1、484 2009 ASHRAEABSTRACTRecently, the interaction between particles retained on HVAC filters and indoor air quality has gained more attention due to their possible relationship to irritation, health outcomes, and odors. This paper focuses on microbial contam-inants and metals captured on HVAC filters
2、 in nine residential and light-commercial buildings. Culturable fungi and bacteria populations captured in the dust were quantified using stan-dard spread plate methods and heavy metal (Pb, As, Cd) concentrations were determined by atomic absorption spec-troscopy. Culturable fungal and fungal spore
3、concentrations ranged from 104to 106and from 102to 103CFU/g, respec-tively, while culturable bacteria and bacterial spore concen-trations ranged from 105107and 103105CFU/g, respectively. Microbial concentrations were consistent across filters having different efficiencies with median concentrations
4、within one order of magnitude. Heavy metal concentrations were as high as 29 g/g for lead, 6 g/g for cadmium, and 7 g/g for arsenic. Variations observed in the metal concentra-tions between different dust samples may be due to particle size differences related to different filter efficiencies and in
5、door sources. This investigation provides insight into possible metal sources and concentrations of biological and heavy metal contaminants present in indoor environments. INTRODUCTIONIndoor air quality researchers typically focus their atten-tion on biological, chemical and particulate contaminatio
6、n of indoor environments and the health effects and discomfort that these contaminants may cause. Indoor environmental investi-gations typically rely on short-term sampling techniques that provide only a snapshot of contaminant concentrations in the indoor environment at the time of sampling. HVAC f
7、ilter dust is a potential resource that has received less attention and may enhance our understanding of indoor occupant exposure. Filters are typically in place for extended periods of time and have the potential to serve as long-term samplers of the indoor environment. Furthermore, HVAC filter dus
8、t can be collected with minimal effort and analyzed for a broad range of contam-inants. This paper focuses on bacteria, fungi, and heavy metals captured on HVAC filters and investigates how these param-eters vary with filter and building characteristics. Several studies have measured the concentrati
9、on of bacte-ria and fungi in indoor environments, especially in air and settled dust (e.g., Bouillard et al., 2005; Dales et al., 1997; Verhoeff and Burge, 1997). However, the reported concentra-tions are difficult to compare because they vary considerably depending on sampling technique and samplin
10、g location, among other factors. An alternative approach for investigating air and settled dust would be to analyze the dust that collects on HVAC filters. A recent study has suggested that HVAC dust may provide an integrated measure of airborne contamination levels in an indoor environment (Tringe
11、et al., 2008). HVAC filters are able to retain biological particles and microorgan-isms can survive, accumulate, and, under certain conditions, multiply on HVAC filters (Farnsworth et al., 2006; Foarde and Hanley, 2001; Kemp et al., 1995; Kemp et al., 2001; Moritz et al., 2001; Simmons and Crow, 199
12、5). In addition, a number of studies suggest a relationship between Sick Building Syndrome (SBS) symptoms and the presence of microorgan-isms on filters (e.g., Schleibinger and Ruden, 1999). Several researchers have also studied heavy metal concentrations in house dust and the correlation with poten
13、tial indoor and outdoor sources and particle size distributions (Al-Rajhi et al., Biological and Metal Contaminants inHVAC Filter DustFederico Noris Jeffrey A. Siegel, PhD Kerry A. Kinney, PhDStudent Member ASHRAE Member ASHRAEF. Noris is a graduate student and J.A. Siegel is an associate professor
14、and K.A. Kinney is a professor in the Department of Civil, Architectural, and Environmental Engineering, The University of Texas, Austin.LO-09-045 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115,
15、part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 4851996; Chattopadhyay et al., 2003; Decker et al., 2002; Kim et al., 1998; Tong, 1998). Despite these e
16、fforts, we are not aware of any research that utilizes HVAC filters as samplers to char-acterize metal concentration levels indoors or that examined the influence of HVAC systems and potential sources on metal concentrations found on the HVAC dust.While both microbial populations and metals found in
17、doors have been studied, the relationship between their pres-ence in HVAC filter dust and critical characteristics of both the particular HVAC system and the building remains unclear. This research compares the contaminant levels found in HVAC filters with different filter efficiencies and provides
