**2.1 How filters work**

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

*Low-temperature Technologies*

with range of 17–9100 CFU/m3

of spores isolated from 85 buildings was 913 cells/m3

air-conditioning system contaminants [11–13].

maintenance, or due to temporary malfunction [15].

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

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

. Dust is formed during the passage of organic and inorganic particles

. In addition, Baxter [4] found that the average number

, ranging from 68 to 2307 cells/

,

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

. 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

cells/m3

1 × 105

m3

**180**

molds and bacteria [16, 17].

The first of these mechanisms applies mainly to mechanical filters and is influenced by particle size. **Figures 6** to **10** illustrate the five mechanical principles of particle capture and their contribution to the retention of particles of different sizes.

Straining (sieving) occurs when the opening between the media components (e.g., fibers, screen mesh, and corrugated metal) is smaller than the diameter of the particle the filter is designed to capture. This principle spans across most filter designs and is entirely related to the size of the particle, media spacing, and media density (**Figure 2**).

Interception occurs when a large particle, because of its size, collides with a fiber in the filter that an air stream is passing through (**Figure 3**).

Diffusion occurs when the random (Brownian) motion of a particle causes that particle to come into contact with a fiber. When a particle vacates an area within the media, by attraction and capture, it creates an area of lower concentration within the medium into which another particle diffuses, only in turn to be captured itself. To enhance the likelihood of this attraction, filters employing this principle operate at low media velocities and/or high concentrations of microfine fibers, glass, or otherwise (**Figure 4**).

Inertial separation is based on a rapid change in air direction and the principles of inertia to separate particulate matter from the air stream. Particles moving at a certain velocity tend to remain at that velocity and travel in a continuous direction. This principle is normally applied when there is a high concentration of coarse particulate matter and in many cases represents a pre-filtration stage prior to the passage of air through higher-efficiency final filters (**Figure 5**).

Electrostatic attraction is obtained by charging the media as a part of the manufacturing process (**Figure 6**). However, it plays a minor role in mechanical filtration. After fiber contact is made, smaller particles are retained on the fibers by a weak electrostatic force. The force may be created through a manufacturing process or be dependent upon airflow across media fibers. The force is eradicated as media fibers collect contaminants that act as an insulator to a charge. Electrostatic filters, which are composed of polarized fibers, may lose their collection efficiency over time or when exposed to certain chemicals, aerosols, or high relative humidity. A decrease in pressure in an electrostatic filter generally increases at a slower rate than it does in a mechanical filter of similar efficiency.

Inertial separation and interception are the dominant collection mechanisms for particles greater than 0.2 μm in size, whereas diffusion is dominant for particles less than 0.2 μm in size.

As mechanical filters become loaded with particles over time, their collection efficiency and reduction in pressure typically increase. Eventually, the decrease

**183**

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

*Model of interception effect mechanism, depends on the collision between the fiber and the particle passing* 

in pressure significantly inhibits airflow, and when this occurs, the filters must be replaced. For this reason, a decrease in pressure across mechanical filters is often monitored, as this can provide an indication of when the filters need to be replaced. Thus, unlike mechanical filters, a decrease in the pressure of electrostatic filters is a poor indicator of the need to change filters. When selecting an HVAC filter, these differences between mechanical and electrostatic filters should be borne in mind because they will have an impact on filter performance (collection efficiency over

*Model of inertial separation mechanism, depends on the collision between the fiber and the small particles for* 

*Model of diffusion mechanism, depends on the motion of the particle causing contact with a fiber.*

time), as well as on maintenance requirements (changeout schedules).

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

**Figure 3.**

**Figure 4.**

**Figure 5.**

*reducing its velocity.*

*through the filter.*

**Figure 2.** *Model of straining (sieving) mechanism, depends on the space between the fibers.*

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

#### **Figure 3.**

*Low-temperature Technologies*

density (**Figure 2**).

otherwise (**Figure 4**).

than 0.2 μm in size.

The first of these mechanisms applies mainly to mechanical filters and is influenced by particle size. **Figures 6** to **10** illustrate the five mechanical principles of particle capture and their contribution to the retention of particles of different sizes. Straining (sieving) occurs when the opening between the media components (e.g., fibers, screen mesh, and corrugated metal) is smaller than the diameter of the particle the filter is designed to capture. This principle spans across most filter designs and is entirely related to the size of the particle, media spacing, and media

Interception occurs when a large particle, because of its size, collides with a fiber

Diffusion occurs when the random (Brownian) motion of a particle causes that particle to come into contact with a fiber. When a particle vacates an area within the media, by attraction and capture, it creates an area of lower concentration within the medium into which another particle diffuses, only in turn to be captured itself. To enhance the likelihood of this attraction, filters employing this principle operate at low media velocities and/or high concentrations of microfine fibers, glass, or

Inertial separation is based on a rapid change in air direction and the principles of inertia to separate particulate matter from the air stream. Particles moving at a certain velocity tend to remain at that velocity and travel in a continuous direction. This principle is normally applied when there is a high concentration of coarse particulate matter and in many cases represents a pre-filtration stage prior to the

Electrostatic attraction is obtained by charging the media as a part of the manufacturing process (**Figure 6**). However, it plays a minor role in mechanical filtration. After fiber contact is made, smaller particles are retained on the fibers by a weak electrostatic force. The force may be created through a manufacturing process or be dependent upon airflow across media fibers. The force is eradicated as media fibers collect contaminants that act as an insulator to a charge. Electrostatic filters, which are composed of polarized fibers, may lose their collection efficiency over time or when exposed to certain chemicals, aerosols, or high relative humidity. A decrease in pressure in an electrostatic filter generally increases at a slower rate

Inertial separation and interception are the dominant collection mechanisms for particles greater than 0.2 μm in size, whereas diffusion is dominant for particles less

As mechanical filters become loaded with particles over time, their collection efficiency and reduction in pressure typically increase. Eventually, the decrease

in the filter that an air stream is passing through (**Figure 3**).

passage of air through higher-efficiency final filters (**Figure 5**).

than it does in a mechanical filter of similar efficiency.

*Model of straining (sieving) mechanism, depends on the space between the fibers.*

**182**

**Figure 2.**

*Model of interception effect mechanism, depends on the collision between the fiber and the particle passing through the filter.*

**Figure 4.** *Model of diffusion mechanism, depends on the motion of the particle causing contact with a fiber.*

#### **Figure 5.**

*Model of inertial separation mechanism, depends on the collision between the fiber and the small particles for reducing its velocity.*

in pressure significantly inhibits airflow, and when this occurs, the filters must be replaced. For this reason, a decrease in pressure across mechanical filters is often monitored, as this can provide an indication of when the filters need to be replaced. Thus, unlike mechanical filters, a decrease in the pressure of electrostatic filters is a poor indicator of the need to change filters. When selecting an HVAC filter, these differences between mechanical and electrostatic filters should be borne in mind because they will have an impact on filter performance (collection efficiency over time), as well as on maintenance requirements (changeout schedules).

**Figure 6.**

*Model of electrostatic attraction mechanism, depends on charging the fiber to retain the small particles by a weak electrostatic force.*
