USE OF IN-DUCT UVC LAMPS TO INACTIVATE AIRBORNE ENVIRONMENTAL BACTERIA AND FUNGI

Transcription

1 USE OF IN-DUCT UVC LAMPS TO INACTIVATE AIRBORNE ENVIRONMENTAL BACTERIA AND FUNGI DW VanOsdell 1* and KK Foarde 2 1 Aerosol Technology Group, Engineering and Technology Division, RTI, RTP, NC, USA 2 Microbiology Laboratory, Environmental and Industrial Meas. Div., RTI, RTP, NC, USA ABSTRACT Germicidal UV (UVC) lamps have a long history of use for inactivation of microbial aerosols. The majority of the literature has considered control of infectious diseases, such as tuberculosis (TB) in medical facilities. Emphasis has recently been on ventilation duct use of UVC. Under these conditions, infections agents are usually of less concern than environmental organisms. Much less information is available regarding common environmental or- ganisms. The present work reports the ability of UVC lamps to inactivate 7 representative microbial aerosols in ventilation duct conditions. Substantial inactivation of airborne environmental fungi was accomplished with UVC dose levels readily achievable using multiple lamps. The vegetative bacteria tested were relatively easy to inactivate, while the bacterial spores tested displayed an intermediate response. Fungal spores were difficult to inactivate. INDEX TERMS Ultraviolet, UVC, ventilation, environmental organisms, microbiological inactivation INTRODUCTION UV lamps have been used to inactivate airborne microorganisms for many years, with increasing activity in the 1960 s. Shechmeister (1991) has reviewed the literature. Much of the early work was directed at control of very infectious diseases (particularly TB), often in medical facilities. This work showed wavelengths within the short wave, or C band of UV light (UVC), to be the most effective germicidal light wavelength. UVC is practically generated by electrical discharge through low-pressure mercury vapor (primary wavelength of nm) enclosed in a glass tube that transmits UVC light. UVC has been shown to inactivate viruses, mycoplasma, bacteria, and fungi, whether suspended in air or deposited on surfaces. Because exposure to UV radiation is a common environmental hazard, cells have developed a number of repair mechanisms to counteract UV-induced damage that must be considered when experimentally measuring UV effects. Numerous studies of UVC to inactivate microorganisms have been conducted, for a variety of purposes and with a variety of methods. While the overall result (UVC destroys microorganisms) has been the same, the studies have differed with regard to some of the secondary parameters. The published duct design guidance and methods require lamp and system design parameters that may not be available and are complicated. Between manufacturers, design guidance is not entirely consistent in format or content. The objective of the research was to evaluate UV lamps to inactivate microbial aerosols in ventilation equipment using established bioaerosol control device performance measures. * Contact author dwv@rti.org 377