18、insight into potential sources of contamination. This investi-gation is part of a broader evaluation of the utility of using filters as samplers for the indoor environment.METHODOLOGYEight residential and one commercial building in Austin, Texas were selected for this investigation. These sites repr
19、e-sent a sample of convenience and not a random sample. To characterize the sites considered, data was collected regarding the year the buildings were built, number of occupants, past or current presence of smokers, proximity to major highways, presence of attached garage, filter location, and condi
20、tioned volume. Two sets of HVAC filters were collected from each site, approximately three months apart. All filters were stored in a 4 C (39 F) environmental chamber maintained at a rela-tive humidity (RH) of approximately 70% until the analyses were performed within a few weeks following collectio
21、n.Characterization of Sites and FiltersThe filters were categorized according to the minimum efficiency reporting value (MERV) as determined by ASHRAE Standard 52.2 (ASHRAE, 2007) and reported by the manufacturers. The sample included seven low-efficiency filters (MERV 5), seven mid-efficiency filte
22、rs (MERV 5-8) and four high-efficiency filters (MERV 9-14). Filter pressure drop measurements were performed at filter installation and removal using an Energy Conservatory DG700 digital manometer, and the mean value of these two measurements characterized the mean filter pressure drop. Mean flow ra
23、tes across each filter in fan-only mode were measured using an Energy Conservatory True Flow Plate. By monitoring the HVAC systems two or three times during the cooling season for 24 hours approximately every month, we measured the cooling duty cycle, which is an estimate of the fraction of time tha
24、t the HVAC system is running during the cooling season. In addition, during the monitoring events, the temperature and RH in the HVAC system return plenum were also recorded. To estimate the mass accumulated on each filter, we subtracted the mean weight of three unused filters from the weight of the
25、 used filter using a balance (Sartorius B310S). Table 1 summa-rizes the instruments used during the investigation.Microbial and Metal AnalysesTwo samples of dust from each filter were acquired by shaking and scraping the dust material off the filters. The samples were subsequently analyzed for micro
26、bial and heavy metal concentrations. The enumeration of culturable bacteria and fungi was completed using the standard spread plate method 9215C (APHA, 1998). The microorganisms present in the HVAC filter dust were transferred into a phosphate buffer solution (PBS, 8 g/L NaCl, 0.2 g/L KCl, 1.44 g/L
27、Na2HPO4, and 0.24 g/L KH2PO4) by sonication and vortexing for 10 minutes each. For bacterial enumeration, a 0.1 ml aliquot of PBS was plated on R2A agar plates containing 0.04% cyclo-heximide. For fungal determinations, a 0.1 ml aliquot of PBS was plated on Sabouraud Dextrose Agar (SDA) plates conta
28、in-ing 0.01% chloramphenicol. Bacterial plates were incubated for 3-7 days at 30 C (86 F), while fungal plates were incu-bated for 7-14 days at room temperature (approximately 23 C/73 F). After incubation, the number of bacterial and fungal colonies formed was counted and the results were used to es
29、ti-mate the microbial concentration in the dust, expressed as colony forming unit (CFU) g-1dust. The analysis was performed three times for each dilution and the average number of colonies formed was recorded. The ability of the microorganisms to form spores was also tested by pasteurizing an aliquo
30、t of the samples for 15 minutes at 75 C (167 F) and then plating the samples as described above. Any colonies that formed were assumed to have originated from spores and to represent the spore-forming fraction of the population. Heavy metal concentrations in the HVAC filter dust were determined by a
31、tomic absorption spectroscopy (Perkin Elmer AAnalyst 600). Dust samples were digested via the micro-wave-assisted digestion method 3030K (APHA, 1998). This method consists of a nitric acid digestion under controlled pressure and temperature conditions that facilitate the transfer of the metals prese
32、nt in the particles into the liquid extract. The liquid extract from each sample was analyzed for selected Table 1. Summary of InstrumentationMeasurement Manufacturer Model AccuracyTemperature Onset Hobo U10 0.4C (0.7F)Relative humidity Onset Hobo U10 3.5% Pressure drop Energy Conservatory DG 700 1%
33、 or 0.2 Pa (0.0008 IWC)Air flow Energy Conservatory True Flow Plate 7%Weight Sartorius Balance B310S 0.001 g486 ASHRAE Transactionsheavy metals (Pb, As, Cd) according to method 3111B (APHA, 1998). To ensure the accuracy of the measurements, reagent blanks and periodic calibration checks were also an
34、alyzed.A nonparametric statistical method, the Wilcoxon Rank-Sum Test, which does not assume any specific distribution of the data, was applied to compare and identify dissimilarities between the different data groups. When comparing the differ-ent data groups, a significance level of 0.1 was assume
