1. What is UV light?

UV Basics

Ultraviolet light belongs in the electromagnetic spectrum with a wavelength in the range of 200 to 400 nm (nanometers), which is shorter than that of visible light, but longer than X-rays. All UV rays and bands are invisible to the human eye.

The UV spectrum can be subdivided into the following bands:

  • UV-A (long-wave; 400 – 315 nm): used for black lights, skin tanning, ink/resin curing.
  • UV-B (medium-wave; 315 – 280 nm): used for psoriasis therapy, can cause sunburn, skin cancer.
  • UV-C (short-wave; 280 – 200 nm): most effective for germicidal disinfection.
  • UV-V (vacuum UV, below 200 nm): can produce ozone in the air.

All the above wavelengths of UV light are emitted by the sun, but only the longer wavelengths, UV-A and UV-B reach the earth. The UV-C rays, which are the shortest wavelength but highest energy are blocked by the ozone layer.


Since UV-C rays are blocked by the ozone layer, microorganisms have not developed a natural defense against UV-C energy. When the DNA of a microorganism absorbs UV-C energy, molecular instability occurs, resulting in the interruption of the DNA sequence. This makes the cell unable to grow or reproduce. Without the ability to reproduce, the cell cannot infect and quickly die.

The application of UV-C energy to inactivate microorganisms is known as Germicidal Irradiation or UVGI. It has been used for this purpose since the early 1900s. Artificial UV-C energy is produced in germicidal ultraviolet lamps which produce UV radiation by ionizing low pressure mercury vapor. These lamps are similar to typical fluorescent household lamps, but do not have the phosphorescent coating which imparts the soft white light. Most commercial UV-C lamps are low pressure mercury lamps that emit UV energy at 253.7 nm, which is an ideal wavelength for disrupting the DNA of microorganisms. UV-C lamps and devices are being increasingly used all over the world in various configurations and applications to disinfect water, air and surfaces.


The amount of UV-C energy needed to inactivate a given microorganism is measured by dose, which is determined by a combination of irradiation energy and exposure time. A key difference between surface inactivation and airstream inactivation of micro-organisms is exposure time. Residence time in the UV field for microorganisms in the air stream is in the order of seconds and would require a much higher UV-C dose as compared to a surface application. Scientists have determined the rates at which various microbial populations decline due to exposure to biocidal factors such as UV-C irradiation. Organisms differ in their susceptibility to UV-C inactivation; in general, viruses are the most susceptible to UV-C followed by bacteria with molds and fungal spores being the least susceptible.

UV-C irradiation also obeys the inverse–square law of light, where the intensity at a given point is inversely proportional to the square of its distance from the light source.

Based on mathematical modeling, PHS engineers have developed proprietary and third party validated computer modeling programs to estimate deactivation rates for target microorganisms and subsequently design UV-C systems that will efficiently and effectively disinfect the air, surface or water situation of interest


Prolonged exposure to UV can cause photo degradation of organic and synthetic materials. Owing to the short wavelength, UV-C transmissivity tends to be very low for most materials; therefore, most of the photo degradation may occur only on the immediate surface of a material, and/or manifest itself as fading or discoloration. The degree to which an object might be susceptible to UV degradation needs to be considered in any application where exposure may be prolonged.


Ultraviolet germicidal irradiation used for water, air and surface disinfection is biocidal to microorganisms but presents a health hazard to humans as well. Excessive exposure to UV can result in damage to the eyes in form of photo keratitis and conjunctivitis. These symptoms usually appear within 6 to 12 hours after UV exposure and resolve within 24 to 48 hours. Ultraviolet radiation exposure can also affect the skin and cause erythema (skin-reddening). Most of the UV-C gets reflected and absorbed by the outer dead layer of the human skin, thus minimizing the UV-C transmitted through the epidermis layer

CDC and NIOSH have recommended permissible exposure limits for different UV wavelengths. For UV-C, at 253.7 nm wavelength, the recommended exposure limit (REL) is 6 mJ/cm2 for a daily 8 hour work shift. The use of proper personal protective equipment (PPE) is recommended when personnel may be exposed to UV radiation.

