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A low-pressure mercury-vapor discharge tube floods the inside of a biosafety cabinet with shortwave UV light when not in use, killing microbes on irradiated surfaces.
Ultraviolet germicidal irradiation (UVGI) is a disinfection technique employing ultraviolet (UV) light, particularly UV-C (180-280 nm), to kill or inactivate microorganisms. UVGI primarily inactivates microbes by damaging their genetic material, thereby inhibiting their capacity to carry out vital functions.[1]
The use of UVGI extends to an array of applications, encompassing food, surface, air, and water disinfection. UVGI devices can inactivate microorganisms including bacteria, viruses, fungi, molds, and other pathogens.[2][3] Recent studies have substantiated the ability of UV-C light to inactivate SARS-CoV-2, the strain of coronavirus that causes COVID-19.[4][5][6][7][8][9]
UV-C wavelengths demonstrate varied germicidal efficacy and effects on biological tissue.[9][10][11] Many germicidal lamps like low-pressure mercury (LP-Hg) lamps, with peak emissions around 254 nm, contain UV wavelengths that can be hazardous to humans.[12][13] As a result, UVGI systems have been primarily limited to applications where people are not directly exposed, including hospital surface disinfection, upper-room UVGI, and water treatment.[14][15][16] More recently, the application of wavelengths between 200-235 nm, often referred to as far-UVC, has gained traction for surface and air disinfection.[11][17][18] These wavelengths are regarded as much safer due to their significantly reduced penetration into human tissue.[19][20][21][22]
Notably, UV-C light is virtually absent in sunlight reaching the Earth's surface due to the absorptive properties of the ozone layer within the atmosphere.[23]
History [ edit ] Origins of UV germicidal action [ edit ] The development of UVGI traces back to 1878 when Arthur Downes and Thomas Blunt found that sunlight, particularly its shorter wavelengths, hindered microbial growth.[24][25][26] Expanding upon this work, Émile Duclaux, in 1885, identified variations in sunlight sensitivity among different bacterial species.[27][28][29] A few years later, in 1890, Robert Koch demonstrated the lethal effect of sunlight on Mycobacterium tuberculosis, hinting at UVGI's potential for combating diseases like tuberculosis.[30]
Subsequent studies further defined the wavelengths most efficient for germicidal inactivation. In 1892, it was noted that the UV segment of sunlight had the most potent bactericidal effect.[31][32] Research conducted in the early 1890s demonstrated the superior germicidal efficacy of UV-C compared to UV-A and UV-B.[33][34][35]
The mutagenic effects of UV were first unveiled in a 1914 study that observed metabolic changes in Bacillus anthracis upon exposure to sublethal doses of UV.[36] Frederick Gates, in the late 1920s, offered the first quantitative bactericidal action spectra for Staphylococcus aureus and Bacillus coli, noting peak effectiveness at 265 nm.[37][38][39] This matched the absorption spectrum of nucleic acids, hinting at DNA damage as the key factor in bacterial inactivation. This understanding was solidified by the 1960s through research demonstrating the ability of UV-C to form thymine dimers, leading to microbial inactivation.[40] These early findings collectively laid the groundwork for modern UVGI as a disinfection tool.
UVGI for air disinfection [ edit ] The utilization of UVGI for air disinfection began in earnest in the mid-1930s. William F. Wells demonstrated in 1935 that airborne infectious organisms, specifically aerosolized B. coli exposed to 254 nm UV, could be rapidly inactivated.[41] This built upon earlier theories of infectious droplet nuclei transmission put forth by Carl Flüugge and Wells himself.[42][43] Prior to this, UV radiation had been studied predominantly in the context of liquid or solid media, rather than airborne microbes.
Shortly after Wells' initial experiments, high-intensity UVGI was employed to disinfect a hospital operating room at Duke University in 1936.[44] The method proved a success, reducing postoperative wound infections from 11.62% without the use of UVGI to 0.24% with the use of UVGI.[45] Soon, this approach was extended to other hospitals and infant wards using UVGI "light curtains", designed to prevent respiratory cross-infections, with noticeable success.[46][47][48][49]
Adjustments in the application of UVGI saw a shift from "light curtains" to upper-room UVGI, confining germicidal irradiation above human head level. Despite its dependency on good vertical air movement, this approach yielded favorable outcomes in preventing cross-infections.[50][51][52] This was exemplified by Wells' successful usage of upper-room UVGI between 1937 and 1941 to curtail the spread of measles in suburban Philadelphia day schools. His study found that 53.6% of susceptibles in schools without UVGI became infected, while only 13.3% of susceptibles in schools with UVGI were infected.[53]
Richard L. Riley, initially a student of Wells, continued the study of airborne infection and UVGI throughout the 1950s and 60s, conducting significant experiments in a Veterans Hospital TB ward. Riley successfully demonstrated that UVGI could efficiently inactivate airborne pathogens and prevent the spread of tuberculosis.[54][55][56]
Despite initial successes, the use of UVGI declined in the second half of the 20th century era due to various factors, including a rise in alternative infection control and prevention methods, inconsistent efficacy results, and concerns regarding its safety and maintenance requirements.[14] However, recent events like a rise in multiple drug-resistant bacteria and the COVID-19 pandemic have renewed interest in UVGI for air disinfection.[57][58][59][60]
UVGI for water treatment [ edit ] Using UV light for disinfection of drinking water dates back to 1910 in Marseille, France.[61] The prototype plant was shut down after a short time due to poor reliability. In 1955, UV water treatment systems were applied in Austria and Switzerland; by 1985 about 1,500 plants were employed in Europe. In 1998 it was discovered that protozoa such as cryptosporidium and giardia were more vulnerable to UV light than previously thought; this opened the way to wide-scale use of UV water treatment in North America. By 2001, over 6,000 UV water treatment plants were operating in Europe.[62]
Over time, UV costs have declined as researchers develop and use new UV methods to disinfect water and wastewater. Several countries have published regulations and guidance for the use of UV to disinfect drinking water supplies, including the US[63][64][65] and the UK.[66]
Method of operation [ edit ] UV light is electromagnetic radiation with wavelengths shorter than visible light but longer than X-rays. UV is categorised into several wavelength ranges, with short-wavelength UV (UV-C) considered "germicidal UV". Wavelengths between about 200 nm and 300 nm are strongly absorbed by nucleic acids. The absorbed energy can result in defects including pyrimidine dimers. These dimers can prevent replication or can prevent the expression of necessary proteins, resulting in the death or inactivation of the organism. Recently, it has been shown that these dimers are fluorescent.[68]
: fig 5.5 This process is similar to, but stronger than, the effect of longer wavelengths (UV-B) producing sunburn in humans. Microorganisms have less protection against UV and cannot survive prolonged exposure to it.[citation needed]
A UVGI system is designed to expose environments such as water tanks, rooms and forced air systems to germicidal UV. Exposure comes from germicidal lamps that emit germicidal UV at the correct wavelength, thus irradiating the environment. The forced flow of air or water through this environment ensures exposure of that air or water.[citation needed]
Effectiveness [ edit ] The effectiveness of germicidal UV depends on the duration a microorganism is exposed to UV, the intensity and wavelength of the UV radiation, the presence of particles that can protect the microorganisms from UV, and a microorganism's ability to withstand UV during its exposure.