35、d owing to the small sample size and the conservative nature of this statistical test. RESULTS AND DISCUSSIONTable 2 characterizes the nine sites and the presence of likely sources of contamination. Site 9 was the one light commercial building included in the study. All the sites were relatively clo
36、se to major highways and five sites had attached garages. Sites 2, 3, 4, and 9 were attached to other dwellings. Four sites had the filter located at the unit, while five were located at the return register. In Sites 3 and 9, multiple filters were present. Cooling duty cycles of the sites ranged fro
37、m 9 to 34%. There were no smokers occupying any of the sites, although Site 2 had smokers in the past. Sites 3 and 4 were located in the same residence with two separate and indepen-dent HVAC systems for different floors of the residence. The sites summarized in Table 2 represent a range of HVAC sys
38、tems and operating characteristics for this region of the country.Table 3 summarizes the characteristics of the 18 HVAC filters, two from each site, that were evaluated during the proj-ect. The mean pressure drop across the filter, P, and the mean volumetric airflow through the HVAC system, Q, were
39、obtained by averaging the values obtained at installation and at removal. For a few filters, we were not able to measure the filter pressure and supply plenum pressure (required for the flow measurement) at filter installation, so the measurements collected at the time of filter removal are reported
40、. For Filter 2 of Site 2, the value reported represents the observation acquired at installation. The mean temperature and RH observed at the HVAC return plenum during the monitoring events are also reported. These values do not represents mean levels during the period the filters were in place, but
41、 only what was observed during the monitoring visits.For some filters, the days in service was not known, because it was the filter the homeowner had in place when we started the investigation. For seven filters it was possible to estimate the mass accumulated over the service life because these fil
42、ters were weighed before use. As expected, we observed a correlation between filter efficiency and particle mass accumulated on the filter. The mean mass accumulated on the low-efficiency and mid-efficiency filters was 1.7 and 4.0 g, respectively. There may also be a correlation between the mass of
43、particles accumulated on filters and the presence of carpet in the house. The mean mass accumulated on the filters from the sites with and without carpet was 3.9 and 0.8 g, respectively. Carpets tend to accumulate more dust than bare floors because they are harder to clean than other types of floor.
44、 As a consequence, particle resuspension from carpet is expected to be greater than from other floor surfaces (Yoon and Brimblecombe, 2000). As demonstrated by Corsi et al.(2008), resuspension of PM10is much larger than PM2.5suggesting that even the low MERV filters can retain many of the larger par
45、ticles from vacuuming activities. Figure 1 presents the mean culturable microbial concen-trations in the HVAC filter dust from the nine sites investi-gated, expressed as CFU/g dust. Since two filters were collected and analyzed from each site, 18 total samples are represented in Figure 1 and the mea
46、n value for each site is shown. For each site, the left bar indicates the culturable concentration of bacteria while the right bar represents the Table 2. Site CharacteristicsSite #Year BuiltNumber of OccupantsProximity to Highway,km (miles)Attached GarageCarpet Filter LocationConditioned Volume, m3
47、 (ft3)Cooling Duty Cycle, %1 1975 2 1.0 (0.62) Yes No Unit 422 (14,900) 142 1973 2 0.6 (0.37) Yes Yes Unit 309 (10,900) 163 1998 1 0.2 (0.12) Yes No Register2114 (4,020) 94 1998 1 0.2 (0.12) Yes Yes Register 227 (8,010) 275 1949 2 1.8 (1.12) No No Register 276 (9,740) 326 1941 4 1.1 (0.68) No Yes Re
48、gister 324 (11,400) 297Late 70s14 0.6 (0.37) No Yes Unit 259 (9,140) 348 1984 3 0.5 (0.31) Yes Yes Unit 308 (10,900) 159 1995 3 0.2 (0.12) No Yes Register3656 (23,200) 191Estimated based on neighborhood and nearby homes.2Three filters in different return grilles were present at this site.3Two filter
49、s in different return grilles were present at this site.ASHRAE Transactions 487culturable fungal concentration. The height of each bar indi-cates the mean culturable concentration and originated from the counts of the microbes with the ability to form colonies on the specific agar plates described in the Methodology section. The bottom section of each bar represents the spore forming fraction of the population, which is the fraction of the viable microbial concentration able to survive the pasteurization treatment. Only the error bars for the total height of the columns
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