**7.1 The relationship between microbes and traditional filters**

We have previously examined the nature of the relationship between filters and airborne microbes, using small pieces (1 × 1 cm) of traditional filters (sponge, polyester, and HEPA) These materials were sterilized with alcohol, then dried, and subsequently moistened with glucose yeast extract medium. We then prepared suspensions of the examined fungal strains (*Aspergillus niger*, *Aspergillus flavus*, *Cladosporium* sp*.*, and *Rhizoctonia* sp.). These were retained in sterilized petri dishes, whereas other groups were prepared for carbon-free sources. They were monitored for 1 to 3 months and thereafter examined under a microscope. Heavy growth of mycelium was observed. The fungal filaments are looped around filter fibers also assembled into the filter cavities, forming a tangled knot of fungal mycelium and filters fibers (**Figure 11**).

**Figures 12**–**15** show that the microscopic structures of the filters shown in **Figure 11** have been colonized by *Aspergillus niger* in (**Figure 12**), *Aspergillus flavus* in (**Figure 13**), *Rhizoctonia* in (**Figure 14**) and *Cladosporium* in (**Figure 15**) which indicate that these fungi use the filters as a support for fungal mycelium.

Microorganisms can exploit various parts of air-conditioning systems, including filters, as sheltered sites, which are conducive to rapid grow and reproduction [16, 17, 56]. The high levels of humidity in air-conditioning systems [16, 57] and the accumulated dust in the filters and other parts of these systems provide an environment that is suitable for the growth of a range of different microbes.

Microorganisms can secrete a diverse array of extracellular enzymes to exploit the various available filter materials, such as cellulose, as sources of nutrition [56, 58].

Kuehn [57] pointed out that moisture promotes fungal growth in filter tissues and can also favor bacterial reproduction leading to subsequent transmission to and dispersal within indoor environments. Such moisture often originates from the drops of condensate that form air-conditioning towers [59].

Maus [60] have suggested that the spores of some bacteria and fungi trapped within air filters can retain their viability and reproduce under the prevailing environmental conditions. These microorganisms can be dispersed through purification and air-conditioning systems and be inhaled by workers and residents in buildings [48].

Microbiological particles constitute one of the most important sources of air pollution that determine the purity of the air. It is known that atmospheric air is a carrier of disease-causing organisms, including fungal spores and microbial

#### **Figure 11.**

*Structure of filters observed under an optical compound microscope. The right-hand panel shows sponge filter cavities of different sizes. These openings are wider than those of other filters. The middle panel shows a polyester filter, which is characterized as a network installation with narrower openings than the sponge filter that are regular in shape. The left-hand panel shows a HEPA filter, characterized by complex knit and numerous narrow openings that increase efficiency by preventing the passage of fine particles.*

#### *Low-temperature Technologies*

cells, the concentrations of which vary widely according to environmental condition, particularly the nature of the internal environment and the various activities of humans who reside or work therein [61]. In order to achieve the desired level of microbial contamination control in air-conditioning systems and develop suitable air purification techniques, it is generally necessary to conduct extensive studies [17].

Hamada and Fujita [62] noted that the contamination of filters tends to be very low during the first year after installation, reaching 257 cells/m3 of room air, whereas by the sixth year of use, they found that the number of contaminating cells had increase threefold to 692.

Durand [63] demonstrated that species of *Aspergillus* and *Penicillium* are the fungi most commonly isolated from the filters of air-conditioning systems, whereas species of *Cladosporium* and other types tend to be detected at relatively low rates.

*The mycelial growth and conidiophores of Aspergillus niger on the studied filters: (a) sponge, (b) polyester, and (c) HEPA.*

**197**

**Figure 13.**

*and (c) HEPA.*

*Impact of Air-Conditioning Filters on Microbial Growth and Indoor Air Pollution*

For bacteria, species of *Actinomycetes* and *Bacillus* (cocci and Gram-negative types)

*The mycelial growth and conidiophores of Aspergillus flavus on the studied filters: (a) sponge, (b) polyester,* 

