Table 2

The effective diameter (µm) of pores for each layer of three different masks based on reflected light images

Effective diameter (µm) of mask pores
	Back up filter	Foreign-manufactured	US-manufactured
Layer 1	25.1	29.4	27.2
Layer 2	23.7	11.0	25.4
Layer 3	Too small to measure*	56.6	Too small to measure*
Layer 4	59.0	26.4	17.5

Foreign-manufactured mask that failed particle filtration efficiency testing was found to have the smallest pore size of 11 µm.
*The masks that passed had smallest pore sizes that were much smaller than 11 µm, beyond the capability of the microscope to reliably resolve for measurement due to the tightly woven fibres and overall material density.