2. How does UV-C disinfect?

Germicidal UV-C systems artificially generate a wavelength of 254 nm of light which destroys the DNA of microorganisms including viruses, bacteria and mold. Once deactivated, these microorganisms are no longer able to reproduce and grow on surfaces.

3. What are the benefits of using UV-C?

In general HVAC applications, UV-C is used to inactivate microorganisms and eliminate the organic build up on cooling coils and drain pans. In addition to improving the indoor air quality, this makes the cooling coil more energy efficient while eliminating the need to clean the cooling coil via traditional methods. In sensitive and public environments, UV-C is not only used for coil and drain pan, but also for disinfecting the air stream of airborne viruses, bacteria and fungal spores.

4. How is a UV-C system sized?

PHS uses proprietary software to precisely size a UV-C system for an application. The software model takes into account various parameters such as the type of microorganism(s), size of the plenum or duct cavity, the air flow rate, wall reflectivity, temperature and humidity.

5. How long does it take for UV to kill microorganisms?

The UV-C dose required to inactivate a specific organism depends on the type of the microorganism that is being targeted, the applied UV-C intensity and the time of exposure to UV-C. PHS has an extensive database of dose values required to inactivate various bacteria, virus and molds.

6. Is UV-C harmful to humans?

Overexposure to UV-C can result in short term skin redness and eye irritation. UV-C does not cause any permanent damage to skin or eyes.

7. If I can’t see UV-C, what is the blue light from the lamp?

UV-C is the invisible part of the light spectrum. The blue color comes from the inert gas inside the UV-C bulb. The bulb can be lit (and blue) yet not produce any UV-C energy.

8. How do I know that the UV lights are working?

The output of the UV lamps can be measured using a UV-C radiometer. Effective of a UV system can be determined by visual inspection of coil and drain pans and microbial sampling. ASHRAE has developed test standards to evaluate the effectiveness of UV for coil and airstream applications.

9. When should I replace lamps?

It is recommended that the UV lamps should be replaced annually or after 8,000 hours of continuous use. Lamps can operate long after their useful life but at reduced intensity as the lamp filaments wear out over time.

10. How do I dispose of UV lamps?

UV lamps should be treated the same as fluorescent lamps and in accordance with local, state or federal regulations.

11. Does UV-C replace traditional methods of cleaning a room with chemical disinfectants?

No, the UV device is an adjunct technology used after the completion of terminal cleaning in operating theaters, intensive care units, patient rooms, oncology areas and restrooms, for example.

12. Will UV-C degrade objects in the room?

UV-C is a short wavelength light and does not penetrate most objects. Hospital room disinfection does not accumulate sufficient exposure time to cause any material degradation.

13. Can someone be in the room during a UV-C cycle?

No, UV-C is a powerful technology and it should only be used when the room is unoccupied.

14. Can the room be occupied immediately after a UV-C disinfection cycle?

Yes, it is perfectly safe to enter the room after the cycle is completed.

15. Does the UV-C device produce chemicals or by-products?

No, however, there is a particular smell which is created due to the breakdown of volatile organic compounds found in the air.

16. What happens when someone walks in the room when the device is operating?

The PHS device has infrared sensors and a people recognition system and the system shuts itself off immediately.

17. How long do the UV lamps last?

Cycling the UV lamps on/off will eventually degrade the UV intensity. In normal use, the UV lamps should be replaced annually.

18. Does it work?

Yes, scientific and anecdotal references abound for UV-C’s efficacy both in literature and in reports of field applications. Of the government reports, NIOSH, OSHA, CDC, GSA, EPA are the most notable. Science in the public forum comes from the University of Cincinnati, Tulsa University, University of Colorado and McGill University (Canada) to name a few. The two labs conducting independent testing and showing very favorable results are ARTI and Battelle. The list of prestigious field studies are too numerous to mention. UVC is used worldwide, more in other countries per capita than in the US. Much of this broader use is for the disinfection of drinking water and treatment of sewage in waste water treatment.

19. What does UV “C” or UV “GI” mean?

The letters “UV” relates to the magnetic wavelength spectrum known as UltraViolet light. This spectrum is more often broken down into four categories: Vacuum, Short Wave, Middle Wave and Long Wave’ or VUV, UVC, UVB & UVA. UVC is the frequency that is the most germicidal and the term UVGI refers to “Ultraviolet Germicidal Irradiation” as used by Federal Agencies such OSHA, NIOSH, GSA, EPA and the CDC when referring directly to UVC.

