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Disinfection performance of ultraviolet germicidal irradiation systems for the microbial contamination on an evaporative humidifier.

Introduction

It is well established that microbes can grow in air-handling units (AHUs), and much research has been conducted investigating the growth of microbes in AHUs and the relationship between contaminations of AHUs and respiratory symptoms (Menzies et al. 2003). In addition to the health-related problems, biofilms forming on the dense fins of cooling coils can increase the pressure drop and even decrease the heat transfer performance of the fins, resulting in increased energy consumption (Kowalski 2009, Witham 2007).

In-duct ultraviolet germicidal irradiation (ID-UVGI) systems have been considered as a countermeasure to disinfect AHUs and are widely used or considered for use primarily in hospitals and public or office buildings. Several field studies have shown that microbial contamination of AHUs can be disinfected using ID-UVGI systems (RLW Analytics, Inc. 2006, Witham 2007). Levetin et al. (2001) experimentally illustrated that the level of microbial contamination in AHUs was correlated with the concentration in indoor air and that it could be reduced using ID-UVGI systems. In addition to ID-UVGI systems, upper room (UR)-UVGI systems have been considered for airborne disinfection of infectious microbes and viruses (Memarzadeh et al. 2010).

The germicidal effect of UVGI systems is determined by the ultraviolet C-band (UVC) intensity (I, [W/[m.sup.2]]), exposure time (t, [sec]) and UVC susceptibility (k,[[m.sup.2]/J]) of each microbe based on the following experimental equation:

KR = 1 - exp(-kIt). (1)

UVC intensity and exposure time can be controlled during the design ofthe UVGI system. However, UVC susceptibility, which is also called the UV rate constant, is a characteristic of each microbe and virus. Many studies have been conducted to identify the UV rate constants of various of microbes and viruses; Kowalski (2009) summarized the data as shown in Table 1. Fungal spores are mostly resistant to the UVC, but bacteria and viruses are comparatively easy to disinfect using UVC rays. Some bacterial spores are more resistant than those in a vegetative condition. Nevertheless, their UV rate constants are still lower, on average, than those of fungi.

Humidifiers have been applied to most AHUs for buildings, not only to maintain a comfortable indoor environment, but also to prevent respiratory diseases from worsening due to extremely low humidity. Typically, steam humidifiers, atomizing humidifiers, and evaporative humidifiers (EH) are used with AHUs (ASHRAE 2000). Of these, EH have been considered, because, unlike steam humidifiers, they require no steam generator and thus use less energy, although the response to the humidifying load and the humidifying capacity of EHs has yet to be refuted. Moreover, the EH element can act as an air filter to eliminate airborne contaminants, such as particles, microbes, and even chemical compounds.

However, microbial contamination of the EH surfaces and the potential dispersion of such contamination into the air have been verified in several studies (Strindehag and Josefsson 1991, Tanaka et al. 2009, Tsukami et al. 2009). Yamazaki et al. (1991) showed that airborne bacteria were detected downstream of the EH when the bacteria count in circulating water exceeded a certain degree. Even if the circulating water was free of bacteria, airborne bacteria could be detected, thus indicating that bacteria contaminating EH elements can originate from sources other than the circulating water.

In this study, the microbes present in an AHU with an EH installed were measured to verify their contamination mechanism. Furthermore, microbe levels were monitored for approximately six months while ID-UVGI systems were in operation to identify the germicidal effect of the UVGI system on the contaminated EH elements.

System setup

The EH system studied in this investigation was installed in AHUs that were operated 24 h/day in a laboratory building, with a maximum airflow of 6420 [m.sup.3]/h (3779 cfm) and an air velocity of 1.92 m/s (3.28 ft/s). The AHU contained a pre-filter and a HEPA filter set for animal laboratories and only handled outdoor air intake. An EH composed of eight element cells was installed between the preheating coil and the cooling coil. The EH element that was applied to eliminate chemical pollutants from the outdoor air in addition to the original humidification function was made of inorganic silica glass fibers, and its resistance to UV was verified through a UV degradation test. There were almost no changes in the physical properties of the EH after more than 2000 h of exposure to the UVC with an intensity of approximately 50 W/[m.sup.2]. The circulating water and additional city water purified using a reverse osmosis filter were treated by the UV filter and supplied to the EH, as shown in Figure 1. During the experiment, water supplied to the EH was sampled and proven to contain almost no culturable microbes.

