**1. Introduction**

Microorganisms are among the most important sources of poor indoor air quality, and contamination of indoor air by microbial pollutants is being increasingly recognized as a public health problem and as one of the factors contributing to sick building syndrome. Bioaerosols, such as those comprising fungi, bacteria, and viruses, in indoor air can cause allergic and infectious diseases, respiratory problems, and hypersensitivity reactions. People who are sensitive to indoor environmental problems complain of a wide variety of symptoms, ranging from headache, tiredness, nausea, and sinus congestion to eye, nose, and throat irritations [1].

Although invisible to the naked eye, the atmosphere is populated by a diversity of microorganisms, including bacteria, fungi, algae, and protozoa. Researchers have estimated that the total bacterial count within the troposphere layer ranges from 1 × 103 to 1 × 105 cells/m3 . Dust is formed during the passage of organic and inorganic particles from external and internal resources, which subsequently aggregate and precipitate. House dust, for example, consists of cotton fibers, hair, bacteria, molds, and remaining paint particles [2, 3]. The findings of a previous study have indicated that the average number of fungi contaminating 820 indoor air-conditioning units was 1252 CFU/m3 , with range of 17–9100 CFU/m3 . In addition, Baxter [4] found that the average number of spores isolated from 85 buildings was 913 cells/m3 , ranging from 68 to 2307 cells/ m3 . Daily and seasonal numbers of contaminant microorganisms in the air vary and depend primarily on environmental factors, such as vegetation, human activities, and seasonal fluctuations [5]. Most of these microorganisms are bacteria and fungi [6].

These microbial contaminants affect the residents of enclosed and humid buildings, particularly in the case of toxic hygrophytic fungi, such as *Phoma* sp., *Exophiala* sp., *Aureobasidium pullulans*, *Acremonium* sp., and *Sporobolomyces*, that are frequently isolated from the cooling pipes of air-conditioning systems. Gram-negative bacteria and their toxins are also isolated from leaks in air-conditioning pipes. Yang [7], for example, identified *Legionella pneumophila*, which is the causal agent of legionnaires disease, as a dominant bacterium in the water leaking from cooling systems. In addition, *Pseudomonas aeruginosa*, which has also been isolated from water leaking from air-conditioning systems, is an opportunistic bacterium responsible for several diseases. Many studies have proven that the heating, ventilation, and air-conditioning (HVAC) systems can become contaminated with organic pollutants, bacteria, and fungi, as well as by particulate matter derived from mice, insects, and nematodes. The bacteria and fungi colonizing these systems tend to saprophytic and thrive in areas that meet their environmental requirements [7]. Fungi have been proven to be a source of airborne contamination in air-conditioning systems [8], including *Alternaria*, *Aspergillus flavus*, *Aspergillus fumigatus*, *Aspergillus niger*, *Aspergillus ochraceus*, *Aspergillus versicolor*, *Botrytis cinerea*, *Cladosporium herbarum*, *Epiccocum purpurascens-sterilia*, *and Penicillium* spp., among which *A. fumigatus*, which has been isolated from air-conditioning filters, is responsible for many dangerous infections. With regard to bacteria, *Propionibacterineae*, *Staphylococcus*, *Streptococcus*, and *Corynebacterineae* (17, 17.5, 20, and 3%, respectively) have been detected in aeration pipes and air filters installed in indoor areas [9, 10]. In addition small percentages of species from the genera *Fusobacterium* and *Veillonella* (0.02 and 0.1%, respectively), which are associated with the mouth cavity and saliva, have also been identified as air-conditioning system contaminants [11–13].

With a view toward providing clean indoor air, several studies have been conducted to investigate measures that can be used to control the levels of microorganisms that colonize filtering, heating, ventilation, and air-conditioning systems. In this regard several types of air filters have been studied with the aim of preventing the penetration of particles. However, although high-efficiency particulate air (HEPA) filters are widely used in hospitals, *Aspergillus*-associated infections continue to occur [14]. Currently, most indoor air-conditioning systems contain internal filters that extract microorganisms from the air (**Figure 1**). However, these microbes often remain viable and can be returned to the surrounding atmosphere under certain circumstances, such as inefficient operation, during periods of maintenance, or due to temporary malfunction [15].

It is widely acknowledged that air-conditioning filters do not remove all the particles from the air. Even the use of HEPA filters will not completely eliminate the problem of microbial contamination, as this material will only retain particles of a minimum of 3 microns in size. Thus, dust particles with sizes smaller than 3 microns will pass through unhindered. Furthermore, when the filters become excessively wet, they can provide a fertile environment for the proliferation of molds and bacteria [16, 17].

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attraction.

**Figure 1.**

**2. Principle of air filtration**

*Accumulated dust on discarded polyester filters.*

**2.1 How filters work**

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

In this chapter, several new topics related to environmentally sustainable buildings were presented, as it clearly describes the potential impact of HVAC systems on the indoor air quality with the aim to enhance the healthy buildings. The chapter is structured as follows: besides the Introduction (Section 1), Section 2 introduces the principle of air filtration, Section 3 is concerned with presenting the traditional air filters, Section 4.5 demonstrates a comparison between the most common and modern HVAC filters, Section 6 provides the impact of HVAC filters on indoor air quality, and Section 7 is concerned with presenting several results for research progress about the relationship between microbes and traditional filters and microbial colonization of the types of filters commonly used in air-conditioning systems.

There are five different collection mechanisms that determine air filtering performance: straining, interception, diffusion, inertial separation, and electrostatic

*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 1.** *Accumulated dust on discarded polyester filters.*

In this chapter, several new topics related to environmentally sustainable buildings were presented, as it clearly describes the potential impact of HVAC systems on the indoor air quality with the aim to enhance the healthy buildings. The chapter is structured as follows: besides the Introduction (Section 1), Section 2 introduces the principle of air filtration, Section 3 is concerned with presenting the traditional air filters, Section 4.5 demonstrates a comparison between the most common and modern HVAC filters, Section 6 provides the impact of HVAC filters on indoor air quality, and Section 7 is concerned with presenting several results for research progress about the relationship between microbes and traditional filters and microbial colonization of the types of filters commonly used in air-conditioning systems.
