Results
Figure 3A–F shows images of the unmasked dummy and with five different face coverings. The frames shown here are the brightest of the sequence of 30 frames, corresponding to the peak particle emission. All other frames, together with raw data, are available in the supplemental materials. Distinct differences in the effectiveness of the various materials can be clearly seen. The unmasked dummy should have the highest particle emission, but the scattering plume appears relatively dim (Figure 3A). This is because the narrow jet crosses the laser sheet quickly, so that at any one-time there are few particles in the laser sheet. The three-layer mask shows some fabric penetration directly in front of the mouth and some turbulent leakage from under the eyes which crosses the laser cone above the forehead (Figure 3B). The single-layer mask shows very high penetration and some leakage (Figure 3C). The brightness of the penetrating aerosol is around five-times greater than even the unmasked dummy, but this is due to the slower, broader flow which fills the laser beam with aerosol.
Figure 3.  Testing mask fabric and fit.
Top row: (A) no mask, (B) three-layer textile mask, (C) single-layer textile mask. Bottom row: (D) CE marked FFP1 PPE mask, (E) CE marked Type 2 medical mask, (F) bandana. Intensity scaled to 100–20,000 counts with ‘bb’ colourmap. Similarly, the bandana shows high penetration (Figure 3F). In this case, the leakage appears less intense, likely as there is less resistance from the fabric that would otherwise force the breath to find a path of lower resistance. The medical mask (Figure 3E) and FFP1 (Figure 3D) mask display no detectable penetration but instead show a few turbulent jets from the cheek and eyes. The leakage at the cheek is easier to discern from inspection of the whole time series (see supplemental materials). In the image of the FFP1 mask, the wisp in front of the mask can be seen to originate at the cheek and did not penetrate the fabric. These leakage jets tend to be directed over the forehead, or across the cheek, and are only partly moved into the laser cone by the extraction air current.