**6. Impact of HVAC filters on indoor air quality**

Filtration technology is currently an integral aspect of air purification techniques that focus on particulate matter, the most common examples of which are fibrous filters [38]. The use of glass fiber filters is another mature filtration technique with a proven high efficiency (99.0%), similar to that of the HEPA filters [39]. Furthermore, wire mesh filters can also provide good filtration efficiency down to sizes of 2–10 μm [40].

On the basis of particle filtration efficiency, air filters can be divided into four types: prefilters, medium filters, HEPA filters, and ultralow particulate air (ULPA) filters [41]. The filtration efficiency of ULPA filters is greater than 99.999%, with particles of diameters down to 0.12–0.17 μm being effectively trapped [42]. Similarly, HEPA material has a strong ability for trapping particles and can remove 99.97% of particulate matter, smog, and bacteria with sizes down to 0.3 μm (**Figure 8**). In contrast, the efficiency of medium filters is only between 60 and 90% [27, 43]. Most air purifiers currently on the market incorporate HEPA filters, as these are internationally recognized as the most efficient filters, capturing particles of different diameters. HEPA filters are designed to be over 99.99% efficient and are used in a diverse range of situations, including in theaters, hospitals, respirators, and vehicles [44]. Furthermore, filtration based on the use of non-woven nano-fiber material is an emerging filtration technique with an extremely high efficiency that is comparable to, or even superior to, HEPA-based filtration in the smaller particle size range [45].

According to the Environmental Protection Agency (EPA), pollutant levels may be two to five times higher indoors than outdoors, which indicates that the poor quality of indoor air is mainly attributable to the inefficient circulation of air. In regions characterized by hot summer, there is a heavy reliance on air-conditioning systems for maintaining comfortable conditions during the hot summer months. Accordingly, windows tend to remain shut, and little fresh air enters into our homes and places of work. The EPA warns that high temperatures and humidity can increase the concentrations of certain pollutants, with young children and the elderly being at particular risk from the detrimental effects of indoor air pollution.

Mechanisms for trapping dust in air using a standard filter, killing almost of all airborne microbes using UV lamps, and removing fine particles (dust) and died microbes using a high-efficiency particulate air filter (**Figure 8**).

*Low-temperature Technologies*

affective control of test microbes.

prevention.

K+

Exposure to bioaerosols can cause various adverse health effects, including infectious and respiratory diseases and hypersensitivity. Consequently, controlling the exposure to bioaerosols constitutes an important aspect of disease control and

Photocatalytic oxidation systems use a UV light source and a titanium dioxide photocatalyst to produce oxidants that destroy gaseous contaminants. When the photocatalyst is irradiated with UV light at wavelength of 254–365 nm, a photon from the light excites a catalyst electron in the valence band to jump to the conduction band, leaving a hole. This photocatalytic oxidation process converts organic pollutants into carbon dioxide and water. Using this technique, pollutants, particularly volatile organic carbons, are preferentially adsorbed on a catalyst surface and oxidized. The hole generated by photocatalysis can further react with surrounding water to produce a hydroxyl radical (ÆOH), whereas the electron in the conduction band reacts with oxygen to yield a superoxide radical anion (ÆO2). These radicals can attack the cell membranes of microorganism, thereby causing the release of

 ions, RNA, proteins, and other important components and eventually resulting in cell death [28]. Given these properties, researchers have applied photocatalytic oxidation to many substrates and achieved impressive results, indicated by the

To date, however, photocatalytic oxidation has yet to be applied to HEPA filters in HVAC systems [27]. HEPA filters have been mandated for use in the removal of airborne microorganisms in many codes adopted in the field of healthcare, including the American Institute of Architects Guidelines for Design and Construction of Hospital and Health Care Facilities (AIA Guidelines), the American Society of Heating, Refrigerating and Air-Conditioning Engineers standards, the Joint Commission on Accreditation of Healthcare Organizations Environment of Care standards, the Centers for Disease Control and Prevention (CDC) guidelines, and recommended practices [29]. Although HEPA filters can efficiently capture aerosolized microorganisms, the area downstream of the filter can become a breeding ground for microbes. Under conditions of suitable temperature and humidity, microbes retained within a filter can multiply using particulates adhered to the filter as a food source, and the microbial progeny can ultimately disperse into the filtered air [30, 31]. Thus, instead of being an apparatus to control air quality, these systems can potentially become a source of pathogens. Efforts have accordingly been made to eliminate the breeding ground problem. For example, Goswami [32] examined four microbial species (*Aspergillus niger*, *Penicillium citrinum*, *Staphylococcus epidermidis*, and *Bacillus subtilis*) that are representative of the genera most commonly detected in hospitals in Thailand, a country characterized by a hot and humid climate, with an average temperature of 27°C and average relative humidity ranging from 62 to 84% [33, 34]. Very high relative humidity not only reduces the probability of microorganisms coming into contact with hydroxyl radicals but also provides sites conducive to microbial survival. Excessive amounts of water can also occlude the reactive sites of filter surfaces and subsequently reduce the photocatalytic oxidation efficiency

[32, 34, 35]. Hence, the effect of relative humidity has been investigated.