2 Effectiveness of UVC Against Bioaerosol For uniform UVC irradiance, the effect on a bioaerosol can be expressed as (Phillips, 1992): N t /N 0 = exp(- k dose) (1) Where N t and N 0 are the number of microorganisms at any time t and at the start, respectively, and k is a microorganism-dependent rate constant. The dose is the product of E eff t, in ΦW s /cm 2, where E eff is the effective (germicidal) irradiance received by the microorganism, in ΦW /cm 2 and )t the exposure duration, s, between the start and time t. Eq. 1 shows that a population of microorganisms has a distribution of resistance to UVC, just as that population would have to any other disinfectant process. For bioaerosols in a ventilation duct, two other distributions are superimposed on the microbial resistance: 1) that of the UVC irradiance field, strongest near the lamps and falls off sharply with distance from the lamps, and 2) that of the particle trajectories traced by the bioaerosol moving through the duct. The effective irradiance received by a bioaerosol particle is therefore a function of position and dose is the integral of the irradiance over the trajectory. In practice, the UVC field falls off roughly as the inverse of distance from the lamps, so for most lamps only the nearest meter is of interest. The direct UVC irradiance field at any position can be calculated for cylindrical lamps using a view factor equation developed originally for heat transfer from cylinders (Modest, 1993). The irradiance from multiple lamps is additive. The effect of duct reflectance is generally included as a factor on the direct irradiance (Phillips, 1992). Dose calculations have been reported using an assumption of total mixing with irradiance computed at a fixed distance to use of the view factor equation to compute irradiance at multiple points along an assumed or computed particle trajectory. METHODS Apparatus All experiments in this paper were conducted in a square (61 cm by 61 cm), stainless steel test duct having the ASHRAE 52.2 closed loop design (ASHRAE, 1999). It was capable of operating at volumetric flow rates of 0.27 to 1.37 m 3 /s while maintaining the air temperature between 13 C and 30 C and relative humidity between 35% and 80%. Two types of UV lamps were used in these experiments. Both were 61 cm long overall, with 57 cm of exposed glass that was 1.5 cm in diameter. They were single-ended lamps mounted such that they projected into the duct, perpendicular to the airflow. Multiple lamps were positioned one above the other in a single vertical column. The low output lamp tested was characteristic of less expensive UVC lamps, and utilized a magnetic ballast. The high output lamps were driven by an electronic ballast at a higher power output. One, 3, or 6 lamps were used during any given experiment, and for some experiments the single lamp was wrapped in 3 layers of window screen (7 wires/cm) to further reduce the available irradiance. UV irradiance was measured with an International Light IL1700 radiometer and SED 240 detector, calibrated with a neutral density filter and quartz filter, and measuring total UVC. Estimation of UV Dose Analysis of the experimental data required estimation of UVC dose. The method used during this work followed the outline above. The UVC irradiance, as a function of position, was cal- 378

3 culated for 100 time steps along the centerline path of each of 36 cells of a 6 by 6 grid within the duct, using the view factor model. The irradiance-time product for each time step was accumulated for each path to obtain the dose for that path. In effect, the centerlines of these cells are the assumed trajectories. The mean dose for each treatment was the mean of the 36 path doses. Lamp surface UVC output is required for the view factor model. This value was obtained by calibrating the model by making duct center measurements of irradiance at fixed distances and manually adjusting the lamp surface output such that the calculated and measured irradiance matched. Reflectance was not included in this model. Once the mean dose had been computed, the microorganism constant k was computed using Eq. 1. This approach includes a number of approximations (single trajectories taken to represent all bioaerosol in a cell, no reflections, unable to compute irradiance at lamp wall.) The model was adequate for dose estimation, with the estimated lamp efficiency values and the estimated microbial inactivation values agreeing reasonably well (~ 20%) with the literature. Microbial Challenge Aerosol Preparation of the challenge suspensions was similar for the bacteria and the fungi. The challenge suspensions were prepared by either: 1) inoculating the test organism onto solid media, incubating the culture until mature, wiping a wetted sterile swab across the surface of the pure culture and eluting from the swab into sterile 18-megohm/cm water to 15% transmission on the spectrometer; or 2) purchasing a spore suspension from a commercial vendor and diluting 1:10 in sterile water. Both corresponded to a concentration of approximately 1.5 x 10 7 CFU/ml. Resuspension in sterile 18-megohm/cm water was essential to minimize the particle counts from sources other than the organisms themselves (e.g.-dissolved solids). All aerosol injection was upstream of mixing devices, and approximately 4 m upstream of the UV devices. In all cases, investigation of the challenge aerosol showed that the aerosol at the UV device was uniformly mixed. The test aerosols were also stable in size at the UV device as evidenced by no size change detected by optical particle counter between 3 m (upstream) and 15 m (downstream), and were therefore considered to be dry. Additional discussion of humidity effects on the challenge aerosol is given below. Measurement of UVC Device Microbiological Inactivation Efficiency The basic measurement used for comparison of microbiological inactivation efficiency consisted of replicated microbial efficiency runs for the UVC device while challenged with a bioaerosol. Measurements were made with the UVC lamps both ON and OFF to account for losses in the duct downstream of the lamps. A single measurement run consisted of two sets of 3 simultaneous, 1-minute, single-stage Andersen upstream samples averaged to give the upstream challenge count, C up, and two sets of 3 downstream samples averaged to provide C down for both the light ON and light OFF cases. The fractional penetrations, P ON and P OFF, were computed as the respective C up /C down. The inactivation efficiency was computed as: Inactivation efficiency = 100 ( 1 - P ON /P OFF ) (2) The two replicate runs are compared and averaged to arrive at a final test result for the device, challenge organism, and operating conditions. The exposure model described above was then used to compute the k-constant for the organism used in the test. 379

