# Performance Comparison among KN95-Certified Face Masks by Classical Techniques and Innovative Test
## 1 Introduction
The WHO itself estimates that approximately 89 million masks are required each month to respond to the virus [20] (assessed to be 3.4 billion masks for the whole duration of the pandemic so far [23]), and this highlighted that the demand for PPE dramatically surpassed the global production capacity of manufacturers, especially during the beginning of the pandemic. In this frame, the use of facial masks and respirators increased in all countries but not all the masks used could be considered protective equipment. In general, high costs together with the low availability brought most people to direct towards cheaper and more accessible alternatives such as surgical masks and homemade ones (tissue, cotton, dusting cloth) but the majority of these homemade designs have not been tested yet, thus it is not easy to make any statement about their efficacy [17; 24; 25; 26]. To face this unmet demand, a dramatic increase in PPE mass production capacity was established as well as new manufacturing, nevertheless the lack of certification on masks' performances did not mitigate this issue [27]. Still, determining mask efficacy is a complex topic, made even more complicated because the infection pathways of SARS-CoV-2 are not yet thoroughly understood, and many factors, such as correct fit and usage of masks, and environmental variables further influence this issue [17]. Face masks must balance two main features: efficiency and breathability. Regarding the efficiency of the face masks, there are two main considerations: the performance of the material at filtering and the fitting of the design to the wearer's face. Leakage must be assessed on an individual-by-individual basis while filtration efficacy can be measured using well-defined test methods. Filtration is achieved by forcing the air to pass through a layer that can retain the target particles or droplets. The efficiency depends on the type and the strength of the interaction between the mask's materials and the particles to be retained in it (mechanical, electrostatic, chemical, biochemical, etc.) and on the factors that can enhance the possibility of this interaction. These factors depend on intrinsic pore properties such as the porosity, the pore size, the pore tortuosity, the internal surface area, and on other external factors such as the thickness of the membrane and the use of several layers. They also depend on the particles to be retained: size, shape, state (solid or fluid), concentration, electrostatic charge, etc. On the other hand, breathability can be defined as the degree to which a fabric permits air and water vapor to pass through it, proportionally to the ease with which an individual can normally breathe without a feeling of asphyxia. In some aspects, filtration efficiency and breathability can be the opposite [28]. Particle removal efficiency is mainly dependent on particle size and velocity, and the removal mechanism is related to impaction, diffusion, settling, and electrostatic attraction. Breathing resistance is associated with a pressure drop as the air passes through the mask, and can be affected by air velocity, particle loading, particle type (hygroscopic or non-hygroscopic), and relative humidity (RH) [16]. The characterization of mask efficacy represents a key point, as it brings the necessary knowledge to address different types of face masks to different types of risks and professions. Respirators, medical masks, and barrier face coverings all filter airborne particles using similar physical principles. However, they are tested for certification using a variety of standardized test methods, creating challenges for the comparison of differently certified products [15]. Disposable respirators are subjected to various regulatory standards all around the world, defining their physical properties and performance characteristics.