# Antimicrobial Air Filter Coating with Plant Extracts Against Airborne Microbes
## 1 Introduction  
As people engage in indoor activities for increasing amounts of time, keeping indoor air clean is becoming increasingly important [1]. Fine dust pollution and air quality deterioration induce a negative effect on humans, including contagious diseases, allergies, acute toxicity, and cancer [2]. To reduce these adverse effects, air filtering is used to improve indoor air quality. An air filter allows clean air to pass and rejects fine particles, including bioaerosols. Bioaerosols are airborne particles consisting of bacteria, viruses, fungi, their fragments, and various antigens [3]. Bioaerosols can accumulate in the filters of air purifiers, air conditioners, heaters, etc., causing a variety of diseases, including hypersensitivity pneumonia and respiratory problems, such as rhinitis and asthma [4, 5, 6, 7]. Methods for removing biological contaminants from indoor environments include UV irradiation [8], photocatalytic oxidation [9], negative air ionization [10], thermal inactivation [11], ozone [12], and electrostatic precipitation [13]. Among these methods, the most noticeable is an antimicrobial filtration systemthat involves filter surface treatments with antibacterial material to physically capture the bioaerosols entering the air filter and then inactivate them on the surface [14]. The antimicrobial materials used include silver (Ag) nanoparticles [15], carbon nanotubes (CNT) [16], iodine powders [17], titanium-based nanoparticles [18], copper [19], and other inorganic materials. However, it is well-known that exposure to nanoparticles causes various problems [20]. In some cases, copper oxide nanoparticles result in DNA damage and oxidative stress on cells [21]. Silver nanoparticles have been found to induce toxicity in mammalian cells and organs, because these penetrate the skin [22]. Moreover, CNTs have also been reported to initiate unwanted mechanisms, such as disturbance of electron transfer through the cell membranes, penetration through the cell envelope, and oxidation of cell components [23]. In addition, the prolonged exposure or inhalation of these nanoparticles can lead to respiratory illness or abnormalities [24]. To overcome the disadvantages of these chemicals and inorganic materials, various materials have been proposed [25]. Natural plant extracts are regarded as antimicrobial agents that can improve indoor air quality because they typically exhibit less toxicity than other chemicals or nanomaterials [26]. Their biological activities, such as anti-inflammatory, antiviral, and antibacterial effects, depend on the type or characteristics of natural plant extracts and are well documented [27]. _Sophora flavscens_, used in oriental medicine and cosmetics for anti-inflammatory, detoxification, and neuralgia relief, was applied to air filters with successful antimicrobial activity against gram-positive bacteria such as _Staphylococcus epidermidis_ and _Bacillus subtilis_[25]. An air filter using _Mukdenia rossii_ showed an inactivation rate of approximately 70% against _S. epidermidis_ when treated with natural plant extracts, the inactivation increasing by 20% when combined with unipolar ion emission [28]. Several plant extracts are good candidates for applying to antimicrobial air filters even if they have not been used to air filter yet. Graprefruit seed extract has a strong antimicrobial activity due to its flavonoids and polyphenol compounds [29; 30; 31]. Shiitake mushrooms are well-known for antimicrobial potential as well as hypocholesterolemomic, hypoglycemic, antitumor, antiviral properties [32]. In this study, tea-tree oil, rosemary, and garlic extracts were selected to prepare antimicrobial air filters. These plant extracts are readily available and widely used as topical antiseptics and complementary medicine for bacterial and fungal infections [33]. Tea-tree oil, as reported in many studies, possesses antifungal, antiviral, antiprotozoan, and anti-inflammatory properties.