W.J. Kowalski, PE, PhD, The Pennsylvania State University
Charles E. Dunn, Sr. President, CEO, LumalierTM

Ultraviolet germicidal irradiation (UVGI) has been in use for air and surface disinfection for the better part of a century and yet for the past few decades it was considered to have doubtful and unpredictable effectiveness for the control of airborne contamination. Things have changed, and with the advent of improved equipment and new analytical methods, UVGI design has been transformed from an art into a science. Current concerns about bioterrorism have also added new impetus to research and development of this and other immune building technologies.

Current Applications

Four major applications exist today for UVGI systems - health care, shelters, prisons, and commercial/residential applications. The latter market is currently the smallest but may have the largest potential for growth. The type of air disinfection systems installed can be subdivided into three categories; in-duct air disinfection, recirculation units, and upper air disinfection systems. In addition there are at least four types of surface disinfection systems; microbial growth control systems, laboratory disinfection systems, portable area disinfection systems, and mail decontamination systems.

Microbial growth control systems have a proven record of maintaining . . . cleanliness, to the point that short payback periods can be established in terms of energy saved.

Area disinfection or decontamination systems can be used to eliminate surface contamination in open areas, either for remediation or for prevention of potential hazards. Portable UVGI systems are available to decontaminate open areas and these are, of course, operated only in unoccupied areas. After hours UVGI systems are permanently mounted on walls or ceilings and are engag4ed during times when no occupants are present. The latter type of system is engaged either by timers or disengaged by motion detectors, and provides options for mailrooms or laboratories where contamination potential exists.

Mail disinfection systems are similar to medical disinfection equipment except that they can be as large as rooms. Several manufactures have already produced such systems as the result of requests following the 2001 anthrax mailings.

Sizing UVGI Systems

Previous methods for sizing UVGI systems were limited to cloning successful applications or the use of inexact catalog guidelines. Current methods developed at Penn State employ computer programs to resolve the three-dimensional intensity field of UV lamps inside reflective enclosures. Although such software is only available on a proprietary basis or for academic research, the methods have been detailed in various publications and such tools will soon become more widely available t designers.

Finally, guidelines are in the planning stage at Penn State under an industry funded program that will facilitate the selection and sizing of UVGI systems tailored for specific applications. A novel aspect of these proposed guidelines involves the specification of a UVGI Rating Value (URV) for air disinfection systems that parallels the ASHRAE 52.2-1999 method for testing and rating filters known as MERV (minimum efficiency reporting value). The proposed URV rating system consists of 20 separate levels of average UVGI intensity, of which several are shown in Table 1.

Table 1

The average UVGI intensity within any enclosure is the critical determinant of any engineered UVGI system. Any UVGI system designed or installed can be assigned a URV based on analysis or by field testing with a photosensor. Given the URV, the dose can be determined for any exposure time, as shown in Table 1 for a 0.5 second exposure time. The exposure time depends on the air velocity and duct length. The kill rates can be computed form the dose for any microbe for which a UVGI rate constant is available. In Table 1, the kill rates for airborne TB bacilli are computed as an example.

The URV should result in standardization of system sizes and eliminate confusion about system performance. A side benefit of this approach is that when selecting a combination filter/UVGI system, and approximate match can be formed between MERV and URV values that will provide a complementary combination of both technologies. For example, a MERV 11 filter is roughly comparable to a URV 11 system in terms of the broad range of microbes removed although they operate on different groups of microbes. That is, this filter removes spores efficiently while the UVGI system does likewise for viruses and together they provide a balanced removal rate across approximately the entire array of airborne pathogens.

Lamps and Reflective Materials

A wide variety of UV lamps and refl3ective materials are available today to suit most any application. Reflective paneling offers an economic means of boosting the average intensity field without additional power consumption. Polished aluminum panels provide high levels of reflectivity (i.e., 75-90%) inexpensively. New materials such as ePTFE can provide extraordinary reflectivity (>99%) and can amplify intensity fields many times depending on the total enclosed surface area.

Bioterroism Applications

Perhaps the greatest interest today is in the use of UVGI as a means of protecting indoor environments against the threat of bioterrorism as apart of an integrated immune building protection system. In combination with filtration, UVGI can provide a cost-effective means of protecting building occupants if biological weapon (BW) agents are intentionally released inside or outside buildings.

The release of BW agents in the outside air intakes in the ventilation system, or in any general area, will be recirculated in building air. The presence of even moderate levels of filtration and air disinfection can have a major impact on the reduction casualties. Simulations of BW agent released performed at Penn State using multizone air mixing models suggest that approximately 90-99% of building occupants can be protected through the use of MERV 11-15 filters in combination URV 11-15 UVGI systems. In these simulations the BW agents; anthrax, smallpox, and botulinum toxin were used to model attack scenarios in multistory office buildings. No additional reduction in potential fatalities resulted when higher levels of filtration, i.e., HEPA filters, or higher wattage UVGI systems were used, indicating that there may be only one size of a MERV/URV system that is appropriate for each specific building. The critical determinants of the size of the UVGI/filter system appear to be the ventilation flow rate and the building volume.

A number of buildings around the nation have already implemented protective systems as a result of the concerns following the anthrax mailings in late 2001. One such implementation involving UVGI was retrofit into the Memphis Light, Gas, and Water (MLGW) main administration building in Memphis, Tennessee. This system made extensive use of UVGI lamps located in the air chases that return air from office spaces and in the air handling unit. The photograph below shows an array of UV lamps located inside one of the building's air handling units.

UVGI lamp array installed inside one of the five MLGW building air handling units

Performance of the MLGW building UVGI system was analyzed in combination with the existing 20% DSP (MERV 6) filters in terms of the removal rates for three design basis pathogens. Because of the high UVGI intensity, an URV has not yet been defined for this system. Table 2 shows the results of these simulations for the attack scenario in which the agent is released continuously in the air intakes. The first two rows show the predicted removal rates for both pathogens and the third row shows the combined removal rate. Note that the combined removal rate is not simply the addition of the two removal rates since the downstream component operates only on the survivors of the upstream component.

Table 2: Simulation Results for Air Intake Release

The simulation results shown in Table 2 show the predicted fatalities for anthrax and smallpox and the predicted infections for TB bacilli. The baseline condition represents a quantity of BW agent released that will cause 99% fatalities. The last row shows the predicted casualties that will result from the installation of a combined air cleaning system. The result is a major reduction in total casualties for all agents and indicates that the system offers considerable protection to building occupants in the scenario. Although this is not the most severe attack scenario, simulations of other scenarios show similar results. These results suggest that relatively moderate levels of filtration and UVGI can provide a significant amount of protection to building occupants.

Any air cleaning system capable of removing BW agents will, of course, also be successful at eliminating the more mundane threats of everyday indoor microbial contamination. For more information on the above subjects, the readers are invited to visit the Penn State Aerobiological Engineering website at www.engr.psu. edu/ae/wjkaerob.html and Lumalier's website at www.lumalier.com.