## 1 Introduction  
Nowadays, not everyone can breathe clean air [1]. The problem concerns mainly high urbanized [2] and industrialized [3] areas, where contamination levels significantly exceed the World Health Organization (WHO) guidelines and recommendations [4]. Studies indicate that people spend most of their time (\(\approx\)90%) indoors [5]. In general, pollutants can get into human settlements through the filters assembled in the Heating, Ventilation and Air-Conditioning (HVAC) systems; the supply air can come entirely from the outside (European countries) or can be the mixture of the outside and recirculated air (USA) [6]. Even small amounts of pollutants can be dangerous due to long-term exposure [7]. The origin and characteristics of air pollution is varied and complex [8]. Particulate matter consists of both solid particles and liquid droplets, which are a mixture of organic and inorganic substances [9]. Two main fractions of particulate matter can be distinguished: (i) PM\({}_{2.5}\), which consists of compounds up to 2.5 \(\upmu\)m in aerodynamic diameter, and (ii) PM\({}_{10}\), which consists of compounds up to 10 \(\upmu\)m in aerodynamic diameter. The division due to fraction size is very important because, the smaller the dimensions of the PM, the greater the threat to the respiratory system [10]. Therefore, HVAC filters able to capture specified air contaminants are desirable for public health considerations [11]. The most popular solution refers to electret/charged filter media; several studies have addressed the role of such materials in HVAC systems. [PERSON] et al. [12] explained how the applied approach can influence the particle separation efficiency. The study was subjected to typical commercial flat-sheet electret materials and particles of sizes of 3-500 nm. The results revealed that, depending on the particle size, different electrostatic mechanisms have different impacts on filtration enhancement; for particles of \(\approx\)20 nm, polarization force; for \(\approx\)100 nm, image force; and for \(\approx\)300 nm, Coulombic force. The filtration electret media produced by [PERSON] et al. [13] was characterized by 99.96% filtration efficiency and low pressure drop (54 Pa). [PERSON] and coauthors [14] developed an electret membrane with 99.992% filtration efficiency and pressure drop of 61 Pa. Both types of membranes were tested against particles of 300 nm in size. Dust loading, particle size, or filter geometry can limit the filtration efficiency of the electret media. Thus, it is worth considering the worst-case scenario assuming a complete lack of charge. The common discharging standards are based on the fiber treatment with isopropanol liquid immersion or isopropanol saturated vapor [5,15]. [PERSON] and coauthors [6] used a combination of electret and mechanical particle capture processes in their work. The developed filter consisted of commercial electret flat-sheet HVAC filter media and nanofibers layer. The results indicated that nanofiber addition enhanced filtration efficiency against very small particles (10-30 nm). The electrospinning method allows us to produce fibers of any size [9,16]. However, the fabrication of ultrathin structures with diameters of \(\approx\)100 nm, especially uniform and beaded-free, is difficult [17]. The presence of beads is considered unfavorable and may disturb the filter operation [18]. The bead structures are formed due to the rivalry between electrical stress and capillary forces, which leads to electrohydrodynamic instabilities [17]. As the stretching of the electrospinning jet is dependent on the solution conductivity, it is possible to add some ionizable substances to enhance the stability of the polymer jet and to fabricate thin fibers [17,19]. Moreover, the added substances are not always safe for human health and may stay in the final products [18]. Some studies have shown that, among numerous polymers, poly(acrylonitrile) (PAN) is often used as the solution component in the electrospinning process.