4 RESULTS AND DISCUSSION Effect of Duct Reflectance for Lamps in Cross-Flow Arrangement Duct reflectance was investigated in a 61 cm square static duct section by measuring the irradiance from 3 LP lamps mounted across the duct as described above. The duct section was lined with either new galvanized steel or with photographic background black flock paper (near zero reflectance.) Nine measurements were made within one quadrant of the duct at 30, 50, 75, and 100 cm from the plane of the lamps, for a total of 36 measurements. Irradiance was highest near the duct center at all distances. As distance increased, the irradiance became lower and more uniform in the measurement plane. The galvanized duct had a more uniform UV distribution. Reflected light contributed an increased fraction of the total irradiance as distance from the lamp increased. The ratio of total irradiance in the galvanized duct to that when flock paper-lined increased linearly from 1.5 at 30 cm to 3 at 100 cm. Effect of Humidity, Air Temperature, and Air Velocity on Lamp UV Output Increasing air absolute humidity was shown to have a small (statistically significant) negative effect on irradiance at a point in the duct. The regression line was: irradiance (W/cm 2 ) = absolute humidity (g H 2 O/g dry air) The effect of airflow rate and temperature on irradiance is shown in Figure 1 for the high output lamp types. The 3 lamps were positioned perpendicular to the flow, at duct center and at ±20.3 cm. The UV detector was positioned at duct center, 50 cm from the lamps. Irradiance measurements were made over the duct velocity and temperature range shown in Figure 1. (Temperature control was problematic at velocities below 0.5 m/s because the flow stratified.) As shown, the irradiance produced by the lamps increased from a minimum at no flow (and consequent high lamp temperature) to a maximum at a particular flow rate (the optimum lamp operating temperature), and then decreased as the flow rate increased further and the lamp was cooled more than Irradiance, W/cm^ Proceedings: Indoor Air C 21C 30C Velocity in Duct, m/s Figure 2. Effect of Air Flow and Temperature on Irradiance from 3 "high" output lamps 4 was optimum. The reduction in output due to over-cooling at temperatures achievable in an HVAC system has been shown to be as much as a factor of two, which could have a significant effect on inactivation for bioaerosols. Airflow had its strongest impact at the lowest temperatures, where the available temperature difference (between the air and the lamp wall) to drive heat transfer is greatest. The reduction in the effect of flow rate at high flow rates (decreasing nega- 380

