that make actual calculating of the UV dosage very difficult (air flow, humidity, distance of microorganism to the UV light and time). However, it is proven that UV light will kill any DNA-based organism given enough UV dosage and that UV light 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 cleanses the air. If a microorganism is not effectively eradicated on the first pass through the ductwork, the UV light will continue to break its DNA down on subsequent passes. In addition, microorganisms typically do not sit in a static environment in HVAC systems except on coils which can be exposed to our lights also. In fact, microorganisms breed microorganisms if not controlled. The UV light helps to reduce incidences of inhaled pathogens for persons who reside or work in indoor environments.


The following are incident energies of germicidal ultraviolet radiation at 253.7 nanometers necessary to inhibit colony formation in microorganisms (90%) and for complete destruction:

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.



Energy dosage of Ultraviolet radiation in µW/cm2needed for kill factor 




Bacillus anthracis - Anthrax



Bacillus anthracis spores - Anthrax spores



Bacillus magaterium sp. (spores)



Bacillus magaterium sp. (veg.)



Bacillus paratyphusus



Bacillus subtilis spores



Bacillus subtilis



Clostridium tetani



Corynebacterium diphtheriae



Ebertelia typhosa



Escherichia coli



Leptospiracanicola - infectious Jaundice



Microccocus candidus



Microccocus sphaeroides



Mycobacterium tuberculosis



Neisseria catarrhalis



Phytomonas tumefaciens



Proteus vulgaris



Pseudomonas aeruginosa



Pseudomonas fluorescens



Salmonella enteritidis



Salmonela paratyphi - Enteric fever



Salmonella typhosa - Typhoid fever



Salmonella typhimurium



Sarcina lutea



Serratia marcescens



Shigella dyseteriae - Dysentery



Shigella flexneri - Dysentery



Shigella paradysenteriae



Spirillum rubrum



Staphylococcus albus



Staphylococcus aerius



Staphylococcus hemolyticus



Staphylococcus lactis



Streptococcus viridans



Vibrio comma - Cholera






Aspergillius flavus



Aspergillius glaucus



Aspergillius niger



Mucor racemosus A



Mucor racemosus B



Oospora lactis



Penicillium expansum



Penicillium roqueforti



Penicillium digitatum



Rhisopus nigricans






Chlorella Vulgaris



Nematode Eggs









Bacteriopfage - E. Coli



Infectious Hepatitis






Poliovirus - Poliomyelitis



Tobacco mosaic






Brewers yeast



Common yeast cake



Saccharomyces carevisiae



Saccharomyces ellipsoideus



Saccharomyces spores




UVC Production by Germicidal UV Lamps

There are many types of lamps that artificially produce UV. There are UV lamps for tanning, for counterfeit money detection, blacklight stage lamps and lamps for mineral displays, lamps that produce Ozone and germicidal UV lamps. The focus of this document are the germicidal UV lamps, which emit shortwave UV light in the ultraviolet section of the specter also known as UVC or germicidal UV. Here we will discuss the artificial UV production by the different UV lamps and the specs of the different types of UV lamps. Many times, people refer to the UV lamps as UV bulbs as in regular light bulb. Even though bulb is not the correct term, replacement bulb, UV bulb or bulbs are widely accepted in the industry as a reference to the UV lamps. UV Lamps – History and Development


UV is artificially produced by mercury vapor low and medium pressure lamps. The low-pressure lamps are most effective, because they emit most of the radiant energy in the germicidal wavelength of 253.7nm also known as the UVC part of the specter. This is the reason low pressure lamps are mostly used in germicidal UV applications. These lamps are sometimes called “amalgam” lamps because they contain solid amalgam “spots” (an amalgam is an alloy of mercury with another element, such as indium or gallium) that controls the mercury vapor pressure.

All lamps have secondary emissions, including small amounts of UVA, UVB, visible light (above 400nm wavelength) and heat. The blue glow of the germicidal UV lamps is not indicative to the effective germicidal output they produce – that could be only determined with a properly calibrated UV sensor and monitor.


As with all gas discharge lamps, the UV output of germicidal lamps is reduced when the temperature of the lamp surface deviates from the optimum. The performance data of the various lamp types and the influences of air or water cooling play an important part in an effective and reliable UV disinfection. If this is neglected it may lead to an inadequate UV installation.


For effective UV disinfection not only the temperature but the transparency of the medium for UVC (253.7nm wavelength) is of great importance. The greater the energy lost through absorption; the less energy remains to kill microbes. Tests have shown that there is a reduction in the UV lamps disinfecting performance if there is a high humidity level. For the effectiveness of UV water purification systems transmittance of the water is very important.


It is important to take the reducing factors under consideration when sizing the UV lamps for an effective UV disinfecting process.


