This study investigates filtration of air as potential mitigation method of thoron progeny exposure. The experiments were conducted in a model room (volume 7.1 m(3)) which was equipped with a pump and an HEPA (high efficiency particulate air) filter. Filtration at a rate of 0.2, 0.4, 0.5 and 0.8 h(-1) during 88 h proved an effective practice in reducing the total indoor thoron decay product concentration. The results indicate that 0.4-0.8 h(-1) filtration rate had almost the same filtration efficiency in decreasing the total thoron EEC (equilibrium equivalent concentration) by 97% while 80% of total thoron EEC were reduced by 0.2 h(-1) filtration rate; meanwhile, the unattached thoron EEC rose significantly by 190, 270, 290%, respectively under 0.4-0.8 h(-1) filtration rate, whereas 0.2 h-1 filtration rate increased unattached thoron EEC by 40%. The aerosol number size distribution variation reveals that filtration operation removes smaller particles faster or earlier than the larger ones. The annual effective dose calculated was reduced by 91-92% at a filtration rate of 0.4-0.8 h-1 while 75% reduced at 0.2 h(-1) filtration rate after 88 h filtration process.