Chuaybamroong [27] examined the application of photocatalytic oxidation to HEPA filters for disinfection of airborne microorganisms. Experiments were conducted at two TiO2 loadings on HEPA filters irradiated with UV-A under two relative humidities. They assessed the inactivation of two fungal (*Aspergillus niger* and *Penicillium citrinum*) and two bacterial (*Staphylococcus epidermidis* and *Bacillus subtilis*) isolates and found that, on average, 60–80% of microorganisms retained on a photocatalytic filter were inactivated, although in the case of *S. epidermidis,* 100%

**190**

inactivation was observed.

#### **Figure 8.**

*A standard filter traps large dust particles in internal structure of an air purifier. HEPA filter captures microbes.*

#### **6.1 Air duct cleaning services**

Given that dirty HVAC units have been proven to be less efficient, it is essential that air-conditioning ducts are periodically cleaned through employing an air duct cleaning service. Such cleaning should include complete care of the internal elements of the HVAC unit.

Ductwork would only be essential if there has been renovation, asbestos abatement, lead paint removal, or a significant accumulation of dust debris. Cleaning would be considered essential in the presence of the following: animal feces, mold, foul odors, noticeable debris, or pet hairs. Furthermore, if occupants suffer from an unexplained allergy, then it would be advisable to consider cleaning. Abe [46] noted that it is possible to remove fungi and bacteria from filters by washing with water and detergent; however, if the fan and heat exchanger are also contaminated, specialist cleaning would be required (**Figure 9**).

Kujundzic [47] mentioned that cleaning room air could contribute to reducing the levels of particulate matter within the home and that this can be achieved by using filters that retain the filtered particles.

#### **6.2 Air-conditioning systems and mold**

Mold is a pervasive problem, of which many property owners are fully unaware. In the Eastern Province of Saudi Arabia, the average annual humidity level is approximately 74%, but can be notably higher during certain times of the day and year. Many types of mold require a humidity of only 50% to commence growth, and air-conditioning systems are a common source of mold in many households. One of the causal factors in this respect is the fact that HVAC systems do not operate continuously, which can result in a fluctuation in humidity levels. The EPA warns that if an HVAC system is turned off before occupants perform tasks such as mopping, the humidity levels can suddenly surge and cause moisture and mold problems. If an HVAC system is improperly programed (which is a

**193**

**Figure 9.**

*Ducts should be cleaned periodically.*

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

common problem), then the air-conditioning system may cycle off when the air is cooled but before it has had time to dry sufficiently, causing moisture problems. HVAC maintenance issues can be a further source of mold-related problems, such as when excessive moisture accumulates on the air-conditioning coils, resulting in the growth of mold. This mold can subsequently be blown through the air-conditioning ducts and released into the surrounding air. Holes or gaps in air ducts can also lead to the formation of condensation, which creates a perfect breeding ground for mold to grow. Such mold can cause numerous health-related problems, including respiratory problems, skin rashes, and allergic reactions [17]. In a study designed to examine the efficiency of various filters used for trapping microorganisms, Al-Abdalall and Al-Abkari [20] isolated the bacteria and fungi colonizing air-conditioning systems in different types of buildings during each of the four season s, in the provinces of Dammam and Qatif in eastern Saudi Arabia, and determined the respective frequency distributions. The air-conditioning systems were found to be contaminated by different types of bacterial and fungal species. Specifically, the isolated bacteria included *Serratia* 

*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*

common problem), then the air-conditioning system may cycle off when the air is cooled but before it has had time to dry sufficiently, causing moisture problems. HVAC maintenance issues can be a further source of mold-related problems, such as when excessive moisture accumulates on the air-conditioning coils, resulting in the growth of mold. This mold can subsequently be blown through the air-conditioning ducts and released into the surrounding air. Holes or gaps in air ducts can also lead to the formation of condensation, which creates a perfect breeding ground for mold to grow. Such mold can cause numerous health-related problems, including respiratory problems, skin rashes, and allergic reactions [17].

