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Patho control systems use UV light to kill viruses, unlike hepa filters which only capture particles.

UV light has been shown to have a more deadly and efficient effect, comparatively on viruses, than common air filtration methods including HEPA. Learn what air treatment method is best for your environment and which are more effective in destroying viruses, as well as being economically viable.

The PathoControl P150 SYSTEM is a self-contained air restoration system that utilizes UV light, designed for your home or office. 

HEPA filtration vs. UV light treatment

From the article, Airborne Respiratory Diseases and Mechanical Systems for CONTROL OF MICROBES By W. J. Kowalski, PE, Graduate Researcher, and William Bahnfleth, PhD, PE, Assistant Professor, The Pennsylvania State University, Architectural Engineering Dept., University Park, Pa

These excerpts are an attempt to compare air filtration methods—(total purge of air, and HEPA filtration), versus Ultraviolet germicidal irradiation (UVGI) treated air—against common pathogens: viruses (including coronaviruses), bacteria, and fungi. We have added emphasis for ease of comparison.

Introduction

Airborne transmission of respiratory diseases in indoor environments remains a problem of indoor air quality (IAQ) with few engineering alternatives and for which performance goals and design parameters are unclear. The engineer who attempts to deal with microbial IAQ finds that pertinent microbiological information exists in abundance but not in easily digestible forms. This article summarizes the relevant literature of medical microbiology and aerobiology in a manner that engineers may find useful and informative and that will facilitate the design of HVAC systems intended to reduce the threat. The general principles presented here can be applied to any indoor environment, including office buildings, schools, residences, hospitals, and isolation wards.

Classification of pathogens

Pathogens are any disease-causing microorganism, but the term applies to any microbial agent of respiratory irritation, including allergens or toxigenic fungi. Respiratory pathogens fall into three major taxonomic groups: viruses, bacteria, and fungi.

The single most important physical characteristic by which to classify airborne pathogens is size since it directly impacts filtration efficiency.2

Fig. 1 presents a graphic comparison of airborne respiratory pathogens in which the spores, bacteria, and viruses can be observed to differentiate well, based on size alone. The left axis indicates the “average” or typical diameter or width. The areas of the circles do not represent the actual sizes of the microbes, but each represents the diameter in proportion to one another. The span of diameters is seen to be almost four orders of magnitude. Some microbes are oval or rod-shaped, and for these only, the smaller dimension is indicated.

Figure 1. Relative size of airborne respiratory pathogens.

Respiratory Pathogens

Table 1 lists all respiratory pathogens under these three categories, along with major diseases, common sources, and average diameters. In the column identifying microbial group, the term actinomycetes refers only to the spore-forming actinomycetes. Some general observations can be made from these charts such as the fact that most contagious pathogens come from humans, most non-contagious pathogens come from the environment, and most primarily nosocomial infections tend to be endogenous. These tables are not necessarily inclusive since a number of pathogens, such as E. coli, Bacillus subtilis, and some other strains of Legionella, can, on rare occasions, cause respiratory disease or allergic reactions.3 The abbreviation “spp.” denotes that infections may be caused by more than one species of the genera but does not imply that all species are pathogenic. Table 1 lists only respiratory pathogens, although non-respiratory pathogens can also be airborne. Certain infections of the skin or eyes, nosocomial infections of open wounds and burns, and contamination of medical equipment may occur by the airborne route. Although these types of infections have not been well studied, any pathogen that transmits by the airborne route will be subject to the same principles and removal processes described in this article.

Table 1

Communicable diseases

Table 1 lists all the main respiratory diseases that can transmit between human hosts via the airborne route. Humans are the natural reservoir for most contagious pathogens but some notable exceptions exist. Pneumonic plague and Arenavirus epidemics originate with rodents or other mammals.1 In regards to the mysterious origin of Influenza, humans apparently share the function of natural reservoir with birds and pigs, as strains of this virus periodically jump between species.3 Many contagious respiratory pathogens also transmit by direct contact through the exchange of infectious droplets or particles called fomites.4 The eyes and nasal passages are vulnerable to fomite transmission. The predominance of these direct routes in comparison with the inhalation route has not been well established but can be very species-dependent.5 Infectivity is also lost upon drying, and therefore hand or surface contact may require the exchange of moisture as well as an infectious dose.1,6

Comparisons

In hospital settings, protection from potential pathogens requires the reduction of microbial contaminants below normal or ambient levels. This is usually accomplished through the use of isolation rooms, HEPA filters, UVGI (Ultraviolet light), and strict hygiene procedures.15 In the health care environment, particular attention must be paid to the possibility of microbial growth indoors and in the air handling units, even if levels are not a threat to healthy people.