In many systems, redundancy in exposing microorganisms to UV is achieved by circulating the air or water repeatedly. This ensures multiple passes so that the UV is effective against the highest number of microorganisms and will irradiate resistant microorganisms more than once to break them down.
"Sterilization" is often misquoted as being achievable. While it is theoretically possible in a controlled environment, it is very difficult to prove and the term "disinfection" is generally used by companies offering this service as to avoid legal reprimand. Specialist companies will often advertise a certain log reduction, e.g., 6-log reduction or 99.9999% effective, instead of sterilization. This takes into consideration a phenomenon known as light and dark repair (photoreactivation and base excision repair, respectively), in which a cell can repair DNA that has been damaged by UV light.
The effectiveness of this form of disinfection depends on line-of-sight exposure of the microorganisms to the UV light. Environments where design creates obstacles that block the UV light are not as effective. In such an environment, the effectiveness is then reliant on the placement of the UVGI system so that line of sight is optimum for disinfection.
Dust and films coating the bulb lower UV output. Therefore, bulbs require periodic cleaning and replacement to ensure effectiveness. The lifetime of germicidal UV bulbs varies depending on design. Also, the material that the bulb is made of can absorb some of the germicidal rays.
Lamp cooling under airflow can also lower UV output. Increases in effectiveness and UV intensity can be achieved by using reflection. Aluminum has the highest reflectivity rate versus other metals and is recommended when using UV.[71]
One method for gauging UV effectiveness in water disinfection applications is to compute UV dose. The U.S. Environmental Protection Agency (EPA) published UV dosage guidelines for water treatment applications in 1986.[72] UV dose cannot be measured directly but can be inferred based on the known or estimated inputs to the process:
In air and surface disinfection applications the UV effectiveness is estimated by calculating the UV dose which will be delivered to the microbial population. The UV dose is calculated as follows:
The UV intensity is specified for each lamp at a distance of 1 meter. UV intensity is inversely proportional to the square of the distance so it decreases at longer distances. Alternatively, it rapidly increases at distances shorter than 1 m. In the above formula, the UV intensity must always be adjusted for distance unless the UV dose is calculated at exactly 1 m (3.3 ft) from the lamp. Also, to ensure effectiveness, the UV dose must be calculated at the end of lamp life (EOL is specified in number of hours when the lamp is expected to reach 80% of its initial UV output) and at the furthest distance from the lamp on the periphery of the target area. Some shatter-proof lamps are coated with a fluorated ethylene polymer to contain glass shards and mercury in case of breakage; this coating reduces UV output by as much as 20%.
To accurately predict what UV dose will be delivered to the target, the UV intensity, adjusted for distance, coating, and end of lamp life, will be multiplied by the exposure time. In static applications the exposure time can be as long as needed for an effective UV dose to be reached. In case of rapidly moving air, in AC air ducts, for example, the exposure time is short, so the UV intensity must be increased by introducing multiple UV lamps or even banks of lamps. Also, the UV installation should ideally be located in a long straight duct section with the lamps directing UVC in a direction parallel to the airflow to maximize the time the air is irradiated.
These calculations actually predict the UV fluence and it is assumed that the UV fluence will be equal to the UV dose. The UV dose is the amount of germicidal UV energy absorbed by a microbial population over a period of time. If the microorganisms are planktonic (free floating) the UV fluence will be equal the UV dose. However, if the microorganisms are protected by mechanical particles, such as dust and dirt, or have formed biofilm a much higher UV fluence will be needed for an effective UV dose to be introduced to the microbial population.
Inactivation of microorganisms [ edit ] The degree of inactivation by ultraviolet radiation is directly related to the UV dose applied to the water. The dosage, a product of UV light intensity and exposure time, is usually measured in microjoules per square centimeter, or equivalently as microwatt seconds per square centimeter (μW·s/cm2). Dosages for a 90% kill of most bacteria and viruses range between 2,000 and 8,000 μW·s/cm2. Larger parasites such as Cryptosporidium require a lower dose for inactivation. As a result, US EPA has accepted UV disinfection as a method for drinking water plants to obtain Cryptosporidium, Giardia or virus inactivation credits. For example, for a 90% reduction of Cryptosporidium, a minimum dose of 2,500 μW·s/cm2 is required based on EPA's 2006 guidance manual.[65]: 1–7
Strengths and weaknesses [ edit ] Advantages [ edit ] UV water treatment devices can be used for well water and surface water disinfection. UV treatment compares favourably with other water disinfection systems in terms of cost, labour and the need for technically trained personnel for operation. Water chlorination treats larger organisms and offers residual disinfection, but these systems are expensive because they need special operator training and a steady supply of a potentially hazardous material. Finally, boiling of water is the most reliable treatment method but it demands labour and imposes a high economic cost. UV treatment is rapid and, in terms of primary energy use, approximately 20,000 times more efficient than boiling.[citation needed]
Disadvantages [ edit ] UV disinfection is most effective for treating high-clarity, purified reverse osmosis distilled water. Suspended particles are a problem because microorganisms buried within particles are shielded from the UV light and pass through the unit unaffected. However, UV systems can be coupled with a pre-filter to remove those larger organisms that would otherwise pass through the UV system unaffected. The pre-filter also clarifies the water to improve light transmittance and therefore UV dose throughout the entire water column. Another key factor of UV water treatment is the flow rate—if the flow is too high, water will pass through without sufficient UV exposure. If the flow is too low, heat may build up and damage the UV lamp.[74]
A disadvantage of UVGI is that while water treated by chlorination is resistant to reinfection (until the chlorine off-gasses), UVGI water is not resistant to reinfection. UVGI water must be transported or delivered in such a way as to avoid reinfection.
Safety [ edit ] Skin and eye safety [ edit ] Many UVGI systems use UV wavelengths that can be harmful to humans, resulting in both immediate and long-term effects. Acute impacts on the eyes and skin can include conditions such as photokeratitis (often termed "snow blindness") and erythema (reddening of the skin), while chronic exposure may heighten the risk of skin cancer.[12][13][75]
However, the safety and effects of UV vary extensively by wavelength, implying that not all UVGI systems pose the same level of hazards. Humans typically encounter UV light in the form of solar UV, which comprises significant portions of UV-A and UV-B, but excludes UV-C. The UV-B band, able to penetrate deep into living, replicating tissue, is recognized as the most damaging and carcinogenic.[76]
Many standard UVGI systems, such as low-pressure mercury (LP-Hg) lamps, produce broad-band emissions in the UV-C range and also peaks in the UV-B band. This often makes it challenging to attribute damaging effects to a specific wavelength.[77] Nevertheless, longer wavelengths in the UV-C band can cause conditions like photokeratitis and erythema.[22][78] Hence, many UVGI systems are used in settings where direct human exposure is limited, such as with upper-room UVGI air cleaners and water disinfection systems.