ID-UVGI systems produced by two different manufacturers were installed facing either the upstream or downstream surface of the EH elements. Three UVC germicidal lamps were used for each surface, and the distance between the lamps and the element surfaces was about 20 cm (7.87 in.), as shown in Figure 2. The ID-UVGI systems were operated continuously with the AHU during this experiment.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Measurement

UVC intensity

UVC intensity is the most important factor for determining the germicidal efficiency of an ID-UVGI system, because exposure time can easily be controlled by adjusting the system operation time. Therefore, UVC intensity was measured to predict the germicidal efficiency of the system using UVC sensors at six points (a-f) on the surface of the EH elements for each upstream and downstream side, as shown in Figure 2. Lau et al. (2009) measured the change of UVC lamp output in various conditions of air velocity and temperature. They found that the output of a UVC lamp could decrease drastically as air velocity increased and air temperature decreased, which caused the lamp surface temperature to drop. Differences in UVC intensities between the operating and nonoperating conditions of the AHU were also tested in this study. The surface temperatures of the UVC lamps in both operating and nonoperating conditions of the AHU were measured using an infrared thermography camera (Ti25, Fluke Corp., USA) with an accuracy of [+ or -]2[degrees]C and were correlated with the UVC intensities measured by the UVC sensors. Another infrared thermography camera was effectively used to measure the surface temperature of the UVC lamps (Lau et al. 2009). The lowest temperature, i.e., the cold spot temperature, on the lamp surface faces the direction of airflow; however, the temperature of the lamp side was measured in these tests because it was impossible to measure the cold spot temperature on the downstream side. All of the measurements of UVC intensity and surface temperature were performed after the measured values had been verified as stable.

Microbial contamination

Bacteria and fungi were monitored before and after the ID-UVGI system was operated, as shown in Table 2. Microbes were sampled five times after the ID-UVGI system was operated to monitor the consistent germicidal effect of the system.

Microbes on the surface of the EH element were sampled from twelve points (1-12) on the upstream and downstream sides of the element, as shown in Figure 2. The samples were taken by touching target surfaces for 10 s with stamp-type soybean casein digest agar (SCDA) media with an area of 10 [cm.sup.2] (1.55 [in.sup.2]) to culture bacteria, and with potato dextrose agar (PDA) media with 100 mg/L (0.22 lbs/L) chloramphenicol added to culture fungi. After sampling, the SCDA media were incubated at 32[degrees]C (90[degrees]F) for two days and the PDA media at 25[degrees]C (77[degrees]F) for five days. Two hundred fifty liters of air before and after the EH were also sampled using a pinholes impactor sampler for 2.5 min on SCDA and PDA media and incubated under the same conditions as those collected from the surfaces. As the air was sampled with the AHU operating, air sampling started 30 s after closing the inspection door to avoid the contamination by ambient room air. The water drained from the EH was also sampled using sterilized pipettes, and 50 [micro]L of the sample was spread on SCDA and PDA media using a spiral distributor in the laboratory and incubated via the same methods.

Results

UVC intensities

The UVC intensities on the upstream side of the EH element surface were from 9.97 to 10.1 W/[m.sup.2] in front of the UVC lamps when the AHU was not operational, as shown in Table 3. However, the UVC intensity at point f was only 2.6% of the maximum, meaning some parts of the EH surface could not be irradiated by the UVC ray. Moreover, these decreased to about 85% when the AHU was operated, because the surface temperature of the lamp dropped from 38[degrees]C (100[degrees]F) to 33[degrees]C (91[degrees]F). Lau et al. (2008) also showed that the output of UVC lamps was drastically reduced with increasing air velocity and decreasing temperature.