20. Do UV-C lamps produce ozone?

Not UV-C but UV-V. UVC does provide exceptionable conditioning of the air much like the sun does outdoors. UVC fixtures are an air conditioning component that’s in addition to other system parts. Those include the filters, coil, heating core, fan, dampers, humidifiers, etc. All are designed to provide a specific function in the job of processing air for occupied spaces.

21. How are used lamps disposed off?

Currently, most users would dispose of them as they would any glass trash, such as their fluorescent lamps! Large fluorescent lamp users follow EPA and state guidelines, and UV-C lamps would then fall into those same guidelines. If you have a fluorescent lamp disposal program in place, UV-C lamps would simply fall into that same program.

22. Do lamps need cleaning?

UV Resources lamps will usually degrade common organic debris that might accumulate on the tube surface; therefore, periodic cleaning is usually not required.

23. How are lamps cleaned when necessary?

Cleaning could be necessary if a lamp has been exposed to any form of hard water, high levels of damp organic debris or any form of oil (i.e. body oil). Vinegar can be used for mineral deposits, glass cleaner for damp organic debris and pure alcohol and a lint free cloth for oil. Commonly available cleaners should work fine if they don’t leave any residue behind.

24. What is inactivation?

For mold and bacteria, doses of UVC energy may not cause immediate cell death but the microbe could be “inactivated”. What’s meant by this is that while some biological activity may still exist, cell replication is impossible; the microbe is no longer viable. Simply, the subject microbe cannot multiply, rendering it harmless! What’s more, small doses of UV-C over time have been shown to hasten cell death. As viral particles are not a life form, we depend solely on inactivation to rid ourselves of their impending harm.

25. If I can’t see UVC energy, what do I see?

Around 90% of the energy generated by a UV-C lamp is in fact UVC energy. The remainder is visible light (blue hue) and a small amount of infrared (heat). Given the brightness from UV-C lamps, that 3-4% of visible light gives one an idea of the amount of UVC energy a lamp produces.

26. If I see blue the lamp’s working?

Not necessarily, the blue color comes from an inert gas within the lamp that doesn’t produce UVC. The lamp can be lit (blue) yet not produce much, if any UV-C energy. This would be a poor indicator.

27. Will UVC kill dust mites?

There’s no evidence that a UV-C dose suitable for a given microbe kill will have any effect dust mites at all. Very high UVC energy or long term exposure to UV-C should disrupt some of the mite’s biological functions, which may lead to death. However, there’s also no evidence anywhere that dust mites live in A/C ductwork.

28. Can I look at the device through a window while it is operating?

Yes. The UV-C waves cannot penetrate through the glass. Therefore, it is safe to be on the other side of a window or wall.

29. Do shading or shadows in the room impact PHS efficacy?

Studies in the literature show that the concentration of pathogens, the organic load and the shading from the direct radiation field did not reduce the effectiveness of PHS. However, it is recommended to operate the robot in multiple locations in a room to maximize the number of surfaces that are directly exposed to Full Spectrum UV-C and to minimize the dependence on surface reflectivity that varies from room to room and from surface to surface. Historic UV-C technologies that have been used in one location in the room did not achieve the multiple reductions in C.diff and MRSA rates that hospitals using the same technologies reported in peer-reviewed journals.

30. How long does the disinfection last?

After disinfection, the room will remain at very low microbial load until new pathogens enter the room from a visitor, patient, care provider or the air handling system. A hospital does not need to repeat the PHS decontamination procedure in a properly cleaned and disinfected room as long as there has been no re-contamination.

31. What is the effectiveness of PHS against endospores like C. diff and bacillus strains?

The UV-C intensity of PHS Robots allows for the deactivation of endospores in 5 minutes! This has been demonstrated in lab studies, hospital environment studies, and validated in outcome studies proving reductions of C. diff rates reported by several hospitals utilizing same technologies.