In air stream UV irradiation reflective materials with high UVC reflection properties should be used as these materials will multiply the UV efficiency of the germicidal lamps.


Development of UV disinfection lamps started in the early forties when Westinghouse began the development of the Cold Cathode lamps in an economical production. After that UVC lamps were tried out for disinfection everywhere – surfaces, goods, water and air. The early extensive testing still applies today as a basic knowledge, underlining the UV technology.

Types UV Lamps

Cold Cathode Germicidal UV Lamps


The Cold Cathode Germicidal UV Lamps are instant-start lamps with a cylindrical cold cathode type of electrode. These lamps are available in different sizes and may be operated either from single lamp transformers or in series trough the medium of high voltage transformers.


The combination of Vycor glass tubing, used in most Cold Cathode lamps, and sturdy electrode construction make lamp life considerably longer than other types of lamps. Good ultraviolet maintenance is provided at lower temperatures and lamp life is unaffected by frequent starts.

Although the amount of radiant energy at 253.7nm wavelength emitted is the same for both high and low ozone lamps, the high ozone lamps use a special Vycor glass which transmits a controlled amount of radiation at 184.9nm, which wavelength produces ozone. Ozone has deodorizing properties and is in itself a bactericidal and fungicidal agent. However, tests have shown that the Ozone has a negative health effect if used indoors so the use of Ozone producing lamps is not recommended for most applications.


The Vycor glass Cold Cathode ultraviolet germicidal lamp is the most economical type for the majority of general germicidal applications, because of its long-life electrode and good glass maintenance.


Hot Cathode Germicidal UV Lamps

The Hot Cathode Germicidal UV Lamps are similar in their operation to the standard fluorescent lamps. The Hot Cathode lamp operates from a ballast or transformer and requires a device such as the glow switch starter to preheat 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 may result in excessive lamp blackening and rapid depreciation in ultraviolet output. Starting of the Hot Cathode lamps at low temperature is sometimes unreliable and may require special equipment.


Slimline Germicidal Ultraviolet Lamps

The Slimline Germicidal Ultraviolet Lamp is an instant-start lamp, similar to the Slimline fluorescent lamp. The Slimline lamps are available in low, high and very high ozone types. The lamp life is governed by the electrode life and number of starts.


Because of their high initial ultraviolet emission and good maintenance, Slimline Germicidal ultraviolet lamps are well adapted for applications such as indirect air irradiation, conveyor lines, surface sterilization and other applications which require high intensity lamps.

High Output Germicidal UV Lamps

The most recent addition to the UV lamp line of products is the type of High Output germicidal UV lamps. The HO lamps are the consequent result of constantly applied know-how and the latest lamp manufacturing processes. High UV output over a great temperature spectrum, a long life and good behavioral patterns are the pointers for the High Output UV lamps. Only high-quality raw materials are used in the lamp production. A fine tuning with the automatic electronic ballasts guarantees the lowest tolerance and a maximum UV stability.


With life duration of 12,000 hours and almost linear performance degradation the high output UV lamps are setting the standards for the development of high-performance UV technology.


The most important factor in using germicidal UV lamp technology is the knowledge about their behavior under real working conditions (e.g. the effect of air stream cooling). It is definitely not just the lamp performance under laboratory conditions that count. Only in gaining this knowledge high quality disinfecting technology can be achieved.


Taking the example of air stream cooling the High Output lamps do show their real advantage. While classic UV lamps heavily depreciate under real working conditions inside an air duct, this is not the case with the High Output UV lamps.


Light Emitting Diodes (LED) UV Lamps

Recently a completely new technology for UV production started to emerge. This is the UV Light Emitting Diodes or UV LED lamps. The UV LEDs are the next generation UV producing devises that will compete with the established UV lamps. Some researchers claim that the UV LEDs have better characteristics surpassing the regular UV lamps. However at this time there is no UV LED equipment that can compete with the high output UV lamps in real world production conditions.


UV Lamps Aging

The decrease in UV lamp output over the typical lifespan of 8,000 - 12,000 hours can vary between 15-40%. The manufacturer should be consulted for information on the end-of-life output of UV lamps. The decrease in UV output should be accounted for in the design phase such that the lamp output does not decrease to a point where the air treatment system becomes ineffective. The most conservative approach is to size the system based on the end-of-life of the lamp UV output. Selecting lamps based on end-of-life UV output will avoid the lamps aging problem.


The lamps should be kept clean and free of dust at all times. If dust accumulates on the lamp it will absorb the UV and convert it to heat, therefore lowering the effectiveness of the UV lamp. Appropriate filtration of the air prior to the UV lamps is recommended.


