Measures the filtration efficiency of a respirator against small dry particles of a specific size, usually 0.1 micron. The filtration efficiency is calculated by comparing the results of the test samples to the results of control counts without a respirator in place. (Reference ASTM F 2299) A face mask is considered to be sub-micron filtering if the PFE is greater than or equal to 98% as referenced in ASTM F 2100.