Conversely, UVC intensities on the downstream side approximately tripled when the AHU was operated. The UVC lamp used in the downstream side was intentionally developed to radiate the UVC at its highest output during AHU operation, according to the manufacturer. In this case, the lamp surface temperature dropped from 75[degrees]C (163[degrees]F) to 53[degrees]C (127[degrees]F). The variations in UVC intensities during operation of the AHU could be explained by the fact that the UVC output would typically reach maximum levels when the cold spot temperature of the low pressure mercury UVC lamp surface is around 40[degrees]C (104[degrees]F) (Philips 2008).

If only the germicidal effect on the EH surface is concerned, the time required to reach a 99.9% kill rate for comparatively UVC-resistant fungi, such as Aspergillus niger (k = 0.00051 [m.sup.2]/J; Chick et al. 1963), on the upstream surface would be approximately from 25 min to 15 h, as determined using the following empirical equation:

t = -ln(1 - KR) / kI, (2)

where t is the exposure time (sec), KR is the kill rate (--), k is the UV rate constant ([m.sup.2]/J), and I is the UVC intensity (W/[m.sup.2]). The time required on the downstream surface, where the UVC intensity is muchhigher, would be approximately from 14 min to 9 h. Table 4 shows the exposure times required to reacha99.9% kill rate of representative bacteria and fungi using the measured UVC intensities. It was expected that it would take less time to inactivate bacteria, because bacteria, even those in their spore condition, are generally more susceptible to UVC than fungi. Regardless of the species of microbe, the UVC dose was considered to be high enough to inactivate most microbes on the EH surface, as the ID-UVGI systems were operated 24 hr/day and 7 days/week with the AHU.

Surface microbes

The results of measuring microbes prior to operation of the ID-UVGI systems showed that the EH elements were substantially contaminated by microbes. The microbes sampled on the EH element surface could not be counted because the cultivated bacteria and fungi covered the majority of the media surface, preventing identification of single colonies.

Tables 5 and 6 show that the levels of the microbes isolated from the EH element surface were reduced 12 days after operation of the ID-UVGI systems began, and the visual signs of contamination on the EH element surface were also drastically reduced, as shown in Figure 3. However, microbes could not be eliminated completely, even after continual measurements for two months. The concentrations of microbes isolated from the lower part (5-10) of the EH element surface were larger than those from the higher part (1-4) in almost all measurements, implying that the microbes flow down with the water and gradually contaminate the EH elements.

Airborne microbes

Figure 4 shows the airborne microbes that were sampled in the upstream and downstream sides of the EH system. The number of airborne microbes, particularly bacteria, isolated from the downstream side before operation of the ID-UVGI systems was extremely high, although no airborne bacteria was isolated in the upstream side, from which the microbes isolated in the EH were assumed to be aerosolized downstream. However, the levels of airborne bacteria were reduced soon after the operation of the ID-UVGI systems began. The temporary increase of bacteria observed on May 7 was assumed to be caused by outdoor air, as the concentration of bacteria in the upstream air was also high. This could also explain the increase in bacteria isolated from the EH element surface on the same day. Almost no bacteria or fungi were isolated in upstream air apart from on May 7. The concentration of airborne fungi was comparatively low throughout the monitoring period.

[FIGURE 3 OMITTED]

Microbes in the water

Along with the microbes on the surface and in the air, those in the water drained from the EH were also reduced after the operation of the ID-UVGI systems began, illustrating the disinfecting capabilities of the ID-UVGI systems. However, microbes were continually isolated, and the concentration increased gradually with time, as shown in Table 7, despite continued operation of the ID-UVGI systems. This trend was similar to the gradual increase in microbes on the EH element surface after the operation of the ID-UVGI systems began.

