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

*Low-temperature Technologies*

contamination (**Figure 10**).

**Figure 10.**

*microscope.*

findings of [51, 55].

**6.3 HVAC systems and indoor pollution**

*liquefaciens*, *Bacillus pumilus*, *Bacillus cereus*, *Bacillus subtilis*, *Staphylococcus lentus*, and *Oligella ureolytica,* whereas the common fungal taxa included

*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* 

*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

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

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

**194**

ducts.
