Testing occurred in the University of Thrace in Greece. It was done in an indoor environment with natural air ventilation. Two instruments were utilized: PROMO 2000 (a light scattering particle counter) and SMPS- CPC (particle counter). Each mask was fitted over the inlet of the instrument and was tested for 30 mins with inlets open, and 30 mins with inlets closed (occasionally the mask was moved against the inlet to allow for homogeneous penetration). This was repeated 6 times. Filtration efficiency was calculated as average particle size in background - average particle concentrations per size bin when the instrument sampled indoor air that passed through the mask) / (average particle size in background. Particle sizes tested by adjusting inlet size ranged from size range of 0.006 μm to 10 μm.