Critical control points for the management of microbial growth in HVAC systems

Transcription

1 Indoor Air 2008, August 2008, Copenhagen, Denmark - Paper ID: 707 Critical control points for the management of microbial growth in HVAC systems Sebastian Gommers, Francesco Franchimon * and Johanna E.M.H. van Bronswijk Public health engineering group of Architecture, Building & Planning, Technische Universiteit Eindhoven, Eindhoven, the Netherlands * Corresponding f.franchimon@tue.nl SUMMARY Office buildings with HVAC systems consistently report Sick Building Symptoms that are derived from microbial growth. We used the HACCP methodology to find the main critical control points (CCPs) for microbial management of HVAC systems in temperate climates. Desk research revealed relative humidity (RH) as a main limiting factor, with fungi needing at least 65% RH and bacteria 91%. Particle deposition inside the HVAC system provides the necessary growth substrate. Filters exposed to outdoor climatic conditions are vulnerable to fungal growth 80% of the time in a year, making them the first main CCP. High RH is also found at the cooling section (the 2 nd main CCP). Particle deposition is highest in bends of the ductwork (3 rd main CCP). We recommend microbial monitoring and controlling of the humidity level at these 3 CCPs to manage microbial growth in HVAC systems. KEYWORDS Indoor Air 2008, HVAC, Microbial contamination, HACCP INTRODUCTION Occupants of office buildings with HVAC systems consistently report more SBS (Sick Building Symptoms) than occupants of buildings with natural ventilation (Finnegan et al., 1984; Robertson et al., 1985). This is considered to be a microbial pollution problem (Batterman and Burge, 1995; Bluyssen P.M. et al., 2003). In this study we used the HACCP (Hazard Analysis and Critical Control Points) (Franchimon et al., 2005) methodology to assess microbial growth in HVAC systems in temperate climates. The aim is to identify the main critical control points (CCPs) to be used in microbial management. METHODS This research is conducted for temperate climates, the so called C-climates according to Köppen s (1923) climate classification. These climates have distinct seasonal characteristics and the temperature differences between summer and winter are not extreme. The climate data of a reference year for the Netherlands (ISO ) was used to acquire hourly values of outdoor temperature and humidity values throughout a year. SPSS and MS Excel 2003 SP2 were used to obtain descriptive statistics. Subsequently temperature and relative humidity (RH) during the year were calculated in different sections of the HVAC system.

2 Particle deposition in HVAC systems leads to soiling (dust layer formation), a feeding ground for bacteria and fungi. To characterize particle deposition in HVAC systems a literature search was performed in the database of Science Direct (1998 to present) with different combinations of the key words: particle, deposition, dust, and duct. RH is most influential for growth of fungi and bacteria on soiled surfaces (Bronswijk, 1981). A literature search revealed taxa of fungi and bacteria that were previously found or could be expected to reside in HVAC systems. Matching the humidity requirements of the organisms with the condition in the HVAC systems should reveal the CCPs to focus on for the management of microbial growth. RESULTS Outdoor climate RH plays an important role in microbial growth and development. Outdoor RH ranges from 30 to 100% (Table 1). Table 1. Percentiles for outdoor relative humidity of the Netherlands reference year (ISO ). N Valid 8760 Missing 0 Minimum 30 Maximum 100 Percentiles Indoor climate HVAC systems are designed to create indoor conditions according to temperature set points, commonly 18ºC (winter) and 20 o C (summer). We took spring and autumn set points as equal to summer and winter set points respectively. HVAC system The air handling system (AHS) under study consists of an air conditioned centralized air handling unit (AHU). The supply side of the AHU contains filters, heating coils, cooling coils and a fan (Figure 1).

3 HVAC h-x diagram A psychrometric chart was used to determine the influence of the AHU on RH in the HVAC system. Hourly outdoor temperature, RH and the design temperature were taken into account. We used the following equations: T 17,6 Pds = 0,813 e 0,2 (1) Where Pds (kpa) is the saturation vapor pressure and T ( C) is the temperature. Pd RH = Pds (2) 100 Where Pd is the partial vapor pressure (kpa) and RH (%) is the outdoor RH. Pd x = 0,622 (3) (101,3 Pd) Where x is the moisture content (g/kg) and the atmospheric pressure is kept constant at kpa. The partial vapor pressure before and after the AHU are assumed to be identical ( Pd 2 = Pd 1 ). Pd RH = 100% (4) Pds Using the above formulas, RH of the air that exits the AHU and flows through the air ducts is calculated. In total 20% of the hours show a RH of 68% or more (Table 2). Condensation (calculated RH value of 100% or higher) occurred 1% of the reference year, when both outdoor temperature and air water content (g/kg) were high. Table 2. percentiles for relative humidity. N Valid 8760 Missing 0 Minimum 9 Maximum 119 Percentiles

