In the standard of the approval test for dust respirator certification, test aerosols are defined as air containing quartz particles of 2 microns or smaller1. However, it is well-known that the filtration efficiency of air filters depends on the size of the test aerosol particles. Alternative test aerosols containing submicron size particles will be studied for use in future approval test of dust respirators. Before searching for alternatives, characterization of currently-used quartz dust, e.g. information about size and electrostatic charge of the aerosol particles, should be determined. The currently-used approval test measures filtration efficiencies with a light scattering photometer. The photometer gives overall aerosol concentration, while Aerodynamic Particle Sizer (APS-33B, TSI Inc., St. Paul, U.S.A.) is able to give the number concentration for each particle size based on the aerodynamic diameter ranging from 0.9 to 2.0 microns. In this study, equivalent diameter was defined as the diameter at which penetration has the same value when measured by APS-33B and by light scattering photometer. The equivalent diameter for the ten filter samples for dust respirator was around 1 microns, irrespective of neutralization of the test aerosol.