This was suspected to be the cause of the residual microbe contamination inside the EH, even after the operation of the ID-UVGI systems began, particularly in the drain water and on the element surfaces. The structure of the EH elements was complex, even consisting of a combination of 150-mm (5.9-in) and 100-mm (3.9-in) thick components giving a total thickness of 250 mm (9.8 in). This design was intentionally used to increase the evaporative efficiency of the EH; however, this may have prevented UVC rays from irradiating some internal components.

Inside of the EH element

An internal inspection of the EH elements, shown in Figure 5, revealed that the visual contamination was not improved, even though the outside surface appeared to be clean. The concentrations of microbes on the inside surface of the EH elements sampled with stamp-type media were uncountable because the cultivated bacteria and fungi covered the majority of the media surface, as was seen when the outside surface of the elements were tested prior to the operation of the ID-UVGI systems.

Discussion

The output of UVC lamps can be changed by increasing air velocity and decreasing air temperature. Surface temperature drop of UVC lamps causes a drop in the vapor pressure of the mercury in the lamps. This vapor pressure drop decreases the mercury steam concentration, resulting in a decrease of UVC output. However, in this study, UVC intensities were measured during AHU operation and nonoperation to assess the effect of surface cooling; because ID-UVGI systems were operated 24 h/day, 7 days/week, the decrease of UVC output was not an important factor to consider here. Even though this effect was not the main issue in this study, it should be considered when designing an ID-UVGI system, because the inside conditions of AHUs commonly have high air velocity and low air temperature.

The results of monitoring microbial contaminations demonstrate the germicidal capability of ID-UVGI systems for EHs, particularly on the element surfaces; however, the complicated internal structures of the elements prevented access of UVC radiation and, thus, still showed microbial contamination. Furthermore, the entire EH element was constantly wet due to the 24/7 operation of the AHUs, conditions that provided an ideal growth environment for microbes. Regardless, even if airborne microbes were dispersed from the EH elements, the HEPA filter (in this case) at the end of the AHU was able to remove most of them.

[FIGURE 5 OMITTED]

Nevertheless, microbial contamination in EH elements should be avoided, because persistent microbe dispersion can shorten the lives of air and water filters and can also cause odor problems. In addition, it can impair the performance of the EH, increasing the pressure drop and leading to a corresponding increase in energy consumption, as in the case of cooling coils (Witham 2007).

EH elements can be dried to partially remove contaminating microbes, because AHUs are typically operated only for 8 h/day. However, AHUs in many buildings, such as hospitals and hotels, are often operated 24 h/day. In those cases, several methods may be considered to prevent microbial contamination of EH elements, such as (a) intermittent operation to allow the EH elements to partially dry using multiple EH element parts, (b) changing the structure of the EH element or making it thinner to allow for effective irradiation by the UVC, and (c) using additional UVC lamps inside of the EH elements to irradiate internal surfaces.

Summary

In this study, the germicidal effectiveness and limitations of an ID-UVGI system for EHs was investigated. UVC lamps on the downstream side of the EH showed an initially high temperature that dropped once the AHU was operating to a level closer to the optimal temperature for UVC lamps; this indicates that the lamps delivered more UVC radiation under actual operating conditions.

Microbial contamination was verified visually on the EH element surface, and considerable amounts of microbes were isolated from the surfaces and the drain water. Airborne microbes, which were assumed to be dispersed from the EH elements, were also detected. Although microbial contamination was reduced drastically in both appearance and levels measured from the EH element surfaces after operation of the ID-UVGI systems began, microbes were persistently observed on the surfaces--in the downstream air and in the drain water. These results were caused by microbial contamination on the insides of the EH elements; these surfaces could not effectively be reached by the UVC radiation. Methods to overcome the limitations of ID-UVGI systems for disinfection of EHs should be considered in the future.

DOI: 10.1080/10789669.2010.541540

References

ASHRAE. 2000. Handbook HVAC Systems and Equipment.Atlanta, GA: American Society of Heating, Refrigerating, and Air-conditioning Engineers Inc.