32. Why can’t cellular repair mechanisms overcome UV-C damage?

The damage caused by PHS’s stops the DNA repair mechanisms for most pathogens in less than 5 minutes. In addition, PHS produces higher energy wavelengths than other UV sources which results in damage to structures in the cell, including ruptured cell walls.

33. What is the difference between a black-light and a blue-light?

A “blacklight” is a fluorescent light tube that emits at about 365 nm – this is just below the wavelengths that humans can see, but it is absorbed by most pigments in clothes so that they “fluorescence”. This is the effect seen in many bars and discos. I’m not sure what you mean by a “blue-light” – perhaps you means “germicidal” low-pressure mercury lamp. They do glow “blue”, but most of their output is at 254 nm, so DO NOT look directly at such a lamp when it is operating. These lamps are used in air and water disinfection, since the 254 nm light is absorbed by DNA in bacteria and viruses causing their inactivation.

34. Is it possible to use UV lamps to protect postal workers from the terrorist threat of Anthrax contamination of mail?

Very little is known about the inactivation of Anthrax spores by ultraviolet light in air. One would have to arrange that the UV irradiance would be high enough so that the spores would receive a sufficient UV dose. If one were to set up such an arrangement, there would have to be safeguards to avoid exposing the workers’ eyes to UV and also the workers should wear latex gloves to block UV from exposing the skin of the workers.

35. When Ultraviolet (UV) radiation is used to treat water, does the water become radioactive?

Ultraviolet is “light” – you can’t see it because our eyes are not sensitive to UV rays; however, it is a form of light with wavelengths beyond the “violet” end (hence the term “ultraviolet”) of the rainbow spectrum. Since UV rays are “light”, they travel through the air and water at the speed of light and when the UV source is turned off, the UV rays disappear. There are no “residues” and the water that has been exposed to UV rays is the same as before exposure, and certainly the water is not “radioactive”. It is like illuminating a bright light in a glass of water. I think you accept that when you turn off the light, the water hasn’t changed.

The UV water disinfection units are designed to provide a sufficient “UV dose” so that all pathogenic microorganisms in the water are made “inactive”. What happens is that UV is absorbed by DNA in microorganisms; DNA is damaged so that the microorganism cannot reproduce. Cells that cannot reproduce cannot cause disease. The nice thing about UV is that it does its job as the water passes through the unit, but after the water has passed, it has been “disinfected”, but its “water quality” has not changed.

36. To purify the water you need 1/100 of watt-second per square cm using UV at 254 nm. What is the dosage for air?

Most regulators now specify a fluence or UV dose of 40 mJ / cm2 to ensure inactivation of at least 4 trunks of pathogenic microorganisms. Since the applied fluence or UV dose is independent of the medium, this requirement would also apply to air. However, I am not yet aware of any regulations relating to UV air treatment.

37. If you want to use a UV germicidal bulb to disinfect PVC objects, what effect would UV light have on PVC?

PVC (polyvinyl chloride) blocks (or totally absorbs) the 254 nm ultraviolet light coming from a “germicidal” UV lamp. Therefore, any object inside a PVC container would not be disinfected at all by a germicidal UV lamp outside the PVC container. Eventually, PVC will degrade due to photochemical attack by ultraviolet light.

Some types of transparent polyethylene and Teflon are transparent at 254 nm, but only for a thin layer.

38. Do all UVC lamps contain mercury to be germicidal? Do all UVC lamps produce an ozone smell?

Most of the UV lamps used for UV disinfection contain mercury. In the case of low pressure lamps, only a few milligrams of mercury are present. In the case of medium pressure lamps, which have a much higher power, there are a few grams. Low and medium pressure lamps generate ozone if they have a very pure (synthetic) quartz casing, which allows the 185 nm mercury emission entering the air. This UV light is absorbed by oxygen in the air to generate ozone. Most UV lamps are made with a form of quartz that contains impurities that completely absorb the emission of 185 nm, so as not to produce ozone.