5 tive slope above 2 m/s) presumably occurs because the temperature difference between the lamp wall and the air is less and the heat transfer rate is consequently dropping. The low output lamp gave similar curve shapes, with a maximum output of about W/cm 2 that occurred at 0.4 m/s. While the behavior of the two lamp types was similar, the optimum operating temperature/air flow points were different for the two lamps. The observed behavior is consistent with literature reports and discussions with manufacturers. Microbiological Inactivation Tests. The preliminary results for microbial inactivation are shown in Table 1. We have conducted microbiological tests with 7 organisms: Serratia marcescens, Staphylococcus epidermidis, Pseudomonas fluorescens, Bacillus subtilis, Aspergillis versicolor, Penicillium chrysogenum and Cladosporium sphaerospermum. The first four are bacteria, and the last three fungi. These organisms cover a broad range of susceptibility to UV. Serratia marcescens, Staphylococcus epidermidis, and Pseudomonas fluorescens are vegetative bacteria. They are readily inactivated. Bacillis subtilis is a spore-forming bacteria and the three fungi also form spores. We expected them to be difficult to inactivate. Most of the tests were conducted at about 75 F, 50% RH, and with the duct velocity at 1.27 or 1.87 m/s. The tests utilized the low output lamps in a cross-flow configuration as described above. In each case irradiance was measured at a known distance from the lamp, on the duct centerline, and that value used to Table 1. Preliminary Results of Microbial Inactivation with Varying Doses of UVC. Organism Lamp Dose % Inactivation Configuration J/m 2 k Factor m 2 /J 1 lamp E-01 Serratia marcesens 1 lamp, 3 screens E-01 1 lamp, 3 screens E-01 1 lamp E-01 Staphylococcus 1 lamp E-01 epidermidis 1 lamp, 3 screens E-01 1 lamp, 3 screens E-01 Pseudomonas 1 lamp E-01 fluorescens 1 lamp, 3 screens E-00 6 lamps E-02 Bacillus subtilis 6 lamps E-02 3 lamp E-02 3 lamp E-02 6 lamps E-03 Aspergillus versicolor 6 lamps E-03 3 lamps E-03 3 lamps E-03 6 lamps E-03 Penicillum chrysogenum 6 lamps E-03 3 lamps lamps E-04 Cladosporium sphaerosperm 6 lamps E-03 3 lamps E-04 Notes: 3 screens indicates that the single lamp was wrapped with 3 layers of household screen to reduce irradiance. 381

6 calibrate the model. The model was then used to compute the mean dose for all the trajectories, and the k factor was computed from this mean dose and measured microbial inactivation. Table 1 shows, for this range of irradiance, all organisms were brought into the measurable range for inactivation efficiency. The vegetative bacteria were readily inactivated. The bacterial spores required a much higher dose; and the fungal spores were even more difficult to inactivate. Agreement between the computed k-values and those reported by Phillips (1992) was generally within a factor of two. Phillips (1992) reported k-values of 2.3E-02 m 2 /J for B. subtilis, a range around ~1.2E-01 m 2 /J for a number of Staphylococcus spp., and a range around ~2.5E-03 for two Aspergillus spp. Effect of Relative Humidity on microbial inactivation We are currently examining humidity effects on the organisms shown in Table 1. The results suggest that some organisms may become less susceptible to UV as the humidity increases. The difference may not be significant in practice. However, it is very interesting in that it may point to some fundamental properties of microbial aerosols that cannot be readily investigated except in the aerosol state. When they are in contact with agar, as has been the case in most of the reported literature, the impact of air RH cannot be studied. CONCLUSIONS AND IMPLICATIONS These data indicate that UVC systems can be used to inactivate a substantial fraction of environmental bioaerosols in a single pass. Much of the variability in the k values reported in the literature is a result of overlooking the importance of the medium the test organism is in during exposure. The susceptibility of an organism could be quite different if it is growing on an agar plate than when it is an aerosol. The use of single pass UVC to inactivate pathogenic or biowarfare aerosols, using the equipment studied, would be problematic because of the high degree of inactivation required and the level of variability observed. The combined effects of the microbial resistance distribution and the dose variability inherent for an aerosol in a duct have a high probability of allowing occasional penetration even using the most thorough design process. Design based on UVC irradiance and bioaerosol exposure models, when properly applied, appear to be sufficiently accurate to allow reasonable design for control of environmental bioaerosols, provided the k-value is available. However, lamp performance values must be known or measured to use the models. Lamp output measurements must be made at the expected use conditions because output varies strongly with temperature and airflow rate. ACKNOWLEDGEMENTS The financial support of ARTI is gratefully acknowledged. REFERENCES ASHRAE ANSI/ASHRAE Standard Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. Modest, M. F Radiative Heat Transfer. McGraw-Hill, Inc. New York, NY. Phillips Lighting Division Disinfection by UV-radiation. Booklet 3222 C Shechmeister, I. L Sterilization by Ultraviolet Radiation. In: Disinfection, Sterilization, and Preservation. Lea & Febiger, Philadelphia. pp