A new method to measure the effective charge density responsible for the enhanced particle capture of electret filters is reported. The experimental method relies on the use of ionizing radiation from a Po-210 radioactive source combined with measurements of the penetration of particles of known sire and charge through the electret filter. The estimated charge density is a function of the ion generation rate, the discharge time and of the filter media mean fiber diameter and packing fraction. Three types of commercially available electret filter media were tested (i.e. A; split-fiber, B; mixed-fiber and C; meltblown media). The total mean charge densities estimated for filters type A, B, and C were 2.16x10(-3), 9.16x10(-4), and 2.58x10(-5) C/m(2) respectively. The ranking of the charge levels was consistent with the efficiencies measured for each type of filter (i.e. higher charge density resulted in higher filter efficiency). The estimated charge density of the split-fiber electret media compared well with values reported by other researchers for the same type of electret media. The proposed method shows good promise but needs to be optimized for improved accuracy in the determination of the total discharge time.