Evaluation of HVAC filters as a sampling mechanism for indoor microbial communities
Research highlights
► Microbial communities in HVAC filter dust not different from those in high surface dust in residences. ► Greater Proteobacteria in HVAC filters than on high surface samples. ► Greater gram-positive bacteria in residences than unoccupied test house. ► Microbial communities in HVAC filter not different from composited indoor air. ► HVAC filters show promise as integrated indoor samplers for microbiological contaminants.
Introduction
The presence of microorganisms indoors has been related to several health and discomfort outcomes including respiratory diseases, odors, and occupant dissatisfaction (Gyntelberg et al., 1994, Verhoeff and Burge, 1997). Some researchers have associated indoor microbial concentrations with asthma symptoms (Park et al., 2006, Ross et al., 2000, Smedje et al., 1997). However, the association between culturable fungal levels in air or dust samples and health problems has been inconsistent (Nelson et al., 1995, Peat et al., 1998, Verhoeff and Burge, 1997). This discrepancy may be attributable to the fact that bioaerosol samples are typically short-term in nature and provide only a snapshot of microbial contaminant levels in air at a particular time and place. Even when collected from the same location, airborne bacterial samples have significant temporal variability (Fierer et al., 2008), highlighting the need to develop an integrative methodology to assess indoor biological contaminants. Floor dust may provide an integrated sample of contaminant levels but these samples are influenced by material tracked-in from the outside and may be skewed towards larger particle-bound contaminants (Lewis et al., 1999).
The majority of previous indoor biological studies have relied on an assessment of culturable microorganisms that represent only a small fraction of the total microorganisms present indoors (Toivola et al., 2002). In recent years, several studies have applied culture-independent, DNA-based approaches to better characterize the diverse bacterial and fungal communities present in indoor environments (Kelley et al., 2004, Pakarinen et al., 2008, Pitkäranta et al., 2008, Rintala et al., 2008, Täubel et al., 2009, Tringe et al., 2008). The application of molecular biology tools to indoor environmental investigations should reveal a much greater fraction of the microbial community present than culturable methods, a finding recently confirmed by Pitkäranta et al. (2008). Vesper et al. (2007) reported an association between asthma symptoms and the Relative Moldiness Index (RMI), an index based on molecular biology tools, confirming that these techniques may provide a better characterization of health effects from microorganisms.
A potential alternative to the use of settled dust and air samples for microbial evaluation is the use of heating, ventilation, and air conditioning (HVAC) filters for indoor environment investigations. Collecting samples of HVAC dust may improve our understanding of indoor occupant exposure by providing an integrated measure of pollutant concentrations associated with indoor particles. Greater than 70% of the residential buildings in the United States have a central forced-air HVAC system (U.S. Bureau of Census, 2005), almost all with a built-in filtration system. These filters essentially serve as passive, long-term samplers that can be collected with minimal effort and analyzed for a broad range of indoor contaminants. Recently, Stanley et al. (2008) utilized filters in two large public buildings as bioaerosol sampling devices to determine the culturable bacteria concentrations for selected species present in air. While most of the molecular-based studies described above focused solely on settled dust, Tringe et al. (2008) investigated the bacterial communities present on the dust that collected on two HVAC filters in two large shopping centers in Singapore. They reported that the two HVAC filters (air samples) have more in common to each other than with environmental (outdoor soil and water) samples collected nearby. They also found greater similarity between the bacterial communities in filter samples and indoor floor dust compared to outdoor ground-level dust suggesting that the filter community originates from an indoor niche. The purpose of the current study is to explore the microbial concentrations and communities on filters and compare them to indoor settled dust and air communities as a first step towards using HVAC filter dust as an integrated measure of microbial levels in residences. This paper focuses on bacterial and fungal culturable concentrations and DNA-based analysis of microbial communities present in HVAC filter dust and other indoor sampling locations in occupied residences and in an unoccupied full-scale test house (UTest House). The investigation was divided into two phases: 1) Investigation of culturable microbial concentrations in settled and HVAC filter dust in eight occupied residences in Austin, Texas; 2) Study of microbial communities, using a culture-independent approach in four sites as well as in the full-scale UTest House where more detailed sampling was performed.
Section snippets
Phase 1: culturable microorganisms
A sample of convenience of eight residential buildings located in Austin, Texas was selected for this part of the investigation. More detailed information regarding the sites and the occupants is reported in Noris et al. (2009). All of the buildings had central air conditioning that recirculated indoor air, as is typical of residences in the Southern U.S.
Sample collection
Floor dust, high surface dust and HVAC filter dust samples were collected 2–3 times over a six-month period from each residence during the
Phase 1: culturable microorganisms
Fig. 1 shows the mean bacterial and fungal culturable concentrations for each of the sampling locations investigated in the residences. Multiple samples at the same site are given equal weighting; there are 21 samples each of floor and high surface dust, 16 HVAC filter dust samples and five air samples shown in the figure.
The culturable concentrations for both fungi and bacteria in Fig. 1 were generally consistent with the published literature. Indoor concentrations for bacteria and fungi vary
Conclusions
This study evaluated the use of HVAC filters as long-term air samplers for indoor biological contamination. Microbial concentrations and communities on HVAC filter dust samples were not statistically different from high surface dust samples in occupied residences. Proteobacteria were present in greater proportion in HVAC filter dust samples than in high surface dust samples and they were present in greater proportion in the unoccupied UTest House than in residences suggesting the outdoor air
Acknowledgments
This study was partially supported by an ASHRAE Grant-in-aid and the NIOSH Pilot Project Research Training Program. The authors would also like to thank Brent Stephens for his help in the filter collection and the Pace group at University Colorado at Boulder for their assistance in the methodology development.
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