## 4 Analysis of the Results and Discussion  
Experimental measurements were carried out on a previously produced half mask, which was placed on a Sheffield head. During the first measurement, the flow rates of the air leaking through the skin-facepiece interface during inhalation were analysed. Prior to the measurement, a vacuum pump was used to reduce the pressure in the pressure container to the value of \(-600\) Pa. After the vacuum pump was disconnected by closing valves V1 through V4 and opening valve V5, the pressure in the system gradually increased, as is shown in Figure 9.  
Due to the pressure drop in the supply pipeline, relative pressure inside the mask was higher than the pressure in the container; however, an analysis revealed that the pressure drop in the distribution pipelines was negligible due to extremely low velocity of the air, and it did not exceed the value of 0.1 Pa. After applying the measured data in formula (4), the curve of the correlation between the leaking air flow and the negative pressure inside the mask was produced (Figure 10).  
Figure 8: Curve of the correlation between the cross-sectional area of the leak and the leak percentage.  
Figure 9: Curve of the changes in the pressure in the container over time during the measurement of inward leakage during inhalation.  
At a maximum permissible negative pressure for FMP3 respirators, representing 420 Pa, the flow rate of the air leaking through the mask was 0.2 L-min\({}^{-1}\). This value represents 0.21% of the flow rate of 95 L-min\({}^{-1}\) during inhalation, as defined by the EN 1827 standard. According to formula (9), the total cross-sectional area of the leak was only 0.14 mm\({}^{2}\).  
The total cross-sectional area of all leaks in inhalation may be different from the total cross-sectional area of the leak in exhalation. That is why the second measurement was carried out. The purpose of this measurement was to identify the flow rate of the leaking air during exhalation, at which the air inside the mask exhibits positive pressure compared to the atmosphere. The second measurement differed from the first one in creating a positive pressure of +600 Pa in the pressure container using a vacuum pump, while the air leakage through the skin-facepiece interface during exhalation was measured. The flow rates of the leaking air during exhalation are shown in Figure 11. To facilitate a comparison of the leaks, the curve of the leakage measured during exhalation was supplemented with the data obtained in the first measurement, which was carried out during inhalation, but the pressure was converted into positive absolute values.  
Figure 11: A correlation between the flow rate of the air leaking through the skin–facepiece interface and positive pressure (exhalation) inside the mask. A comparison of leakage during inhalation and exhalation.  
Figure 10: A correlation between the leaking air flow rate and negative pressure (inhalation) inside the mask.  
At a maximum permissible positive pressure (during exhalation) for FMP3 respirators, which represents 300 Pa, the flow rate of the air leaking into the mask was 0.14 L\(\cdot\)min\({}^{-1}\). This value represents 0.09% of the flow rate of 160 L\(\cdot\)min\({}^{-1}\) during inhalation, as defined by the EN 1827 standard.  
Considering the maximum pressures for inhalation and exhalation, the measured values of leakage were much lower than the threshold of 2% defined by the standard for protective half masks.