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The germicidal effects of ultra violet (UV) radiation have been known and researched since the late 19th century. In the early part of the 20th century, at least one medical equipment manufacturer (Westinghouse) was producing a sterilizing unit for hospital operating rooms which capitalized on the sterilization properties of ultra violet light.

UV for sterilization fell into disfavor by the early 1900’s with the development of chemical sterilization techniques using chlorine, ozone, peroxides, etc. Within the last few decades, however, UV has seen resurgence based on economics and from its lack of toxic chemical byproducts.

In fact, enough interest in the use of UV for a wide variety of applications has resulted in the formation of the International Ultra Violet Association, which was founded in 1999. In addition, the Environmental Protection Agency has decided to include UV disinfection in forthcoming US drinking water regulations.

The Physics of UV Disinfection

Ultra violet radiation consists of high energy photons which occupy the 200 to 400 nanometer (2000 to 400 Angstroms) wavelengths of the electromagnetic spectrum. (See Figure 1) UV light sits just below soft X-ray radiation and just above visible light. UV energy does not directly kill a pathogen, but rather causes a photochemical reaction within its genetic structure which inhibits the organism’s ability to reproduce. (The same phenomena human skin cells to burn and die...not so good if you are a single cell organism.)

The amount of energy delivered by a photon is inversely proportional to its wavelength. Planck’s Law of Radiation provides us with a simple mathematical expression to determine the amount of energy of a given wavelength:

E = h * c/l

Where:

E = -amount of energy in Joules (1 Watt-second)

H = -Planck’s Constant (6.6261 x 10-34 m2 kg/sec)

C = -the speed of light (299792458 meters/sec)

l = -wavelength (meters)

Intuitively, as the amount of energy increases, the germicidal effectivity would also increase. Empirical analysis of the survivability of various pathogens over the UV spectrum have concluded that a wavelength of 253.7 nanometers to be the most effective.

Many types of germicidal lamps are available today, but the most common is a Type UV-C, low pressure mercury arc lamp available in tubes that range from a few inches to 60 inches in length, similar in appearance to a common fluorescent light. A UV-C lamp produces UV light in the range of 200 to 280 nanometers.

Additional empirical work has been conducted to determine the amount of exposure time and UV intensity required to kill various pathogens. Table 1, (available from numerous sources) describes the product of intensity and exposure time required to kill specific pathogens using a standard UV-C source producing 253.7 nm wavelength output.

Typical Uses

It is not uncommon to see ultra violet germicidal irradiation (UVGI) being used in research and laboratory animal facilities today. Most obvious are the quiet blue lights standing guard at various doors and entry ways. These UVGI lamps are constantly sterilizing the air and wall surfaces at entry points where other methods are impractical.

UVGI may also be found in water treatment and purification systems. At least one supplier of automatic watering systems for research animals provides a UVGI component. These components are effective supplementary tools to filtration and reverse osmosis systems which address other elements of water quality like particulates, organics, and metals.

UVGI of laboratory water sources may be considered a preferable method over chemical based techniques like chlorination and acidification. UVGI does not alter the physical properties of the water in taste and odor, and there are no residual chemicals that might interfere or taint downstream research results.

UV is a common method of maintaining water quality for aquatics. A UV sterilization component, with particulate filtration provides a very effective means of providing a clean environment for research aquatics like Xenopus and Zebrafish.

UVGI may be found hidden within the HVAC systems supplying air to the research facility. The need for sterilized air is increasingly important in areas of dense populations where communicable diseases are readily transferred via “communal” air. The ability to control the formation of toxic molds in regions of high humidity has also become a factor in the use of UV in HVAC systems. In the laboratory setting, where communicable pathogens are specifically maintained for study, UV becomes a critical component in a network of pathogen control systems. Currently, CDC guidelines recommend use of UV only in conjunction with HEPA filtration and high rates of purge.

New and Future Uses for UVGI

Earlier it was noted that the use of UV has seen resurgence as a clean and economical method for sterilization. Lab animal managers and facility planners should consider looking outside the vivarium for practical applications of UVGI that may have corollary application in the research facility.