Precautions are commonly implemented to protect users of these UVGI systems, including:
Since the early 2010s there has been growing interest in the far-UVC wavelengths of 200-235 nm for whole-room exposure. These wavelengths are generally considered safer due to their limited penetration depth caused by increased protein absorption.[79][80] This feature confines far-UVC exposure to the superficial layers of tissue, such as the outer layer of dead skin (the stratum corneum) and the tear film and surface cells of the cornea.[22][81][82][83] As these tissues do not contain replicating cells, damage to them poses less carcinogenic risk. It has also been demonstrated that far-UVC does not cause erythema or damage to the cornea at levels many times that of solar UV or conventional 254 nm UVGI systems.[84][85][22]
Indoor air chemistry [ edit ] UV can influence indoor air chemistry, leading to the formation of ozone and other potentially harmful pollutants, including particulate pollution.[86] This occurs primarily through photolysis, where UV photons break molecules into smaller radicals that form radicals such as OH.[87] The radicals can react with volatile organic compounds (VOCs) to produce oxidized VOCs (OVOCs) and secondary organic aerosols (SOA).[88]
Wavelengths below 242 nm can also generate ozone, which not only contributes to OVOCs and SOA formation but can be harmful in itself. When inhaled in high quantities, these pollutants can irritate the eyes and respiratory system and exacerbate conditions like asthma.[89]
The specific pollutants produced depend on the initial air chemistry and the UV source power and wavelength. To control ozone and other indoor pollutants, ventilation and filtration methods are used, diluting airborne pollutants and maintaining indoor air quality.[90]
Exposure limits [ edit ] Exposure limits for UV, particularly the germicidal UV-C range, have evolved over time due to scientific research and changing technology. The American Conference of Governmental Industrial Hygienists (ACGIH) and the International Commission on Non-Ionizing Radiation Protection (ICNIRP) have set exposure limits to safeguard against both immediate and long-term effects of UV exposure.[91][92] These limits, also referred to as Threshold Limit Values (TLVs), form the basis for emission limits in product safety standards.
The UV-C photobiological spectral band is defined as 100–280 nm, with limits currently applying only from 180 to 280 nm. This reflects concerns about acute damage such as erythema and photokeratitis as well as long-term delayed effects like photocarcinogenesis. However, with the increased safety evidence surrounding UV-C for germicidal applications, the existing ACGIH TLVs were revised in 2022.[93]
The TLVs for the 222 nm UV-C wavelength (peak emissions from KrCl excimer lamps), following the 2022 revision, are now 161 mJ/cm2 for eye exposure and 479 mJ/cm2 for skin exposure over an eight-hour period.[94] For the 254 nm UV wavelength, the updated exposure limit is now set at 6 mJ/cm2 for eyes and 10 mJ/cm2 for skin.[94]
UVC radiation damage to materials [ edit ] UVC radiation is able to break chemical bonds. This leads to rapid aging of plastics and other material, and insulation and gaskets. Plastics sold as "UV-resistant" are tested only for the lower-energy UVB since UVC does not normally reach the surface of the Earth.[95] When UV is used near plastic, rubber, or insulation, these materials may be protected by metal tape or aluminum foil.
Uses [ edit ] Air disinfection [ edit ] UVGI can be used to disinfect air with prolonged exposure. In the 1930s and 40s, an experiment in public schools in Philadelphia showed that upper-room ultraviolet fixtures could significantly reduce the transmission of measles among students.[96] In 2020, UVGI is again being researched as a possible countermeasure against COVID-19.[97][98]
UV and violet light are able to neutralize the infectivity of SARS-CoV-2.[99] Viral titers usually found in the sputum of COVID-19 patients are completely inactivated by levels of UV-A and UV-B irradiation that are similar to those levels experienced from natural sun exposure. This finding suggests that the reduced incidence of SARS-COV-2 in the summer may be, in part, due to the neutralizing activity of solar UV irradiation.[99]
Various UV-emitting devices can be used for SARS-CoV-2 disinfection, and these devices may help in reducing the spread of infection.[100] SARS-CoV-2 can be inactivated by a wide range of UVC wavelengths, and the wavelength of 222 nm provides the most effective disinfection performance.[100]
Disinfection is a function of UV intensity and time. For this reason, it is in theory not as effective on moving air, or when the lamp is perpendicular to the flow, as exposure times are dramatically reduced. However, numerous professional and scientific publications have indicated that the overall effectiveness of UVGI actually increases when used in conjunction with fans and HVAC ventilation, which facilitate whole-room circulation that exposes more air to the UV source.[101][102] Air purification UVGI systems can be free-standing units with shielded UV lamps that use a fan to force air past the UV light. Other systems are installed in forced air systems so that the circulation for the premises moves microorganisms past the lamps. Key to this form of sterilization is placement of the UV lamps and a good filtration system to remove the dead microorganisms.[103] For example, forced air systems by design impede line-of-sight, thus creating areas of the environment that will be shaded from the UV light. However, a UV lamp placed at the coils and drain pans of cooling systems will keep microorganisms from forming in these naturally damp places.
Water disinfection [ edit ] Ultraviolet disinfection of water is a purely physical, chemical-free process. Even parasites such as Cryptosporidium or Giardia, which are extremely resistant to chemical disinfectants, are efficiently reduced. UV can also be used to remove chlorine and chloramine species from water; this process is called photolysis, and requires a higher dose than normal disinfection. The dead microorganisms are not removed from the water. UV disinfection does not remove dissolved organics, inorganic compounds or particles in the water.[104] The world's largest water disinfection plant treats drinking water for New York City. The Catskill-Delaware Water Ultraviolet Disinfection Facility, commissioned on 8 October 2013, incorporates a total of 56 energy-efficient UV reactors treating up to 2.2 billion U.S. gallons (8.3 billion liters) a day.[105][106]
With competitive price and timely delivery, UVDF sincerely hope to be your supplier and partner. Ultraviolet can also be combined with ozone or hydrogen peroxide to produce hydroxyl radicals to break down trace contaminants through an advanced oxidation process.
It used to be thought that UV disinfection was more effective for bacteria and viruses, which have more-exposed genetic material, than for larger pathogens that have outer coatings or that form cyst states (e.g., Giardia) that shield their DNA from UV light. However, it was recently discovered that ultraviolet radiation can be somewhat effective for treating the microorganism Cryptosporidium. The findings resulted in the use of UV radiation as a viable method to treat drinking water. Giardia in turn has been shown to be very susceptible to UV-C when the tests were based on infectivity rather than excystation.[107] It has been found that protists are able to survive high UV-C doses but are sterilized at low doses.