39. What is UVV?

UVV light refers to another wavelength in the ultraviolet spectrum. Some UV-C devices also produce light in this wavelength. The manufacturers of these devices promote UVV as an added tool for IAQ control, saying that UVV attacks microorganisms, chemicals, and odors. While this may be true, it is important to understand that UVV (unlike UV-C) will also “attack” occupants in treated spaces by adversely effecting human lungs! The reason for this is that the shorter (185 nanometers) wavelength of UVV light actually generates ozone. This occurs because UVV light reacts with oxygen to break it into atomic oxygen, a highly unstable atom that combines with oxygen to form O3 (ozone). The American Lung Association states that “exposure to ozone causes a variety of adverse health effects, even at levels below the current standard.” And the U.S. Food & Drug Administration says: “In order for ozone to be effective as a germicide, it must be present in a concentration far greater than that which can be safely tolerated by man and animals.” The longer (254 nanometers) wavelength of UV-C light, by contrast, provides highly effective air, surface, and water disinfection without producing any harmful ozone. The materials and methods of UV-C lamp construction determine whether a given UV-C device will produce both UV-C and UVV light or only the safer UV-C wavelength. Bio-Shield UV-C devices do not produce any UVV.

40. UV treatment is a proven method of Ozone destruction. Why is that so important to the beverage industry?

Beverage companies use residual ozone in their water systems to sanitize and disinfect the storage tanks, plumbing and pumps and to ensure they remain bacteria free. The residual ozone needs to be destroyed prior to contact with the product so that it does not contaminate. UV is ideal because it is a non-chemical process and it’s fast.

41. How does ozone works?

The third oxygen atom of ozone is extremely reactive because it is unstable. This atom readily attaches itself to other odor molecules. When contaminants such as odors, bacteria or viruses make contact with ozone, their chemical structure is changed to less odorous compounds. As more ozone attacks the remaining compounds, the odor is eventually destroyed. This process is called oxidation. Ozone essentially reverts back to oxygen after it is used. This makes it a very environmentally friendly oxidant.

42. How long does ozone last?

As soon as ozone is formed in a generator and dispersed in a room, some of it reverts back into oxygen (O2). This step occurs by several processes including the following: Oxidation reacts with an organic material such as odors or smoke. Reactions with bacteria etc., which again consumes ozone by oxidation reactions. Additionally, ozone breaks down thermally. Higher temperatures destroy ozone quicker than lower temperatures. The ozone that remains is referred to as Residual ozone. “Residual” ozone created will return to oxygen usually within 30 minutes, in amounts equal to half its level. What this means is that after each subsequent 30 minute period there would be half as much residual ozone left at the end of the period as was present at the beginning of the period. This is similar to a geometric progression of 16; 8; 4; 2; 1. In practice, the half life is usually less than 30 minutes due to temperature, dust and other contaminants in the air. Therefore, ozone, while very powerful, doesn’t last long. It does its job and then disappears back into safe oxygen.

43. What is the right position?

The UVC dose received in a normally equipped burn ICU room after decontamination with a mobile UVC-emitting unit varies depending on the distance between the light source and the irradiated area and any objects in between that shadows. One must assure that an adequate dose has been received in shadowed and/or critical areas. Disposable indicators can help ensure that an adequate dose has been received.

PHS lamps and other unmeasured UVC lights, while they may use UVC lamps, have no way of calculating the UVC dose needed for each location to determine the positioning or time necessary to disinfect accurately, nor can they reach shadowed or hidden surfaces. As distance from the lamp increases, effectiveness against microorganisms decreases sharply – requiring multiple positioning around the room to achieve even partial disinfection.

PHS is an automated, remotely-operated system that self-adjusts to the size and content dynamics of a room to deliver the proper dose of UVC light energy. PHS technology uses instrument to measure the UVC light in a combined way that is deflected into shadowed and indirect areas for a terminal disinfection. As energy is deflected around the room, shadowed areas are reached and pathogens are eradicated ensuring a thorough disinfection outcome, every time.

44. What is the right level of ozone?

The right level of ozone is when all the generated ozone is being consumed. This only applies to continuous ozone use in occupied environments – not for shock treating. However, this is difficult to attain because it becomes a balancing act. Initially the machine’s output is set high to get rid of the problem odor as quickly as possible. As this is being accomplished, less ozone is required for the diminishing odor, etc., thereby leaving some residual ozone in the air. If the machine output is not turned down, then more residual ozone will remain. If a strong smell of ozone is noticed, then there is more ozone present than is required. Simply turn the rheostat (output level control) down. With ozone, more is not considered better. Note: It is not necessary that ozone be detected by humans for it to be effective. Ozone can work even when humans are not able to smell it.