In a study designed to examine the efficiency of various filters used for trapping microorganisms, Al-Abdalall and Al-Abkari [20] isolated the bacteria and fungi colonizing air-conditioning systems in different types of buildings during each of the four season s, in the provinces of Dammam and Qatif in eastern Saudi Arabia, and determined the respective frequency distributions. The air-conditioning systems were found to be contaminated by different types of bacterial and fungal species. Specifically, the isolated bacteria included *Serratia* 

**Figure 9.** *Ducts should be cleaned periodically.*

*Low-temperature Technologies*

**6.1 Air duct cleaning services**

specialist cleaning would be required (**Figure 9**).

using filters that retain the filtered particles.

**6.2 Air-conditioning systems and mold**

ments of the HVAC unit.

**Figure 8.**

*microbes.*

Given that dirty HVAC units have been proven to be less efficient, it is essential that air-conditioning ducts are periodically cleaned through employing an air duct cleaning service. Such cleaning should include complete care of the internal ele-

*A standard filter traps large dust particles in internal structure of an air purifier. HEPA filter captures* 

Ductwork would only be essential if there has been renovation, asbestos abatement, lead paint removal, or a significant accumulation of dust debris. Cleaning would be considered essential in the presence of the following: animal feces, mold, foul odors, noticeable debris, or pet hairs. Furthermore, if occupants suffer from an unexplained allergy, then it would be advisable to consider cleaning. Abe [46] noted that it is possible to remove fungi and bacteria from filters by washing with water and detergent; however, if the fan and heat exchanger are also contaminated,

Kujundzic [47] mentioned that cleaning room air could contribute to reducing the levels of particulate matter within the home and that this can be achieved by

Mold is a pervasive problem, of which many property owners are fully unaware. In the Eastern Province of Saudi Arabia, the average annual humidity level is approximately 74%, but can be notably higher during certain times of the day and year. Many types of mold require a humidity of only 50% to commence growth, and air-conditioning systems are a common source of mold in many households. One of the causal factors in this respect is the fact that HVAC systems do not operate continuously, which can result in a fluctuation in humidity levels. The EPA warns that if an HVAC system is turned off before occupants perform tasks such as mopping, the humidity levels can suddenly surge and cause moisture and mold problems. If an HVAC system is improperly programed (which is a

**192**

#### **Figure 10.**

*Microorganisms collected by swabbing an air-conditioning duct and using the swabs to inoculate a fungal growth medium. The left-hand panel shows Aspergillus fumigatus growth on inoculated medium. The right-hand panel shows the mycelial growth and conidiophores of A. fumigatus viewed under a compound microscope.*

*liquefaciens*, *Bacillus pumilus*, *Bacillus cereus*, *Bacillus subtilis*, *Staphylococcus lentus*, and *Oligella ureolytica,* whereas the common fungal taxa included *Cryptococcus laurentii*, *Aspergillus niger*, *Aspergillus flavus*, *Cladosporium* sp*.*, and *Rhizoctonia* sp*.* Ironically, the findings of the study indicated that buildings that were in good condition were those likely to have the highest levels of microbial contamination (**Figure 10**).

Among the microparticles suspended in air, there is an abundance of biological material, including fungal spores, pollen grains, bacteria, and viruses. Airconditioning systems can readily become polluted by these biological contaminants, which disperse throughout indoor areas and raise the risk of infection among the occupants [48]. The amounts of bacteria and fungi harbored by these systems tend to differ according to location, and numbers and frequencies also show seasonal differences [49]. Furthermore, differences in the number of microorganisms isolated and the distribution of different types of isolates can also depend on the type of filter used and the frequency of cleaning. In this regard, [20] found *Cladosporium* sp. to be a dominant contaminant and also identified *Alternaria* sp., *Aspergillus flavus*, *Aspergillus niger*, and *Rhizoctonia* sp. with frequencies of 24.16, 12.96, 12.8, 8.29, and 4.96%, respectively. Similar results were obtained by [50–53]. Consistently, Al-Suwaine et al. [54] mentioned that *Aspergillus* and *Cladosporium* spp. were the common isolates detected in closed systems in Riyadh, KSA, whereas other fungal genera, including *Fusarium* and *Rhizopus*, were isolated in low frequency, similar to findings of [51, 55].

### **6.3 HVAC systems and indoor pollution**

Heating, ventilating, and air-conditioning (HVAC) systems function by drawing in air through a network of intake ducts, cooling the air, and then releasing the cooled air back into the home through return ducts. The constant recirculation of air in HVAC systems means that pollutants are continuously blown through indoor areas. In the eastern region of Saudi Arabia, given that the summer is generally very hot and humid, many properties are at risk from the growth of mold within air ducts.

**195**

**Figure 11.**

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

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

**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

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

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

*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.*

indicate that these fungi use the filters as a support for fungal mycelium.

drops of condensate that form air-conditioning towers [59].

**7. The microbial colonization of traditional filters**

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

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

mycelium and filters fibers (**Figure 11**).
