Ultraviolet light is the non-ionizing radiation within the wavelengths of 180 and 400 nanometers (nm.)  This radiation is formed by “arcs, gas and vapor discharges, fluorescent and incandescent sources and solar radiation.”[1][1]

Bactericidal radiation for E. Coli has been shown to be maximal at a wavelength of 160 nm for surface decontamination.[2][2]  UV Light is capable of penetrating the cell wall of the certain microorganisms and is absorbed by the cellular material producing changes in the DNA and RNA that either prevent replication or causes the cell to die.  An important consideration when using UV light is that is has very poor penetrating power and hence, only microorganisms on the surfaces that are directly exposed to the light will be susceptible to the radiation.  Although air is transparent to UV light, the efficiency of UV exposure is dependant on the exposure time of the microorganism, the nature of the biological contamination and the exposure intensity.[3][3]  Germicidal effectiveness (intensity) varies with distance from the organism to the lamp.  At a distance of 2” from the lamp, the intensity factor is 32.30. At 4”, it is 18.60.  At 24”, the intensity factor is 2.33.[4][4]  UV light is most effective at temperatures between 65-70 deg F and a relative humidity of less than 60%.  Air that has a higher flow-rate requires more UV light for effective biocidal properties to be achieved.[5][5] 

Lamp life span is another factor to consider in using a UV air disinfection system.  It is recommended that lamps in industrial HVAC systems be replaced after 9,000 hours of use.  (After this time, effectiveness is reduced to 80%).[6][6]

Some manufacturers recommend replacing lamps 2 times per year.[7][7]  After 100 hours of operation, the output of the lamp decreases rapidly.  Lining the surrounding duct with an aluminum surface can increase the intensity of the lamp 80-90%.[8][8]  Magnesium Oxide also has high reflective properties for UV light (75-90% reflectance).[9][9]  Lamp intensity can also be maintained by keeping it clean and free from dust and debris.[10][10]  Newly manufactured lamps claim to have higher UV-C output that is not affected by colder air and higher flow rates.

Studies of treated and untreated air handler units of a four-story building indicated that reduction of microbial growth varied with operating conditions of the AHU.  Microbial growth in drain pans decreased 3-5 orders of magnitude when treated with UVC.[11][11]

No ozone or formaldehyde was detected in Gauvin’s study.  The only parameter affected was the organism concentration; carbon dioxide levels, temperature and relative humidity were not affected by the use of germicidal UV lamps.[12][12]  Shaughnessy’s study of the new lamps also evaluated O₃ emissions (non-detected), but did not evaluate any other parameters.  There is still more research needed to further evaluate UV light efficacy.

Some models might produce Ozone, so it is important to research the company product before purchasing.  Ozone is produced by wavelengths of less than 250 nm.[13][13]

Threshold limit value (TLV) guidelines are for skin and eye exposure.  Skin exposure can be tolerated in excess of the TLV for conditioned (tanned) individuals.  Exposure to UV radiation in conjunction with certain chemicals or prescription drugs can result in skin erythema even at levels below the TLV.[14][14]  Direct exposure to UV light causes eye irritation.  In theory, long-term exposure could cause skin cancer.  The Conference of Governmental Industrial Hygienists (ACGIH) has established a UV radiation TLV of 60 Joules per square meter of skin (J/m²) for an 8 our period at a wavelength of 254 nm.  At a wavelength of 180 nm, the TLV is 2500 J/m² for an 8-hour period.[15][15]  With UVC lighting operating in the 190-290 nm ranges, even acute operational exposure can be hazardous.  The University of Washington’s Environmental Health and Safety issued a brief statement on UVC related health hazards.[16][16]  In summary, the statement recommends UV-rated Personal Protective Equipment to be worn by operators exposed to UVC lighting. 

The Center for Disease Control (CDC) recommends UV lamps be cleaned weekly and tested periodically with a special meter.  These recommendations are for UV lamps used in Biological Safety Cabinets (BSC).  BSC’s present ideal controlled conditions for evaluating the effectiveness of UV lighting due their sound engineering design, which employ high air filtration and laminar airflow.[17][17]  In 2002, the University of Michigan went so far as to withdraw its support of UV lighting for sterilization due to inefficiency of the product, exposure hazard to occupants, and the creation of an expensive hazardous waste disposal problem (mercury).[18][18]  The University of Michigan was using UV lighting for BSC’s.

While UV wavelengths express biocidal properties, the organisms are not removed from the HVAC system.  Therefore, the allergenic properties of these organisms are still present until they are physically removed from the system.  In older buildings, UV Lighting AHU retrofit packages can actually cause problems due to the fact that many AHU components are not UV rated e.g. wiring (control, electrical, fire), drain pan, drive belts, etc.  Progress of UV lighting as a prevention of indoor air quality problems continues to be researched.  The Environmental Protection Agency (EPA) has established protocols dedicated to researching new technologies and their uses called Environmental Technology Verification (ETV).  ETV has conducted studies verifying the effectiveness of UVC technologies for water purification.  The EPA through ETV has not reported any findings related to the effectiveness of UVC lighting for air purification or as a biocide for aerosolized bio-pollutants.  Until further government and scientific research is performed acknowledging efficacy values of UV lighting, Pure Air Control Services recommends limited use of UV.

For more information on UV lights or other position statements, contact Pure Air Control Services at 800.422.7873 ext. 802.