45. Is ozone harmful and what, if any, are the long term effects?

Several regulatory agencies, including the Occupational Safety and Health Administration (OSHA), have stipulated that the safe allowable level of residual is 0.1 ppm (parts per million). Note that this permissible level is for continuous exposure throughout an entire 8 hour day. The temporary affects of such a low exposure would range from headaches, to sore throats and irritation in the eyes and nose. No long term effects have ever been documented from ozone exposure.

46. How does ozone kill bacteria?

Ozone kills bacteria by destroying the cell wall of the bacteria. Once the cell wall is destroyed, the bacteria will be unable to survive.

47. To be effective, how close to the surface do the lamps need to be?

The exposure of germicidal ultraviolet is the product of time and intensity. High intensities for a short period and low intensities for a long period are fundamentally equal in lethal action on bacteria. The inverse square law applies to germicidal ultraviolet as it does to light: the killing power decreases as the distance from the lamps increases. The average bacterium will be killed in ten seconds at a distance of six inches from the lamp.

48. How do you calculate ultraviolet radiation?

The intensity of UV radiation is measured in the units of milliwatts per square centimeter (mW/cm2) which is energy per square centimeter received per second. Also, it is measured in the units of millijoules per square centimeter (mJ/cm2), which is energy received per unit area in a given time.

49. Do I need ozone-producing lamps?

It depends on your particular need. Most of the time you do not need ozone, unless there are shaded areas that the UVC light cannot reach. Ozone helps to “carry” the ultraviolet radiation in the air to where it normally cannot reach directly.

50. When do I need to use ozone-producing lamps?

Germicidal UVC lamps generate energy at 185 nanometers as well as 254nm. This UVC emission produces abundant amounts of ozone in air. Ozone is an extremely active oxidizer and destroys microorganisms on contact. Ozone also acts as a deodorizer. Another advantage is that it can be carried by air into places that UVC radiation cannot reach directly.

51. What effects does UV light have on surrounding materials?

Long-term exposure of germicidal UVC light to plastics will shorten the shelf life of the plastic by approximately 10%. Example: If the plastic would normally last about ten years, and it’s exposed to germicidal UVC light the entire time, it would probably need to be replaced in 9 years. Plant life may be damaged by direct, or reflected, germicidal ultraviolet rays. Transient dyes and colors may be faded from prolonged exposure to ultraviolet rays.

52. Can germicidal UVC penetrate surfaces or substances?

No – germicidal UVC sterilizes only what it comes in contact with.

53. How do you determine the square footage that one germicidal UVC lamp will cover?

This is determined by the wattage of the lamp. Example: A 15-watt lamp will cover approximately 100 square feet; a 30-watt lamp will cover approximately 200 square feet (60,96 meters).

54. Do the lamps need a ballast to work?

Yes – a germicidal lamp is one part of a system, and the system cannot be fully defined and optimized unless the lamp and ballast combination is determined. It is the interaction of the lamp and ballast that is the true determinant of system performance.

55. How much intensity do I need to kill certain organisms?

The exposure of germicidal ultraviolet is the product of time and intensity. High intensities for a short period and low intensities for a long period are fundamentally equal in lethal action on bacteria. The inverse square law applies to germicidal ultraviolet as it does to light: the killing power decreases as the distance from the lamps increases. The average bacterium will be killed in ten seconds at a distance of six inches from the lamp.

56. Should UVC lamps be cleaned?

Yes – depending on the surrounding environment, UVC lamps should be checked periodically (approximately every three months), and can be cleaned with a dry cotton cloth or paper towel. Wear rubber gloves and clean with alcohol only. This will also help maximize lamp life.

57. Will germicidal UV take care of mold?

Yes. Germicidal UVC lamps will kill up to 99.9% of mold and help prevent future mold growth.

58. Do germicidal lamps kill viruses?

Yes -germicidal UVC lamps kill up to 99.9% of most viruses, airborne bacteria and mold spores.