High-efficiency particulate air (HEPA)

Full outside air systems are often used in health care facilities and TB isolation rooms, subject to CDC guidelines.15 Fig. 9 shows the effect of full purge air flow on the reduction of pathogens in a room with an initial concentration of 100 microbe CFU per cu meter. Comparing this with Fig. 10 shows the results of HEPA filtration at the same recirculation flow rates. The results are practically identical. The use of HEPA recirculation, of course, carries a lower total energy penalty2 in hot or cold climates. But in mild or dry climates, high percentages of outside air can prove economical, especially in applications involving evaporative coolers. Hospitals often have commitments to specific guidelines, but other facilities may select and size systems to suit their goals and budgets. HEPA filters, for example, are not the only choice for controlling microbial IAQ. High or medium efficiency filters are capable of removing airborne pathogens, especially spores, without high operation or replacement costs.2,16 Overall, particle removal efficiency might be improved by locating medium efficiency filters in the recirculation loop vs. the outside air intakes16 or even downstream of the cooling coils. But, this choice will depend on each individual system’s operating parameters. Combining purge air with HEPA filtration results in performance that is essentially additive, and cost optimization becomes straightforward. Energy consumption, replacement costs, and microbial IAQ goals will dictate the economic choice for any particular installation.16 The performance of medium efficiency filters in combination with purge air flow is not directly additive but depends on the filter efficiency vs. particle size curves, the sizes of the pathogens of concern, and the system operating parameters.

Ultraviolet germicidal irradiation (UVGI)

Chronic dosing with UVGI can have a major impact on airborne viruses and bacteria.16 A graphic comparison of the relative effectiveness of the three main alternatives—outside air purge, filtration, and UVGI—is provided in Fig. 11 through 13. The scenario of an initially contaminated room may not be realistic but provides dramatic differentiation of the effectiveness of pathogen removal.

Fig. 11 shows the effect of 1 air changer per hr (ach) of outside air on reduction of room air contaminant concentrations from an initial value. Perfect mixing is assumed, along with 500 CFU per cu meter contamination of each microbial group initially, 100 CFU per cu meter of spores in the outside air, and no internal generation.

Fig. 12 shows the effect of an ASHRAE medium efficiency filter (80 to 85 percent dust spot) to the supply air of the model building while maintaining 1 ach of outside air. The filter model describes filter efficiency vs. diameters in accordance with typical vendor performance curves.16 Spore levels indoors are clearly reduced below outdoor ambient levels. Some reduction of bacteria and viruses can also be noted, but their removal is still dominated by the purging effect of the outside air. The filter used in this analysis provides a baseline for comparison. High efficiency filters, such as the 90 to 95 percent filters used in hospitals,2 would result in even higher removal rates.

Fig. 13 shows the impact of a UVGI system with 25 mW (W=watt) per sq cm placed in the recirculation loop. The outside air is maintained at 1 ach, but no filters are included. Spores are relatively unaffected by the UVGI, but the viruses are markedly reduced. This model incorporates chronic dosing effects from recirculation with an exposure of 0.2 sec for each pass. The decay rate Equation 1 is applied with known rate constants16 for a wide cross-section of the microbial species listed in Table 1. The unusual performance characteristics of each technology have been highlighted in these examples. Inclusion of these characteristics in any evaluation, along with the IAQ design goals, ambient conditions, and internal generation rates, will dictate the choices for any given application—subject only to economic limitations.

Figures 11, 12 & 13

Conclusions

UV light has been shown to have a more deadly and efficient effect, comparatively on viruses, than common air filtration methods including HEPA. Perfect solutions to the problem of airborne disease transmission do not yet exist, but the available technologies—outside purge air, filtration, and UVGI—can be successfully implemented when their characteristic effects are understood and the goals clearly defined. Whether the application involves improvement of microbial IAQ in an office building or minimizing the risk of infection in an operating room, these technologies can be optimized individually or in combination from a cost or performance standpoint. Finally, since microbes will never ignore opportunities provided to them, appropriate design, regular surveillance, and maintenance of these technologies in particular, and HVAC systems in general, should always be proactive.

References

1) Mitscherlich, E. and E. H. Marth. Microbial Survival in the Environment. Berlin: Springer-Verlag, 1984. 2) Burroughs, H. E. “Filtration: An investment in IAQ.” HPAC Aug. 1997: 55-65. 3) Freeman, B. A. ed. Burrows Textbook of Microbiology. Philadelphia: W. B. Saunders Co., 1985. 4) Ryan, K. J. ed. Sherris Medical Microbiology. Norwalk: Appleton & Lange, 1994. 5) Mandell, G. L. ed. Principles and Practice of Infectious Diseases. New York: Wiley, 1985. 6) Hers, J. F. ed. Airborne Transmission and Airborne Infection. Proc. of VIth International Symposium on Aerobiology. Technical University at Enschede. The Netherlands: Oosthoek Publishing Company, 1973. 15) Castle, M. and E. Ajemian. Hospital infection control. New York: John Wiley & Sons, 1987. 16) Kowalski, W. J. “Technologies for controlling respiratory disease transmission in indoor environments: Theoretical performance and economics.” Master’s Thesis. Ann Arbor: UMI Dissertation Services, 1997.