## 1 Introduction  
Submicron particles, regardless of their sources, are very dangerous to human health. Submicron particles from the range of 100 to 1000 nm, and nanoparticles (<100 nm), are reported to have adverse health effects on humans exposed to such particles in indoor (houses, offices, shops, galleries), semi-closed (i.e., with permanent but limited air exchange, like transportation carriages, trains, busses, or subway stations), and urban open air environments, where they spend the majority of time each day. The particles can be of natural origin (fungal, bacteria, viruses, skin remnants) or can be produced by humans' activities, including, cooking, toasting, frying, smoking, fireplace burning, walking, wearing of clothes, machining, wearing of moving parts of various machines, etc. [1, 2, 3, 4, 5, 6, 7, 8, 9], and/or can be infiltrated from the outdoors with the ventilated air [10, 11, 12, 13, 14, 15, 16]. For example, in the case of an underground station, the particles originate from the natural abrasion of the construction elements of trains and rails, or due to the movement of passengers [17, 18, 19, 20, 21, 22, 23, 24, 25]. Such particles have a great propensity to be deposited in the human's airways during inhalation, due to impaction or Brownian diffusion [26, 27]. Such micro- and nanoparticles can circulate in the environment due to the air motion, forced by the natural convection, heating, ventilation, or air-conditioning systems, or by humans' movement. Optimized filtration devices used for the removal of indoor particles are therefore required in order to protect human health by effectively operating HVAC systems. Nonwoven fibrous filters, electret filters [28], or electrostatic precipitators [29; 30; 31] are usually used to remove fine particles from the ambient air to decrease their concentration in the indoor environment. The optimal solution to the problem of nanoparticle removal can be the nanofibrous filters. Nanofibrous filtration is one of the modern methods for the removal of fine inorganic dust or microorganisms (bacteria, viruses, and spores) of a size smaller than one micron from the air [32; 33]. Nanofibrous filters, with nanopores between fibers of usually <200 nm, can be produced by the electrospinning method, which generates polymer fibers of a diameter of <200 nm [34; 35; 36; 37; 38; 39; 40]. Compared to other methods used for the production of commercial filters, the electrospun nonwoven filters are made of fibers of more uniform diameters [41; 42; 43; 44]. Studies of the process of filtration of various aerosol contaminants by nanofibrous filters were carried out by [PERSON] and [PERSON] [45], and [PERSON] et al. [46]. The nanofibers used by these authors were produced by polymer melt blowing [47]. [PERSON] and [PERSON] [48] proposed a model of motion and deposition of particles on the filter's fibers, which could be used for computational modeling. However, melt blowing technology does not offer the possibility to produce nanofibers with controlled fiber diameters and a controlled degree of fibers' alignment. The interest in nanoparticle filtration by fibrous filters with the simultaneous inactivation of microorganisms started at the beginning of this century [49; 50; 51; 52; 53]. The inactivation of microorganisms was carried out via the deposition of active chemical agents onto the nanofibrous substrate [54; 55; 56; 57]. Metal oxide nanoparticles blended with nanofibers produced by electrospray were used in order to act as a catalyst for the protection against chemical contaminants [58; 59; 60; 61]. The current state of knowledge in the field of filtration of viruses by various types of filters was discussed by [PERSON] et al. [62]. The filtration efficiency of the electrospun filters used for the respiratory masks was investigated by [PERSON] and [PERSON] [63]. The filtration efficiency for the aerosol particles of dioctyl phthalate (DOP) was estimated to be >99.999999%.