Developing countries [ edit ] A 2006 project at University of California, Berkeley produced a design for inexpensive water disinfection in resource deprived settings.[108] The project was designed to produce an open source design that could be adapted to meet local conditions. In a somewhat similar proposal in 2014, Australian students designed a system using potato chip (crisp) packet foil to reflect solar UV radiation into a glass tube that disinfects water without power.[109]
Wastewater treatment [ edit ] Ultraviolet in sewage treatment is commonly replacing chlorination. This is in large part because of concerns that reaction of the chlorine with organic compounds in the waste water stream could synthesize potentially toxic and long lasting chlorinated organics and also because of the environmental risks of storing chlorine gas or chlorine containing chemicals. Individual wastestreams to be treated by UVGI must be tested to ensure that the method will be effective due to potential interferences such as suspended solids, dyes, or other substances that may block or absorb the UV radiation. According to the World Health Organization, "UV units to treat small batches (1 to several liters) or low flows (1 to several liters per minute) of water at the community level are estimated to have costs of US$20 per megaliter, including the cost of electricity and consumables and the annualized capital cost of the unit."[110]
Large-scale urban UV wastewater treatment is performed in cities such as Edmonton, Alberta. The use of ultraviolet light has now become standard practice in most municipal wastewater treatment processes. Effluent is now starting to be recognized as a valuable resource, not a problem that needs to be dumped. Many wastewater facilities are being renamed as water reclamation facilities, whether the wastewater is discharged into a river, used to irrigate crops, or injected into an aquifer for later recovery. Ultraviolet light is now being used to ensure water is free from harmful organisms.
Aquarium and pond [ edit ] Ultraviolet sterilizers are often used to help control unwanted microorganisms in aquaria and ponds. UV irradiation ensures that pathogens cannot reproduce, thus decreasing the likelihood of a disease outbreak in an aquarium.
Aquarium and pond sterilizers are typically small, with fittings for tubing that allows the water to flow through the sterilizer on its way from a separate external filter or water pump. Within the sterilizer, water flows as close as possible to the ultraviolet light source. Water pre-filtration is critical as water turbidity lowers UV-C penetration.
Many of the better UV sterilizers have long dwell times and limit the space between the UV-C source and the inside wall of the UV sterilizer device.[111][third-party source needed]
Laboratory hygiene [ edit ] UVGI is often used to disinfect equipment such as safety goggles, instruments, pipettors, and other devices. Lab personnel also disinfect glassware and plasticware this way. Microbiology laboratories use UVGI to disinfect surfaces inside biological safety cabinets ("hoods") between uses.
Food and beverage protection [ edit ] Since the U.S. Food and Drug Administration issued a rule in 2001 requiring that virtually all fruit and vegetable juice producers follow HACCP controls, and mandating a 5-log reduction in pathogens, UVGI has seen some use in sterilization of juices such as fresh-pressed.
Technology [ edit ] Lamps [ edit ] Germicidal UV for disinfection is most typically generated by a mercury-vapor lamp. Low-pressure mercury vapor has a strong emission line at 254 nm, which is within the range of wavelengths that demonstrate strong disinfection effect. The optimal wavelengths for disinfection are close to 260 nm.[65]: 2–6, 2–14
Mercury vapor lamps may be categorized as either low-pressure (including amalgam) or medium-pressure lamps. Low-pressure UV lamps offer high efficiencies (approx. 35% UV-C) but lower power, typically 1 W/cm power density (power per unit of arc length). Amalgam UV lamps utilize an amalgam to control mercury pressure to allow operation at a somewhat higher temperature and power density. They operate at higher temperatures and have a lifetime of up to 16,000 hours. Their efficiency is slightly lower than that of traditional low-pressure lamps (approx. 33% UV-C output), and power density is approximately 2–3 W/cm3. Medium-pressure UV lamps operate at much higher temperatures, up to about 800 degrees Celsius, and have a polychromatic output spectrum and a high radiation output but lower UV-C efficiency of 10% or less. Typical power density is 30 W/cm3 or greater.
Depending on the quartz glass used for the lamp body, low-pressure and amalgam UV emit radiation at 254 nm and also at 185 nm, which has chemical effects. UV radiation at 185 nm is used to generate ozone.
The UV lamps for water treatment consist of specialized low-pressure mercury-vapor lamps that produce ultraviolet radiation at 254 nm, or medium-pressure UV lamps that produce a polychromatic output from 200 nm to visible and infrared energy. The UV lamp never contacts the water; it is either housed in a quartz glass sleeve inside the water chamber or mounted externally to the water, which flows through the transparent UV tube. Water passing through the flow chamber is exposed to UV rays, which are absorbed by suspended solids, such as microorganisms and dirt, in the stream.[112]
Light emitting diodes (LEDs) [ edit ] Recent developments in LED technology have led to commercially available UV-C LEDs. UV-C LEDs use semiconductors to emit light between 255 nm and 280 nm.[70] The wavelength emission is tuneable by adjusting the material of the semiconductor. As of 2019 , the electrical-to-UV-C conversion efficiency of LEDs was lower than that of mercury lamps. The reduced size of LEDs opens up options for small reactor systems allowing for point-of-use applications and integration into medical devices.[113] Low power consumption of semiconductors introduce UV disinfection systems that utilized small solar cells in remote or Third World applications.[113]
UV-C LEDs don't necessarily last longer than traditional germicidal lamps in terms of hours used, instead having more-variable engineering characteristics and better tolerance for short-term operation. A UV-C LED can achieve a longer installed time than a traditional germicidal lamp in intermittent use. Likewise, LED degradation increases with heat, while filament and HID lamp output wavelength is dependent on temperature, so engineers can design LEDs of a particular size and cost to have a higher output and faster degradation or a lower output and slower decline over time.
Water treatment systems [ edit ] Sizing of a UV system is affected by three variables: flow rate, lamp power, and UV transmittance in the water. Manufacturers typically developed sophisticated computational fluid dynamics (CFD) models validated with bioassay testing. This involves testing the UV reactor's disinfection performance with either MS2 or T1 bacteriophages at various flow rates, UV transmittance, and power levels in order to develop a regression model for system sizing. For example, this is a requirement for all public water systems in the United States per the EPA UV manual.[65]: 5–2
The flow profile is produced from the chamber geometry, flow rate, and particular turbulence model selected. The radiation profile is developed from inputs such as water quality, lamp type (power, germicidal efficiency, spectral output, arc length), and the transmittance and dimension of the quartz sleeve. Proprietary CFD software simulates both the flow and radiation profiles. Once the 3D model of the chamber is built, it is populated with a grid or mesh that comprises thousands of small cubes.
Points of interest—such as at a bend, on the quartz sleeve surface, or around the wiper mechanism—use a higher resolution mesh, whilst other areas within the reactor use a coarse mesh. Once the mesh is produced, hundreds of thousands of virtual particles are "fired" through the chamber. Each particle has several variables of interest associated with it, and the particles are "harvested" after the reactor. Discrete phase modeling produces delivered dose, head loss, and other chamber-specific parameters.