Is UV harmful? We are all exposed to the UV in sunlight. UV exposure can be very harmful, or harmless, depending on the type of UV, the type of exposure, the duration of exposure, and individual differences in response to UV. There are three types of UV:

UV-C - Also known as "shortwave" UV, includes germicidal (253.7nm wavelength) UV used for air disinfection. Unintentional overexposure causes transient redness and eye irritation, but does NOT cause skin cancer or cataracts.


UV-B - A small, but dangerous part of sunlight. Most solar UV-B is absorbed by the diminishing atmospheric ozone layer. Prolonged exposure is responsible for some type of skin cancer, skin aging, and cataracts (clouding of the lens of the eye).


UV-A - Longwave UV, also known as "blacklight", the major type of UV in sunlight, responsible for skin tanning, generally not harmful, used in medicine to treat certain skin disorders.

According to the Environmental Protection Agency (EPA), 10% of all colds are caught outdoors, 90% are caught indoors! We’ve all watched helplessly as a cold virus passed from one member of the family to another.


Perhaps you suffer from asthma or allergies and despite desperate attempts to dust more, keep the windows closed, clean your bedding, clothing, carpeting and furniture more frequently, your symptoms still persist.


The EPA states that indoor air can be up to 70 times more polluted than the outdoor environment. One reason for this is that the HVAC (HVAC stands for High-voltage alternating current) duct work is full of airborne germs, their particles and by products. These microbes are alive and thriving inside the furnace or air conditioning systems. The airborne germs adversely affect the air quality as they are blown past the furnace or air conditioning filter and circulated throughout the buildings.


In-duct and upper air UV air cleaners can be utilized to disinfect the indoor air.


UV Health Facts

Why is UV-B harmful while UV-C (germicidal UV) is not? - The difference has to do with the ability of UV rays to penetrate body surfaces. UVC has an extremely low penetrating ability. It is nearly completely absorbed by the outer, dead layer of the skin (stratum corneum) where it does little harm. It does reach the most superficial layer of the eye where overexposure can cause irritation, but it does not penetrate to the top of the lens of the eye and can not cause cataracts. UVC is completely stopped by the ordinary eye glasses and by ordinary clothing.


How much UV exposure is considered safe? The National Institute for Occupational Safety and Health (NIOSH) has established safe exposure levels for each type of UV. These safe exposure limits are set below the levels found to cause eye irritation, eye being the body part most sensitive to UV. For germicidal UV (253.7nm) the exposure limit is less than 0.2µW/cm² over 8 hours.


How can people be certain they are not overexposed to UV? When upper room UV is first installed it must be checked with a sensitive UV meter to make sure reflected UV is less than 0.2µW/cm² at eye level. UV air cleaners must be installed well above eye level - usually 7 feet above the floor. UV tubes (lamps) within the air cleaners should not be directly visible from within 30 feet. Safety is assured if UV measurements at eye level meet NIOSH standards.


What are the symptoms and signs of UV overexposure? UV overexposure causes an eye inflammatory condition known as photokeratitis. For 6 to 12 hours after an accidental overexposure the individual may feel nothing unusual, followed by the abrupt sensation of foreign body or "sand" in the eyes, redness of the skin around the eyes, some light sensitivity, tearing, and eye pain. The acute symptoms last 6 to 24 hours and resolve completely without long-term effects. Overexposure of the skin resembles sunburn but does not result in tanning.


What precautions are needed with overhead germicidal UV? Fixtures must be turned OFF when cleaning, inspecting or changing the lamps. Persons hypersensitive to sunlight may need to wear protective glasses, clothing or use sunscreen on exposed skin. No special protection is needed for most people.


Indoor Air Pollution. UV and Air Pollutants

Most people take the quality of indoor air for granted and assume the air is clean and safe to breathe. According to the American Medical Association 50% of all illness is caused or aggravated by polluted indoor air.


The air in today's buildings can contain different microbial contaminants such as bacteria, viruses, mold, fungi and spores. Bioaerosols are the airborne microbes, their fragments, toxins and waste products. Numerous studies have found high concentrations of such contaminants in the air handling equipment and in the air inside the places where people live and work. These indoor air pollutants can make the air quality less than desirable, may even cause unhealthy effects ranging from allergies to tuberculosis, and are actually the cause of death to an estimated 8.5 million people annually.


Some studies state that properly designed and installed UV air cleaners will eradicate or greatly reduce the microbial contaminants from the indoor air. This is why the indoor air quality in public buildings has been addressed by specific requirements of UV installation inside the HVAC systems.
When the problem of residential indoor air quality became apparent, ultraviolet technology became the proven answer to effectively controlling airborne microbial pollutants. The artificially generated UV can reduce or virtually eliminate all DNA based air pollutants that regular filtering systems do not catch.