As an example, at least one new facility has looked to the application of UV in the food service industry for methods of sterilization of inbound feed and bedding inventories. Traditional methods of cleaning and sanitizing inbound bags of feed, bedding, and animal shipping cartons have relied on wet chemical treatment. Typically, a technician is required to mist down incoming bags or boxes with a hand sprayer containing a dilute chlorine or alcohol solution. However, two manufacturers are now producing automated “spray boxes” with integrated misting manifolds and automated conveying systems. (Figure 2 represents a large format, built-in disinfecting unit.) While effective, the need to handle and dispose of hazardous chemicals still exists.

A similar issue of sanitization exists in the food preparation and service industry. Outbound meats and raw vegetables will have longer shelf life when sterilized prior to packaging. Most food processing facilities can be generalized as manufacturing plants, and the use of automated processing equipment is critical to maintain the volume of output required to meet consumer demand. These facilities have implemented UV “tunnels” to automate the sterilization process. Foods travel on automated conveyer systems which passes through a tunnel made up of numerous UV light sources. (The best analogy here would be a tanning bed with a conveyer system.) UV intensities are generally fixed in these systems; however, conveyer travel rates can be optimized for varying sterilization needs.

In at least one large, new vivarium, this same food sterilizing technology has been implemented at the inbound freight dock. Feed and bedding delivered to the dock are placed on a conveyer system which passes through a UV sterilizing system similar to the one shown in Figure 3. Materials exiting the UV system pass through a simple barrier onto another automated conveyer in a clean side storage room. This system eliminates all the hazards and waste disposal issues associated with chemical sterilization techniques, while providing automation of the material handling issue.

A separate consideration that must be addressed is the amount of UV radiation exposure that employees and operators may receive. The American Conference of Governmental Industrial Hygienists have established standards for exposure of not more than three millijoules per square centimeter over an eight hour period for a wavelength of 270 nanometers. Hazard metering tools are available on the open market to help assess and monitor exposure against this standard.

Additional work continues in improving the use of UV for air handling systems. The previously mentioned International UV Association formed an Air Disinfection Committee in 2003 whose charter is to provide guidelines for the effectiveness, use, and safety of UV systems in air handling systems. Availability of electronic ballasts for UV lamps is improving the implementation of UV systems by allowing smaller form factors, better tolerance to voltage variance, and the ability to vary lamp output.

The ability to vary lamp output implies that environmental sensors for temperature and humidity can be fed into a Programmable Logic Control (PLC) device which can subsequently manage or optimize the output intensity of a connected UV light source. UV radiometers may also be included in these systems that automatically monitor and report UV system performance and can provide a warning or alarm when the UV lamp output is below the levels necessary for effective pathogen control. These advances allow integration of UV as a supplemental pathogen control mechanism for isolation cubicles, fume hoods, changing stations, etc.

More recently, the process of photocatalytic oxidation has emerged with several beneficial attributes to indoor air quality control. A semiconductor photocatalyst like Titanium Dioxide (TiO2) or Zinc Oxide (ZnO) can become highly reactive when irradiated with a UV light source (less than 385 nanometers). Air pollutants, especially volatile organic carbons (VOC) are readily absorbed by the activated surface and are then oxidized into carbon dioxide. The features of low energy consumption, long life and low maintenance makes this element of UV even more interesting. This process is currently being developed for use in clean room systems where VOCs are an important safety consideration, but its applicability is much broader.

Yet another potential benefit of UV for the laboratory animal environment is the formation of ozone. UV light at 185 nanometers creates low concentrations of ozone. Many commercial UV light sources include a quartz filter designed to impede the output at 185 nanometers and prevent the creation of ozone. However, the known qualities of ozone as a deodorizer could be beneficial in the laboratory animal environment. The challenge is to control the ozone at safe levels; Federal limits require not more than 0.04 ppm.

Conclusion

The efficacy of ultra violet radiation as a sterilization method is well documented. Technologies developed in the last decade have shed “new light” on the implementation of UV for water, air and materials sterilization. By looking outside the vivarium at how other industries are using UV, new ideas can be brought back that will continue to improve the laboratory animal facility’s ability to manage bio-hazards in safer and more economical ways.