Chick, E.W., A.B. Hudnell, and D.G. Sharp. 1963. Ultraviolet Sensitivity of fungi associated with mycotic keratitis ando ther mycoses. Medical Mycology 2(4):195-200.

IUVA. 2005. IUVA Draft (Guideline IUVA-G01A-2005: General Guideline for UVGI Air and Surface Disinfection Systems. Scottsdale, AZ: International Ultraviolet Association.

Kowalski, W. 2009. Ultraviolet Germicidal Irradiation Handbook. Heidelberg, Germany: Springer-Verlag.

Lau, J., W. Bahnfleth, and J. Freihaut. 2009. Estimating the effects of ambient conditions on the performance of UVGI air cleaners. Building and Environment 44:1362-70.

Lau, J., W. Bahnfleth, P. Kremer, and J. Freihaut. 2008. Investigation of surface temperature distributions of UVC lamps under variable flow conditions using infrared camera measurements. Proceedings of Indoor Air, Copenhagen, Denmark, p. 113.

Levetin, E., R. Shaughnessy, A.C. Rogers, and R. Scheir. 2001. Effectiveness of germicidal UV radiation for reducing fungal contamination within air-handling units. Applied and Environmental Microbiology 67(8):3712-5.

Memarzadeh, F., R.N. Olmsted, and J.M. Bartley. 2010. Applications of ultraviolet germicidal irradiation disinfection in health care facilities: Effective adjunct, but not stand-alone technology. American Journal of Infection Control 38(5):S13-24.

Menzies, D., J. Popa, J. Hanley, T. Rand, and D. Minton. 2003. Effect of ultraviolet germicidal lights installed in office ventilation systems on workers' health and wellbeing: Double-blind multiple crossover trial. Lancet 362:1785-91.

Philips. 2008. UV purification range brochure. http://www. lighting.philips.com.

RLW Analytics, Inc. 2006. Advanced HVAC systems for improving indoor environmental quality and energy performance of California K-12 schools. Project 3 Final Report, UVC Technology, California Energy Commission.

Strindehag, O., and I. Josefsson. 1991. Emission of bacteria from air humidifiers. Environment International 17(4) 235-41.

Tanaka, H., H. Niwa, and H. Baba. 2009. Study on performance evaluation of evaporative humidifier for air-conditioning system. Part 1. Comparative experiments of humidification performance and humidified air cleanliness. Conference of The Society of Heating, Air-Conditioning and Sanitary Engineers ofJapan, Kumamoto, Japan, pp 189-192 (in Japanese).

Tsukami, S., S. Nakai, and A. Ito. 2009. Investigation concerning energy supply system of the next generation type hospital that considered load characteristic and type. Part 4. Investigation and a comparison inspection experiment about the humidification method of the hospital air-conditioner, Conference of the Society of Heating, Air-Conditioning and Sanitary Engineers Of Japan, Kumamoto, Japan, pp. 1319-22 (in Japanese).

Witham, D.L. 2007. Ultraviolet-A three edged sword; cuts maintenance, cuts energy costs and improves IAQ. Proceedings of ASHRAE IAQ 2007, Baltimore, MD.

Yamazaki, S., H. Kimura, R. Funakubo, and M. Tsukeshita. 1991. Control of bacterial dispersion from humidifier to the humidified air. Proceedings of the Future Practice of Contamination Control, London, pp. 303-7.

Minki Sung, (1),* Shinsuke Kato, (1) U Yanagi, (2) Minsik Kim, (3) and Mitsuo Harada (4)

(1) Institute of Industrial Science, The University of Tokyo, Tokyo, Japan

(2) Department of Architecture, Kogakuin University, Japan

(3) Department of Architecture, The University of Tokyo, Japan

(4) Tokyo Electric Power Company, Japan

* Corresponding author e-mail: mkii@iis.u-tokyo.ac.jp

Minki Sung is a researcher. Shinsuke Kato, PhD, Fellow ASHRAE, is a professor. U Yanagi, PhD, is a professor. Minsik Kim is a graduate student. Mitsuo Harada is a general manager.

