PAO has 0.1 to 1.0 micron particle size.which range of particle are scanned by scanner to detect the leakage in HEPA filter.
scanning is related to concentration used. its not related to particle size. limit - downstream leakage should not exceed 0.01% concentration
The scanner measuring base on the concentration of the smoke. But the smoke generator create the smoke with the size from 0.2-0.3 micro. And the hepa filter have the efficiency 99.97%at particle size of 0.3 micro.