Reduction Equivalent Dose [ edit ] When the modeling phase is complete, selected systems are validated using a professional third party to provide oversight and to determine how closely the model is able to predict the reality of system performance. System validation uses non-pathogenic surrogates such as MS 2 phage or Bacillus subtilis to determine the Reduction Equivalent Dose (RED) ability of the reactors. Most systems are validated to deliver 40 mJ/cm2 within an envelope of flow and transmittance.[114]
To validate effectiveness in drinking water systems, the method described in the EPA UV guidance manual is typically used by US water utilities, whilst Europe has adopted Germany's DVGW 294 standard. For wastewater systems, the NWRI/AwwaRF Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse protocols are typically used, especially in wastewater reuse applications.[115]
See also [ edit ] References [ edit ]
UVC LED emitting 265 nm compared to E. coli germicidal effectiveness curve.[67]
Public health concerns such as multi- and extensive drug-resistant tuberculosis, bioterrorism, pandemic influenza, and severe acute respiratory syndrome have intensified efforts to prevent transmission of infections that are completely or partially airborne using environmental controls. One such control, ultraviolet germicidal irradiation (UVGI), has received renewed interest after decades of underutilization and neglect. With renewed interest, however, come renewed questions, especially regarding efficacy and safety. There is a long history of investigations concluding that, if used properly, UVGI can be safe and highly effective in disinfecting the air, thereby preventing transmission of a variety of airborne infections. Despite this long history, many infection control professionals are not familiar with the history of UVGI and how it has, and has not, been used safely and effectively. This article reviews that history of UVGI for air disinfection, starting with its biological basis, moving to its application in the real world, and ending with its current status.
The history of UVGI air disinfection has been one of promise, disappointment, and rebirth. Investigations of the bactericidal effect of sunlight in the late 19th century planted the seed of air disinfection by UV radiation. First to nurture this seed was William F. Wells, who both discovered the spread of airborne infection by droplet nuclei and demonstrated the ability of UVGI to prevent such spread. Despite early successes in applying UVGI, its use would soon wane due to a variety of reasons that will be discussed in this article. However, with the enduring research of Riley and others, and an increase in tuberculosis (TB) during the 1980s, interest in UVGI was revitalized. With modern concerns regarding multi- and extensive drug-resistant TB, bioterrorism, influenza pandemics, and severe acute respiratory syndrome, interest in UVGI continues to grow. Research is ongoing, and there is much evidence on the efficacy of UVGI and the proper way to use it, though the technology has yet to fully mature. provides an overview of some of the key studies in the history of UVGI air disinfection.
In-duct UVGI is the other primary application of UVGI air disinfection. Designed to disinfect air as it passes through the HVAC system and before it is recirculated or exhausted, in-duct UVGI irradiates the entire cross-section of a duct at high intensities not accessible to room occupants, and may include the use of highly UV-reflective materials to further increase irradiance levels. Effective room air disinfection depends on circulating maximal room air through the duct and the velocity at which it is circulated. Also, though not designed to disinfect the air in any direct way, UVGI is used to disinfect surfaces inside HVAC systems, such as cooling coils and drip pans. Disinfecting these surfaces may reduce the maintenance requirements for HVAC systems, and it has been suggested that it could also reduce nonspecific building-related illnesses. 1 , 2
Upper-room UVGI is one of two primary applications of UVGI air disinfection. Designed for use in occupied rooms without using protective clothing, upper-room UVGI uses wall-mounted and ceiling-suspended, louvered/shielded UVGI fixtures to confine the germicidal radiation to the entire room area above people's heads and greatly minimizes exposure to occupants in the lower room. Effective air disinfection in the breathing zone then depends on good vertical air movement between the upper and lower room, which can be generated naturally by convection, the HVAC system, or low-velocity paddle fans where needed.
Ultraviolet germicidal irradiation (UVGI) is an established means of disinfection and can be used to prevent the spread of certain infectious diseases. Low-pressure mercury (Hg) discharge lamps are commonly used in UVGI applications and emit shortwave ultraviolet-C (UV-C, 100–280 nanometer [nm]) radiation, primarily at 254 nm. UV-C radiation kills or inactivates microbes by damaging their deoxyribonucleic acid (DNA). The principal mode of inactivation occurs when the absorption of a photon forms pyrimidine dimers between adjacent thymine bases and renders the microbe incapable of replicating. UVGI can be used to disinfect air, water, and surfaces, although surface disinfection is limited by microshadows and absorptive protective layers. Water disinfection is currently the most advanced and accepted germicidal application. Air disinfection is accomplished via several methods: irradiating the upper-room air only, irradiating the full room (when the room is not occupied or protective clothing is worn), and irradiating air as it passes through enclosed air-circulation and heating, ventilation, and air-conditioning (HVAC) systems. UVGI is also used in self-contained room air disinfection units.
The distinction should be made between the biological effect and the penetration depth of UV radiation, a key concept in UVGI safety. UV-C wavelengths are the most biologically active radiation and, ironically, much less dangerous to humans. This is because UV-C radiation is absorbed by the outer dead layer of human skin, while UV-B and UV-A radiation penetrate deeper. 39 While attention to UVGI safety is important, because overexposure to 254 nm radiation can readily cause erythema (“sunburn”) to the skin and photokeratitis (“welder's flash”) to the eyes, the long-term health risks are considered to be negligible compared with common solar UV exposures.
Hollaender and associates, 32 – 34 among others, picked up where Gates left off, and by 1944, Hollaender and Oliphant claimed, “It is quite possible that the high sensitivity of many agents at about [260 nm] is based on the important function desoxyribose nucleic acid plays in biological activities.” 35 Beukers and Berends 36 exposed frozen solutions of thymine to UV-C radiation in 1960, resulting in the formation of thymine dimers. Shortly thereafter, the production of dimers from adjacent pyrimidines was demonstrated after exposure to UV radiation, accounting for “a large part of the effects of ultraviolet radiation on biological systems.” 37 The biological foundation of UVGI had been laid. For a more extensive review on the history of the biological effects of UV radiation on microorganisms, see Hockberger 7 , 38 and Coohill. 29
d The modern action spectra are derived from the Deutsches Institut für Normung (represented with a solid line) and the Illuminating Engineering Society of North America (represented with a dotted line). The relative output of a low-pressure Hg germicidal lamp is overlaid on these action spectra to illustrate why the lamps are highly efficient in germicidal applications.