Received July 14, 2010; accepted November 6, 2010
Table 1. Average UV rate constants by species (surface).

Microbe Type k
 ([m.sup.2]/J)

Bacteria Vegetative 0.14045
Viruses All 0.03156
Bacterial spores Spores 0.01823
Fungal cells and yeast Vegetative 0.00700
Fungal spores Spores 0.00789

Table 2. Schedule of monitoring microbes and operating the
ID-UVGI system.

Date Operation

April 7, 2009 Pre-sampling of microbes
April 10, 2009 Installing and starting operation
 of ID-UVGI systems
April 22, 2009 First sampling of microbes
May 7, 2009 Second sampling of microbes
May 18, 2009 Third sampling of microbes
June 4, 2009 Fourth sampling of microbes and
 measuring UVC intensity
October 5, 2009 Fifth sampling of microbes
 (internal inspection of the EH element)

Table 3. Measurement results of UVC intensity (W/[m.sup.2]).

 Upstream

Point Off On Off/On

a 10.0 8.41 0.84
b 4.13 3.24 0.78
c 10.1 8.69 0.86
d 9.97 8.87 0.89
e 6.36 5.49 0.86
f 0.261 0.241 0.92
Lamp surface temperature 38[degrees]C 33[degrees]C 0.87
 100[degrees]F 91[degrees]F

 Downstream

Point Off On Off/On

a 4.86 13.6 2.80
b 3.2 8.89 2.78
c 5.87 16.2 2.76
d 4.95 15.4 3.11
e 4.39 12.2 2.78
f 0.186 0.438 2.35
Lamp surface temperature 75[degrees]C 53[degrees]C 0.71
 163[degrees]F 127[degrees]F

Table 4. Exposure time (min) needed to inactivate microbes 99.9%.

 Fungi Bacteria

 Aspergillus Cladosporium Fusarium Bacillus
 subtilis, herbarum, solani, subtilis,
Point k = 0.00051 * k = 0.0028 * k = 0.007 * k = 0.0246 *

a 27 (17) 5 (3) 2 (1) <1 (0)
b 70 (25) 13 (5) 5 (2) <1 (1)
c 26 (14) 5 (3) 2 (1) <1 (<1)
d 25 (15) 5 (3) 2 (1) <1 (<1)
e 41 (19) 7 (3) 3 (1) <1 (<1)
f 937 (515) 171 (94) 68 (38) 19 (11)

 Bacteria

 Staphylococcus
 Escherichia aureus,
Point coli, k = 0.01 * k = 0.0886 *

a <1 (1) <1 (<1)
b 4 (1) <1 (<1)
c <1 (1) <1 (<1)
d <1(1) <1 (<1)
e 2 (1) <1 (<1)
f 48 (26) 5 (3)

* Referring to IUVA (2005).

Note: numbers in parenthesis indicate time for the downstream side.

Table 5. Measurement results of surface bacteria on the EH elements
(CFU/10 [cm.sup.2]; mean value and standard deviation)

 Sampling April 7 April 22
 points

Upstream 1-4 TMTC 2.8 [+ or -] 3.6
 5-10 TMTC 14.7 [+ or -] 13.1
 11-12 TMTC 0.0 [+ or -] 0.0
 Average (1-10) TMTC 9.9 [+ or -] 11.8

Downstream 1-4 TMTC 1.5 [+ or -] 2.4
 5-10 TMTC 8.2 [+ or -] 8.4
 11-12 TMTC 0.0 [+ or -] 0.0
 Average (1-10) TMTC 5.5 [+ or -] 7.3

 May 7 May 18 June 4

Upstream 31.0 [+ or -] 48.9 3.8 [+ or -] 3.9 7.5 [+ or -] 4.5
 36.0 [+ or -] 24.9 7.0 [+ or -] 7.6 11.2 [+ or -] 5.5
 0.0 [+ or -] 0.0 0.0 [+ or -] 0.0 2.0 [+ or -] 2.8
 34.0 [+ or -] 33.9 5.7 [+ or -] 6.3 9.7 [+ or -] 5.2