Al-Abkari [17] recommended that air-conditioning filters should be cleaned regularly and that regular maintenance is necessary to prevent an accumulation of contaminants and to remove the suspended dust. In this regard, spongy filters can be readily washed and cleaned with detergents, and be reused after cleaning. In contrast, HEPA filters, which consist of interlocking fibers, are very difficult to wash and clean and should thus be replaced on a regular basis. Furthermore, it has been demonstrated that the number of microbial colonies (bacteria and fungi) growing on culture dishes that were exposed to air that had passed through different filters increased after 30 min and then decreased after 60 min. This indicates an inverse relationship between the period of operation of the air-conditioning system

Al-Abkari [17] examined the extent of microbial growth on the most common types of filter used in the eastern region of Saudi Arabia (sponge, polyester, and HEPA) and found that sponge filters harbored the highest microbial moist mass, reaching 0.177 and 0.257 gm for bacteria and fungi, respectively.

tend to be the most commonly isolated.

and the quantity of air that had been purified [64].

*DOI: http://dx.doi.org/10.5772/intechopen.88548*

*Impact of Air-Conditioning Filters on Microbial Growth and Indoor Air Pollution DOI: http://dx.doi.org/10.5772/intechopen.88548*

**Figure 13.**

*Low-temperature Technologies*

had increase threefold to 692.

studies [17].

cells, the concentrations of which vary widely according to environmental condition, particularly the nature of the internal environment and the various activities of humans who reside or work therein [61]. In order to achieve the desired level of microbial contamination control in air-conditioning systems and develop suitable air purification techniques, it is generally necessary to conduct extensive

Hamada and Fujita [62] noted that the contamination of filters tends to be

whereas by the sixth year of use, they found that the number of contaminating cells

Durand [63] demonstrated that species of *Aspergillus* and *Penicillium* are the fungi most commonly isolated from the filters of air-conditioning systems, whereas species of *Cladosporium* and other types tend to be detected at relatively low rates.

*The mycelial growth and conidiophores of Aspergillus niger on the studied filters: (a) sponge, (b) polyester, and* 

of room air,

very low during the first year after installation, reaching 257 cells/m3

**196**

**Figure 12.**

*(c) HEPA.*

*The mycelial growth and conidiophores of Aspergillus flavus on the studied filters: (a) sponge, (b) polyester, and (c) HEPA.*

For bacteria, species of *Actinomycetes* and *Bacillus* (cocci and Gram-negative types) tend to be the most commonly isolated.

Al-Abkari [17] recommended that air-conditioning filters should be cleaned regularly and that regular maintenance is necessary to prevent an accumulation of contaminants and to remove the suspended dust. In this regard, spongy filters can be readily washed and cleaned with detergents, and be reused after cleaning. In contrast, HEPA filters, which consist of interlocking fibers, are very difficult to wash and clean and should thus be replaced on a regular basis. Furthermore, it has been demonstrated that the number of microbial colonies (bacteria and fungi) growing on culture dishes that were exposed to air that had passed through different filters increased after 30 min and then decreased after 60 min. This indicates an inverse relationship between the period of operation of the air-conditioning system and the quantity of air that had been purified [64].

Al-Abkari [17] examined the extent of microbial growth on the most common types of filter used in the eastern region of Saudi Arabia (sponge, polyester, and HEPA) and found that sponge filters harbored the highest microbial moist mass, reaching 0.177 and 0.257 gm for bacteria and fungi, respectively.

**Figure 14.**

*The mycelial growth of Rhizoctonia sp. on the studied filters: (a) sponge, (b) polyester, and (c) HEPA.*

Comparatively, the bacterial mass recorded on polyester and HEPA reached 0.024 and 0.037 gm, respectively, and the corresponding fungal mass on these filters was 0.072 and 0.047 gm, respectively. Douwes [65] isolated polysaccharide compounds known to be excreted by various fungi that grow on dust in residential homes, and the detection of these compounds is accordingly considered to be a good indicator of the presence of these fungi.

Moray and Williams [66] performed direct microscopic observations of the porous soft filters typically used in air-conditioning systems and accordingly identified pollen grains, cellulose fibers, synthetic fibers, decayed plant leaves, hairs, parts of insects, dust, mites, and numerous organic compounds, all of which can provide a refuge for microbes.