Henri and Henri were the first to show the mutagenic effects of UV radiation. In 1914, they observed modification of the metabolism of Bacillus anthracis upon exposure to sublethal doses of UV radiation. 24 In 1929 and 1930, Gates published a series of articles providing the first analytical bactericidal action spectrum. 25 – 27 Using an Hg arc lamp, Gates produced similarly shaped action spectra for Staphylococcus aureus and Bacillus coli (B. coli), both with peak effectiveness at 265 nm. These action spectra corresponded to the absorption spectrum of nucleic acids, and Gates hinted that his data “… point the way in a further search for the specific substance, or substances, involved in the lethal reaction,” suggesting that nucleic acids may be the genetic material and responsible for cell death—not proteins, as was a common belief 28 at the time. In his article on UV action spectroscopy, Coohill expressed that Gates' bactericidal action spectrum was “… considered by some to be the most crucial action spectrum ever published.” 29 Gates' findings were supported by Ehrismann and Noethling in 1932; 30 in 1935, the Commission Internationale de l'Eclairage (CIE) 31 examined early data and proposed an official bactericidal action spectrum. Gates' historical bactericidal action spectrum for B. coli is plotted in , along with two modern germicidal action spectra and the relative output of a germicidal lamp.
Between 1901 and 1903, Bang reported different sensitivities of Bacillus prodigiosus to UV radiation, with UV-B and UV-C radiation more effective than UV-A radiation. 19 , 20 Employing a prism and different arc lamps, a peak bactericidal effectiveness between 226.5 nm and 328.7 nm was confirmed by Barnard and Morgan. 21 Hertel was the first to provide a thorough quantitative analysis of the effect of light on microorganisms. Between 1904 and 1905, Hertel used a prism and thermoelectric measurement technique to quantify the relative intensity of radiation emitted from arc lamps, varying as a function of wavelength. With these data, Hertel established the degree of germicidal effectiveness between the UV and visible spectral regions. The region of greatest effectiveness was found to be the UV-C, followed by UV-B, UV-A, and visible radiation, respectively, with the dose required for cell death increasing by orders of magnitude in the visible region. 22 , 23
In 1885, Duclaux reported differences in sensitivity to sunlight between different species of bacteria spores. 10 – 12 This finding pointed to another key factor that influences UVGI performance—microbial sensitivity. Different microbes have different sensitivities to UVGI and require varying doses of radiation for the same fraction of inactivation. Many later studies would attempt to quantify the UVGI sensitivity for numerous types of microorganisms. In 1890, Koch demonstrated the lethal effect of sunlight on tubercle bacillus, portending the modern use of UVGI to combat TB infection. 13 Two years later, using a prism, a heliostat, and quartz test tubes, Geisler showed that UV radiation from sunlight and electric lamps was more effective in killing bacteria than longer wavelength radiation; however, he also noted that the lethal effects of longer wavelength radiation were amplified at increased intensities. 14 Buchner dismissed contributions from infrared radiation on the germicidal action of sunlight by passing sunlight through an infrared-absorbing water filter before it reached a bacterial sample. 15 Ward improved upon these results between 1892 and 1894, demonstrating the violet-blue and UV-A portions of the solar spectrum to be the most deleterious to bacteria. 16 – 18
These early investigations pointed toward some key factors (to be later investigated in-depth) that influence UVGI performance. Inactivation of a given fraction of organisms is dependent on the dose of radiation received. Dose (J●m −2 ) is the product of intensity (W●m −2 ) and exposure duration (s). Inactivation is also dependent on the wavelength of received radiation. Much of the work following these initial investigations was devoted to finding the wavelength dependence of the germicidal action of light, with investigations into the following wavelength ranges: UV-C (100–280 nm), UV-B (280–315 nm), UV-A (315–400 nm), visible (400–700 nm), and infrared (700–10 6 nm).
As early as 1845, it was known that microorganisms respond to light. 3 A breakthrough came in 1877, when Downes and Blunt 4 – 6 observed that exposing test tubes containing Pasteur's solution to sunlight prevented the growth of microorganisms inside the tube and, upon increased exposure durations, the test tubes remained bacteria-free for several months. In his 2002 article on the history of UV photobiology, Hockberger called this “one of the most influential discoveries in all of photobiology.” 7 Downes and Blunt went on to demonstrate that the ability of sunlight to neutralize bacteria was dependent on intensity, duration, and wavelength, with the shorter wavelengths of the solar spectrum being the most effective. Tyndall later confirmed these results. 8 , 9
William F. Wells pioneered both the concept of airborne infection by droplet nuclei and the use of UVGI to disinfect the air. In 1933, Wells presented the idea that various-sized droplets containing infectious organisms are expelled into the air and quickly dried by evaporation after an infectious person coughs or sneezes,40 expanding upon an early droplet theory put forth by Flüugge.41 These evaporated droplets, or droplet nuclei, can remain in the air for extended periods of time, and people can breathe them in. The idea of infection via droplet nuclei had been sparked by investigations into respiratory infections associated with dust-suppressive water sprays used in New England textile mills.
While the ability of UV radiation to inactivate microorganisms was known, previous studies had exposed microorganisms on solid media or in liquids, not in the air. In 1935, using aerosolized B. coli, 254 nm radiation, and carefully controlled conditions, Wells went on to demonstrate that airborne infectious organisms could be effectively killed in a short period of time.42 The use of UVGI not only inactivated the infectious organisms in the air, but proved the very concept that infections can be spread via the airborne route. Sharp was the first to confirm these results and documented an example of airstream disinfection, foreshadowing the use of UVGI in in-duct HVAC systems.43,44 These initial investigations would provide the framework and impetus for infection control by the irradiation of air.
