An experiment was designed to quantify the filtration and fit (inhalation) performance of 10 full mask assemblies when donned on a headform 3D printed using Polylactic acid.
The headform was based on the ISO 18526-4:2020 1-M headform but was modified by creating a nominally 42 mm hole, originating at the mouth of the headform, through to the back of the headform; such that an insert could be fitted. The fitted insert was required to collect condensed NaCl on a filter, to prevent drawing it into the pump.
The headform was further modified to allow for a maxillofacial rubber to be used as the face of the headform. The modification of the headform, and manufacture of the silicone rubber face was carried out at King’s College London (see Figure 1). The skin of the face was made from Technovent M511 Addition Platinum Silicone Rubber, with a standard cure hardness of approximately Shore A25. The remainder of the face was packed with Tecnovent M510 Gel Catalyst.
This modification was carried out, in order to mimic human skin on the headform, such that the “fit” of the mask would be more comparable to that of a human wearing the mask than that of a rigid plastic headform.
The masks provided consisted of 2 designs, one with a red face seal and the other with a green face seal, both with removable filter holders. Head straps were not provided with the masks; therefore, straps were retrofitted on the mask from a YINGHUA RM101 FFP2 mask using the connecting loops on the filter holder.
The full mask assembly was made by first removing the filter holder on the mask, then placing either a nominally 80 mm diameter filter cut from a larger sheet, or a pre-cut filter, within the holder and replacing the holder back on to the mask ensuring that the retaining clips on the holder are fully engaged onto the mask. As only two masks with face seals were provided, the masks were washed in clean water, and dried, between each test.
The filter material specimens were pre-conditioned at nominally 21.0 °C and 85 %RH in an environmental chamber for at least 4 hours prior to measurement.
The headform, donned with the full mask assembly under test*, was placed inside an enclosure and attached to a 7-stage Anderson cascade impactor, calibrated mass flow meter, needle valve and vacuum pump via the rear connector of the insert, through a feedthrough port located on the side of the enclosure. For this test, the pressure port of the insert was sealed in order to prevent any challenge aerosol from circumventing the mask.
An ultrasonic mesh nebuliser charged with nominally 8 % NaCl solution (by mass) was used to create a challenge aerosol of particles within the enclosure, though a second feedthrough port on the side of the enclosure.
The aerosol was dispensed at a rate of nominally 0.25 ml min-1 for nominally 1 minute. After this time, the nebuliser was disabled, and the vacuum pump activated, pulling air from the front of the insert though the cascade impactor. The pump was run for nominally 1 minute, at a flow rate of 28.3 l min-1 before it was deactivated.
The cascade impactor was then removed, and the plates were washed, in turn, in nominally 250 ml of deionised water. The conductivity of the water and total mass of the solution were recorded. These values were then used to calculate the mass of salt collected on each plate.
The full mask assembly was then doffed and washed.
This procedure was carried out for the two different masks, denoted as “red” and “green”, and three different supplied filter materials, denoted “material 1”, “material 2”, and “material 3”.
The above procedure was also performed on a commercially available CE marked FFP2 face mask, in order to facilitate comparison with the sample masks provided.
* The tester was not a trained or qualified fit tester
Additionally, 10 control runs were performed periodically, with no mask donned on the headform. This was carried out to allow the baseline, total mass of salt measured, to be calculated when no mask was used. Repeating the control runs throughout the mask measurements also allowed for the stability of the system to be continually assessed.
For each test specimen, full mask assembly, the filtration efficiency (FE) was calculated according to the following equation:
Where 𝑚 is the total mass of salt measured and 𝑚𝑐 is the average of the total masses of salt measured for two control runs, one carried out before the test specimen measurement and one after the test specimen measurement run.
The results of the headform based combination of filtration efficiency and fit test can be found in the table below.
*This result may be due to improper donning of the full mask assembly on to the headform. As the interface between the face seal and the rigid section of the mask was susceptible to becoming misaligned. This may be due, in-part, to the friction between the face seal and maxillofacial rubber face of the headform when donning the full mask assembly.
The reported uncertainties include the following type A uncertainties: The short-term reproducibility of the electrical conductivity meter and the short-term reproducibility of the mass balance in the measurements performed, and the following type B uncertainties: the calibration of the electrical conductivity meter, the resolution of the electrical conductivity meter, and the resolution of the mass balance. The reported uncertainty does not include contributions from the mass flow meter, concentration of the NaCl solution, or the total amount of nebulised NaCl solution during the mask testing.
The results and uncertainties given in this report are valid for the specified samples tested and refer to the measurements made during testing only.