Downstream 10.0 [+ or -] 8.8 4.0 [+ or -] 8.0 3.8 [+ or -] 3.1
 12.8 [+ or -] 8.3 5.8 [+ or -] 10.2 10.0 [+ or -] 3.3
 0.5 [+ or -] 0.7 0.0 [+ or -] 0.0 0.5 [+ or -] 0.7
 11.7 [+ or -] 8.1 5.1 [+ or -] 8.9 7.5 [+ or -] 4.5

 October 5

Upstream 5.0 [+ or -] 0.0
 2.7 [+ or -] 1.5
 -
 3.6 [+ or -] 1.7

Downstream 1 [+ or -] 1.4
 2.7 [+ or -] 2.1
 -
 2 [+ or -] 1.9

TMTC: too many to count.

Table 6. Measurement results of surface fungi on the EH
elements (CFU/10 [cm.sup.2]; mean value and standard
deviation).

 Sampling points April 7 April 22

Upstream 1-4 TMTC 0.0 [+ or -] 0.0
 5-10 TMTC 0.3 [+ or -] 0.5
 11-12 TMTC 0.0 [+ or -] 0.0
 Average (1-10) TMTC 0.2 [+ or -] 0.4

Downstream 1-4 TMTC 0.5 [+ or -] 0.6
 5-10 TMTC 1.3 [+ or -] 1.5
 11-12 TMTC 0.0 [+ or -] 0.0
 Average (1-10) TMTC 1.0 [+ or -] 1.2

 May 7 May 18

Upstream 3.3 [+ or -] 6.5 9.3 [+ or -] 7.6
 11.8 [+ or -] 9.5 27.2 [+ or -] 22.0
 0.0 [+ or -] 0.0 0.5 [+ or -] 0.7
 8.4 [+ or -] 9.1 20.0 [+ or -] 19.4

Downstream 2.0 [+ or -] 2.8 2.8 [+ or -] 3.4
 2.0 [+ or -] 1.4 15.2 [+ or -] 14.4
 0.0 [+ or -] 0.0 0.0 [+ or -] 0.0
 2.0 [+ or -] 1.9 10.2 [+ or -] 12.6

 June 4 October 5

Upstream 16.8 [+ or -] 17.1 0.5 [+ or -] 0.7
 27.3 [+ or -] 21.5 2.7 [+ or -] 3.1
 2.5 [+ or -] 3.5 -
 23.1 [+ or -] 19.6 1.8 [+ or -] 2.5

Downstream 5.8 [+ or -] 5.6 0 [+ or -] 0.0
 10.3 [+ or -] 5.4 2 [+ or -] 1.7
 0.0 [+ or -] 0.0 -
 8.5 [+ or -] 5.7 1.2 [+ or -] 1.6

TMTC: too many to count.

Table 7. Microbes in the drain water (CFU/50 [micro]L).

 April 7 April 22 May 7 May 18 June 4

Bacteria Upstream - 31 810 1180 1088
 Downstream 23,550 20 238 981 1163

Fungi Upstream - 0 4 27 20
 Downstream 995 0 4 26 19

 October 5

Bacteria 637
 32

Fungi 0
 0

Figure 4. Airborne microbes measured in the AHU:
(a) bacteria and (b) fungi.

Date (a) Bacteria (b) Fungi
 [CFU/[m.sup.3] [CFU/[m.sup.3]

 upstream downstream upstream downstream

4/7 0 754 0 12
4/22 2 4 0 14
5/7 64 104 0 8
5/18 4 8 0 4
6/4 2 10 0 4
10/5 10 66 2 12

Note: Table made from bar graph.
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Author:Sung, Minki; Kato, Shinsuke; Yanagi, U.; Kim, Minsik; Harada, Mitsuo
Publication:HVAC & R Research
Date:Jan 1, 2011
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