Immediately perceiving the potential of UVGI, Hart employed direct, high-intensity UVGI for the disinfection of hospital operating room air at the Duke University Hospital in 1936, after traditional methods had failed.45 The setup was designed to irradiate the entire room, with special emphasis on highly irradiating the volume around the surgical site and instrument/supply tables. Hart later reported the reduction in the postoperative wound infection rate in clean cases from 11.62% without the use of UVGI to 0.24% with the use of UVGI.46 Following Hart's lead, colleagues from Duke and other hospitals installed UVGI in their operating rooms and reported similar success.47–51
Following initial successes in the operating room, the application of UVGI in hospitals was soon extended to infant wards by implementing various configurations of cubicle-like UVGI “light curtains” designed to prevent respiratory cross-infections. As in the operating room, high-intensity, direct UVGI was used, assuming that human exposure would be transient in passing through. In 1936, Wells and colleagues designed such UVGI barriers for Charles McKhann at the Infants' and Children's Hospital in Boston. In 1941, Del Mundo and McKhann reported a difference in the infection rate of 12.5% in a control ward and 2.7% in a ward with UVGI barriers.52 Parallel studies evaluating UVGI barriers reported successes similar to that in Boston, including both the reduction of respiratory cross-infections and the reduction of cross-cubicle spread of aerosolized test organisms.53–58
Modifying the original experimental design, other studies of cross-infection in infant wards employed upper-room UVGI instead of light curtains. As discussed previously, upper-room UVGI confines the germicidal radiation to the entire room area above people's heads, and effective air disinfection in the lower room then depends on good vertical air movement between the upper and lower room. Robertson et al. reported nearly one-half the number of infections using only upper-room UVGI in rooms where natural ventilation was impeded; no additional effect from UVGI was found in rooms where doors and windows were left open.57 Several other investigators produced further positive results using upper-room UVGI to prevent cross-infections.58–60
Between 1937 and 1941, Wells successfully used upper-room UVGI to prevent the epidemic spread of measles among children in suburban Philadelphia day schools, where infection outside of school was unlikely—a classic experiment that has been difficult to reproduce. During this study, 53.6% of susceptibles in unirradiated schools were infected, while only 13.3% of susceptibles in irradiated schools were infected (excluding secondary infections from siblings), even with the irradiated schools having a greater percentage of susceptibles.61 These results were supported upon investigation of measles attack rates in other nearby unirradiated schools.62
In 1943, the Council on Physical Therapy accepted UVGI for disinfecting purposes.63 From 1941 to 1943, Lurie exposed two sets of rabbits to air originating from rabbits infected with TB. With sufficient germicidal intensity, none of the rabbits receiving irradiated air developed TB, while the majority of the rabbits receiving non-irradiated air did.64 Beginning in 1943, studies were undertaken to evaluate the ability of upper-room UVGI (the floor was later irradiated also) to prevent respiratory infections in the intermittent aggregations at naval training stations. These studies produced modest success, limited by less-than-ideal experimental designs.65–69
Early investigations by Whisler,70 Wells,71–74 and -others75 evaluated the effect of physical and environmental factors on UVGI efficacy, including humidity and air circulation—two important factors in the performance of UVGI. Microbes were found to be significantly more resistant to UVGI at higher humidity. Luckily, the humidity of most buildings is kept well below adverse levels to provide occupant comfort. Also, as discussed previously, good air circulation is requisite for effective upper-room UVGI. Infected lower-room air must circulate through the irradiated upper room, where inactivation depends on the received dose (the intensity of radiation in the upper room multiplied by how long the microbe remains in the irradiated zone). Air circulation is also an important factor in in-duct UVGI, which requires maximal room air circulation through the duct and is dependent on the velocity of air moving through the duct.
Throughout the 1940s, extensive work by Luckiesh and colleagues provided further evidence for the efficacy of UVGI, while also detailing early designs and guidelines for UVGI air disinfection systems and applications of UVGI.75 This work represented a high water mark in the technical knowledge and expertise of UVGI. The effectiveness of UVGI to disinfect exhaust air in infectious disease laboratories was also demonstrated, including the first use inside an air conditioner.76, 77
In 1955, Wells published the authoritative Air Contagion and Air Hygiene,62 deemed a “landmark monograph on air hygiene” by Edward Nardell.78 Six years later, Riley followed with his Airborne Infection: Transmission and Control.79 These two works may be consulted for greater detail in the early studies using UVGI and all other aspects of airborne infection.
Beginning in the 1930s as a Harvard medical student working in Wells' lab, Richard L. Riley became a disciple of and collaborator with Wells and his work on airborne infection and UVGI. In fact, Wells shared credit with Riley for the droplet nuclei concept. Riley and colleagues conducted two two-year experiments in a Veterans Hospital TB ward during the 1950s and early 1960s. In a preliminary study80 without patients, Escherrischia coli (E. coli) and bovine tubercle bacilli were separately aerosolized into the ward ventilation system with and without UVGI. UVGI effectively inactivated E. coli in the ward and prevented rabbits from developing TB. Conversely, exposed rabbits were infected with TB without the use of UVGI.
In subsequent studies, the TB ward was continually occupied with six infected patients and sealed from the rest of the hospital. The room air was exhausted through ventilation ducts to control chambers housing colonies of guinea pigs; one chamber received air from an irrFadiated duct and one received air from a non-irradiated duct. This method eliminated contagion via means other than through the exhausted air. The results of these studies confirmed both that TB could readily be spread through droplet nuclei and that UVGI could sufficiently inactivate the infected air (100% in the study).81,82 Riley also used the experiments to estimate the concentration of infectious droplet nuclei in the air and study the variability in the infectiousness of different patients. Around the same time, McLean prevented the spread of influenza in Veterans Hospital TB patients using upper-room UVGI during the 1957 pandemic, providing evidence for the airborne transmission of influenza. The infection rate was only 1.9% in an irradiated ward, while it was 18.9% in a non-irradiated ward.83
During the early 1970s, Riley and colleagues published a series of articles detailing the results of using upper-room UVGI in a model room aerosolized with Serratia marcescens. The effects on disinfection rates in the lower room from air mixing via convection and a ceiling fan were studied and mathematically modeled.84–86 It was shown that temperature gradients and ceiling fans could greatly affect air mixing in a room and, thus, the rate of disinfection in the lower room. By supplying air cooler than the lower-room air to the upper room and/or using a ceiling fan, the efficiency of UVGI in disinfecting the lower room was greatly increased. The ability to prevent the spread of infectious organisms throughout a building by placing UVGI in corridors was also demonstrated.87
Additionally, Riley et al. investigated the effect of relative humidity (RH) on the efficacy of UVGI, with a sharp decline found in the fraction of organisms killed at RH values higher than 60% to 70%.88 In 1972, Kethley and Branch conducted model room studies similar to Riley's and studied the effect of aerosol size and sampling location within a mechanically ventilated room. They found smaller particles to be more susceptible to UVGI, and discovered that different sampling locations produced different calculated disinfection rates. This led to the conclusion that lamp locations and air movement patterns within a room need to be considered for optimal disinfection.89
During 1975, Riley et al. found virulent tubercle bacilli and Bacillus Calmette-Guéerin (BCG) to be equally susceptible to UVGI. They then aerosolized BCG into an approximately 200-square-foot model room and measured its disappearance with and without upper-room UVGI, finding a sixfold increase in the disappearance rate using one 17-watt (electrical) fixture, and a ninefold increase using two fixtures of a combined 46 watts (electrical). It was inferred that the results using BCG were directly applicable to virulent tubercle bacilli.90 Riley also equated these results to the removal of contaminated air using ventilation, where one air change (AC) corresponds to a volume of fresh air entering (and contaminated air leaving) a room equal to the volume of the room. One AC equates to removing about 63% of contaminated air in a perfectly mixed room. Using this concept of air changes via ventilation, Riley expressed his results using UVGI in equivalent air changes (Eq AC). One Eq AC corresponds to inactivating about 63% of airborne microorganisms with UVGI in a perfectly mixed room. In his experiment, Riley calculated an increase of 10 and 25–33 AC/hour using the 17-watt and combined 46-watt upper-room UVGI fixtures, respectively. shows Riley's measured disappearance of BCG in the model room with and without the 17-watt upper-room UVGI fixture. This quantitatively illustrated the potential of upper-room UVGI to prevent TB transmission. For decades to follow, these results led to the following rule of thumb: 17 watts (electrical) input to UVGI lamps per 200 square feet of floor area. It was hoped that by following this guideline, similar air disinfection rates would be achieved.
