###### Abstract  
The present article describes the measurements of flow rates of the inward air leakage through the skin-facepiece interface of a protective half-mask with replaceable filters. The measurements were carried out while applying an indirect method in which the pressure drops in a compressed air container were measured, and subsequently, the total flow rate of the leak was calculated. This methodology facilitated measuring extremely low air flow rates at the atmospheric pressure of 3.2 \(\times\) 10\({}^{-6}\) m\({}^{3}\cdot\)s\({}^{-1}\). A numerical analysis of the inward air leakage through the gaps between the face and the facepiece of the mask was carried out with the aim of identifying the cross-sectional area of the leak. With the tested mask, which was made of Santoprene 8281-45 MED, the leakage measured during inhalation was 0.21%, which corresponded to the cross-sectional area of only 0.14 mm\({}^{2}\). pandemic; COVID-19; personal protective equipment; measurement Article The Determination of the Inward Leakage through the Skin-Facepiece Interface of the Protective Half-Mask  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] to 100 nm [6]. This size is similar to the size of the SARS coronavirus, which is also 100 nm [7]. The virus gains entry through the mucous membranes, and the main routes of transmission include: virus-infected droplets and aerosols, or a contact with skin, surfaces, or other items contaminated by fomites [8; 9; 10; 11]. As of the end of July 2021, COVID-19 globally infected as many as 199,194,369 people, out of whom 4,241,981 died. Research, development, and testing of novel materials to be used in air filtration and the manufacture of respirators with replaceable filters, which facilitate efficient protection against the SARS-CoV-2 virus, are still regarded as highly topical due to persisting problems with the present pandemic. The 3D technology was used in the production of PPE by the team of scientists, [PERSON] et al. [12]. The adaptors and the clip were printed using the fused deposition modelling. The results obtained by this research team were transformed into the EasyBreath mask, which was tested in an operating theatre. When compared to the combined use of a headshield and an FFP2/N95 facemask, surgeons reported they felt safer and had better vision of the surgical field thanks to the portable light. The case study demonstrated a new approach to designing and manufacturing PPE in emergency situations that relies on the rapid development and domestic manufacturing of products through 3D printing technologies. The same topic was also discussed in the paper by [PERSON] et al. [13]. A team led by [PERSON] focused their attention on the development of a novel charged PVDF nanofibre filter technology in order to effectively capture the fast-spreading, deadly airborne coronaviruses, especially COVID-19, with the target aerosol size of 100 nm [7]. The virus and its attached aerosol were simulated by sodium chloride aerosols sized 50-500 nm, which were generated from a sub-micron aerosol generator. Filter fibres were subsequently electrostatically charged by a corona discharge. This team designed four new filters in total, whereas one of the filters achieved 90% efficiency and exhibited an ultralow pressure drop of only 18 Pa (1.9 mm water), and another filter met the 30 Pa threshold and exhibited a high efficiency of 94%. Generally, newly developed products may only be marketed if they are certified and tested in accredited laboratories. The highest degree of protection of the respiratory system against infections is provided by products classified as FFP3 protective equipment. Respirators, half masks, and masks that belong to this category must meet a vast number of requirements prescribed by the EN 143, EN 149, and EN 1827 standards, as well as by the relevant standards applicable to respirators protecting against particles.