4 Particle deposition Dust (accumulation of settled air-borne particles) forms a substrate on the inner surface of the HVAC system that functions as a feeding ground for microbes. Aspergillus spp. and Penicillium spp. are known to proliferate on soiled steel surfaces (Pasanen et al., 1992). Particles enter the HVAC system through the air intake of the AHU, from the indoors when air is re-circulated, or originate from fungi that grew through the filters and sporulate at the clean site (Elixmann et al., 1989). Ventilation air ducts and the AHU itself are subject to deposition of air-borne particles. Dust accumulates in air filters and on surfaces in the HVAC system (Batterman and Burge, 1995). It is also found on the bottom surface of straight ducts and especially in bends due to segregation of the boundary layer of air (Wallin and Malmström, 1995). Bacteria and Fungi Spores of microorganisms are always present in the air. A wide range in the number and type of fungi and bacteria are found in outdoor air depending on the time of the day, the weather, season and geographical location. According to Lacey in Building Mycology (Singh, 1994) the Cladosporium and Alternaria are most abundant in outdoor air in temperate climates, but are less abundant in indoor environments (Bronswijk, 1981). The majority of indoor fungal spores are produced indoors in spaces and the HVAC system. Fungi will grow on any substrate that contains nutrients and moisture. The availability of water to fungi depends on the material s water activity (a w ). Below 0,65a w there is no growth. As the water activity increases more species of fungi are able to grow and growth becomes more rapid (Singh, 1994). The a w is a quotient of water vapor pressure over the substrate and the saturation pressure of water at a given temperature. The a w is also known as relative equilibrium moisture. When RH is in equilibrium over the substrate, equation (7) expresses the relationship between RH and the a w -value. RH (%) a 100 (7) = w When the a w exceeds 0.65, fungi are able to grow (Table 3). The lowest a w for bacterial growth is All fungi mentioned in Table 3 are known producers of allergens (Bronswijk, 1981).

5 Table 3. Known fungi found on building materials, paints, finishing and furnishing materials (Schleibinger et al., 2004), soiled surfaces and home textiles in buildings (Bronswijk, 1981), and their required water content. Minimal water activity a w > 0.65 (xerohygrophilic species) a w > 0.80 (mesohygrophilic species) a w > 0.90 (hydrophillic species) microorganism Aspergillus fumigatus group A. repens goup Penicillium brevicompactum Wallemia sebi Aspergillus candidus A. flavus A. versicolor Cladosporium sphaerospremum Penicillium chrysogenum P. cyclopium P. frequentans P. piceum Scopulariopsis brevicaulis Absidia corymbifera Acremonium murorum Alternaria alternata Cladosporium cladosporioides Fusarium spp Gliocladium roseum Mucor spp Penicillium spp Stachybotrys chartarum Trichothecium roseum DISCUSSION The HACCP methodology is applied to air as a consumption commodity. To guarantee quality monitoring is needed of the processing from outdoor air to the air we breathe. By viewing the whole process from a microorganism point of view we are able to locate CCPs in the process. Previous studies focused on identifying microorganisms in the HVAC system (Siegel and Walker, 2001; Pasanen et al., 1992; Chang et al., 1996; Simmons and Crow, 1995; Hugenholtz and Fuerst, 1992). We pinpointed the most risky areas (Figure 1). Microbial risks of filters were reported earlier (Elixmann et al., 1989; Yamada et al., 2006; Batterman and Burge, 1995). Pasanen et al. (, 1991) experimented with filters removed from offices and found that high RH stimulated fungal growth on the filters. Since humidity conditions in filters are suitable 80% of the time in a reference year (Table 1) and the dust collected makes a good substrate for microorganisms (Simmons and Crow, 1995), filters are the first CCP that should be monitored continuously for microbial growth. Figure 1. Most important Critical Control Points in a HVAC system; 1 = filter section; 2 = cooling section; 3 = bends in ducts. The second CCP is the cooling section of the AHU. Hugenholtz and Fürst (1992) found extensive bacterial growth in the biofilm on a cooling coil. Modeling and experimental results

6 by Siegel and Walker (2001) show a potential for indoor bioaerosols to deposit and be viable on heat exchangers. When temperatures and RH are high, the cooling section will decrease the temperature to the given set point. The cooling effect can result in condensation on the cooling coils and on any following components. This provides a RH supportive of even hydrophilic fungal species and bacteria. Psychrothermophilic bacteria grow in these high humidity and low (<15 C) temperature areas. The third CCP can be found in the ductwork. Particles that pass the filter accumulate on the duct inner surface. Due to the large inner surface area of ducts and the long operation time, a significant amount of dust accumulates in the ducts (Bluyssen P.M. et al., 2003), enough to promote the growth and survival of microorganisms (Chang et al., 1996). Pasanen et al. (1993) found fungal spores germinating and sporulating on soiled steel surfaces of the ventilation system, after the surface was moistened for only a few hours. In our study RH in ducts exceeded 68% RH in 20% of the time of the reference year. This will support growth of xerophilic fungi. Occasionally condensation occurs, supporting mesothermophilic (15 C <T<40 C) bacteria. Humidification is not considered in this study since humidifiers are scarcely used in temperate climates. Humidifiers will worsen the situation by providing better conditions for microbial growth (Batterman and Burge, 1995; Bluyssen P.M. et al., 2003). Other environmental factors, such as ph and surface roughness are also of influence on microbial growth but were not taken into account in this study. In addition, calculations were only based on a reference year of the Dutch climate. Field studies in different temperate regions are required to confirm the location of the CCPs and to determine the efficacy of humidity management. CONCLUSIONS There are 3 main CCPs to be managed to limit microbial growth in HVAC systems: filters, cooling section and beds in ducts. We recommend microbial monitoring and controlling the humidity levels at these points. ACKNOWLEDGEMENT We are grateful to Royal Haskoning, division Building Services for financial support. Publication of research results does not imply the endorsement or agreement with these findings by Royal Haskoning.

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