Open in a separate windowaOne AC corresponds to adding a volume of fresh air to the room equal to the volume of the room and equates to a removal of about 63% of airborne organisms in a perfectly mixed room. One AC produced with UVGI represents an equivalent AC and equates to inactivating about 63% of airborne organisms with UVGI in a perfectly mixed room.
bAdapted from: Riley RL, Knight M, Middlebrook G. Ultraviolet susceptibility of BCG and virulent tubercle bacilli. Am Rev Respir Dis 1976;113:413-8.
UVGI = ultraviolet germicidal irradiation
UV = ultraviolet
AC = air change
W = WaH
Despite the early successes in demonstrating the effectiveness of UVGI, the technology was largely abandoned and forgotten in the years following Wells' promising work.78,91 There are several reasons why this occurred. The inability to reproduce the success of Wells in preventing the spread of measles,92–95 along with other failures,96–100 engendered broad disillusionment with UVGI. Around the same time, antibiotics were developed to treat TB, and there was hope that common viral illnesses could be controlled by immunization. Additionally, there was concern regarding the health effects from UV-C exposure and the production of ozone by germicidal lamps. Concerns that UVGI required high maintenance, that UVGI would be ineffective at higher humidity, and that its germicidal efficacy was unproven also contributed to UVGI's second-class status among air disinfection strategies.
It is now known, through successes and failures, where and how UVGI can be effective.78 UVGI is most effective in preventing infections spread chiefly by droplet nuclei, not by direct contact or larger respiratory droplets, although some surface decontamination likely occurs. Also, the location(s) where UVGI is employed must also be the primary location(s) of disease transmission (i.e., there cannot be a high risk of acquiring the same infection outside the location where UVGI is used). From these criteria, the cause of previous UVGI failures can be deduced. The failure to prevent the spread of measles in schools can be explained by infections occurring outside the classroom (e.g., on school buses or through other extracurricular interaction).62,79 Wells successfully prevented the spread of measles in schools because infection occurring outside the school in a wealthy Philadelphia suburb was unlikely.
In the late 1980s, there was a renewed interest in UVGI due to the unexpected rise in TB in 1985 and the emergence of multiple drug-resistant strains, with specific concerns about the homeless, those infected with human immunodeficiency virus (HIV), and those who work with infected populations.101–103 It was then argued that UVGI, along with other measures, could be used to control the transmission of TB.104–109 Though the potential application of UVGI in locations such as hospitals and shelters was recognized, new challenges were also presented. Low ceiling heights and the lack of technical expertise, standards and regulations, and clinical trials all had to be addressed.
Since then, ongoing efforts toward meeting these new challenges have included: aerosol chamber and model room studies110–121 evaluating various environmental and physical factors on UVGI efficacy (e.g., air mixing and ventilation, humidity, microbial sensitivity, fixture irradiance and configuration, and photoreactivation); the mathematical modeling and predicting of UVGI fixture irradiances122–124 and room and duct disinfection/infection rates,123,125–133 including the use of computational fluid dynamics;134– 138 and applying UVGI in real-world studies.139–141 Other efforts have been directed toward establishing the maintenance requirements142 for UVGI fixtures, developing methods of accurate UVGI measurement,122,143–145 and evaluating the safety146,147 of UVGI installations, including the development of more modern “ozone-free” lamps. In 2003, the CIE148 published a technical report on UVGI air disinfection, summarizing the present state of knowledge. At press time, a CIE committee was preparing a report on the risk of photocarcinogenesis from UVGI lamps, including a comparison of the relative risk compared with typical UV-B and UV-A exposures from outdoor sunlight. Additional research has continued to evaluate the use of UVGI in the operating room to reduce postoperative infections.149,150
The Tuberculosis Ultraviolet Shelter Study (TUSS), the first real-world study on the use of UVGI to prevent TB, was conducted from 1997 to 2004.151 TUSS was a double-blind, placebo-controlled field trial that evaluated the use of upper-room UVGI at 14 homeless shelters in six U.S. cities. The results from TUSS were inconclusive due to insufficient numbers of documented TB skin test conversions (i.e., the rise in TB had already been checked); however, much practical experience and other data were gained from the study.147
Preliminary guidelines have also been -published,152–154 and, in 2005, the Centers for Disease Control and Prevention (CDC)155 expanded on its previous recommendation156 that UVGI be used as a supplement for TB infection control in health-care settings. In 2009, building upon initial guidelines and evaluating the influx of new research, CDC produced the first comprehensive guidance document for using upper-room UVGI to control TB in health-care settings.157
In 2009, Escombe and colleagues published the first clinical trial using upper-room UVGI to prevent TB transmission.158 Similar to Riley's classic studies in the 1950s, this study ventilated air from a continually occupied HIV-TB ward in Lima, Peru, to guinea pig colonies housed in rooftop chambers for 535 consecutive days. On alternating UV-on and -off days, one group of guinea pigs breathed air from the TB ward with upper-room UVGI and a mixing fan turned on, and a separate control group of guinea pigs breathed air from the TB ward with upper-room UVGI turned off. Further, air was drawn from the lower room without deliberately passing it through the UV field, simulating air breathed by occupants. Results showed a 34.9% infection rate in the control group and a reduced rate of 9.5% in the group with UVGI. TB disease was subsequently confirmed in 8.6% of the control group compared with 3.6% of the group with UVGI ( ). It should also be noted that the mean RH during the study was about 77.0%, determined by previous studies to be above the maximum level for optimal UVGI efficacy.
Open in a separate windowaOne group was exposed when upper-room UVGI was turned on in the TB ward, and a control group was exposed when upper-room UVGI was turned off in the TB ward.
bAdapted from: Escombe AR, Moore DAJ, Gilman RH, Navincopa M, Ticona E, Mitchell B, et al. Upper-room ultraviolet light and negative air ionization to prevent tuberculosis transmission. PLoS Med 2009;6:e43.
TB = tuberculosis
UV = ultraviolet
UVGI = ultraviolet germicidal irradiation
At press time, Nardell and colleagues were completing a clinical trial using upper-room UVGI to prevent TB transmission similar to that of Escombe et al. (Personal communication, Edward Nardell, Harvard School of Public Health, October 2008). Also at the time of publication, Noakes and colleagues planned to develop a design tool and guidance documents to assist architects and engineers in designing effective and safe UVGI installations in real-world hospital environments (Personal communication, Catherine Noakes, Pathogen Control Engineering Research Group, School of Civil Engineering, University of Leeds, March 2009). Additionally, an interdisciplinary computer-assisted design lighting project promises to help engineers and architects design UVGI installations in a variety of settings (Personal communication, Edward Nardell, Harvard School of Public Health, October 2008). Together, these efforts will contribute even more valuable information, experience, and guidance for the use of upper-room UVGI to prevent airborne infection.
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