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The brick houses,the mud houses and the caves are the three typical rural residential houses in China,usually with naked surface from where the thoron gas easily comes out.In order to evaluate the thoron exposure of those indoor environments,we carried out a series of field measurements using a portable thoron progeny concentration measuring device and a portable thoron progeny size distribution measuring device.The dose conversion factors and annual effective doses of thoron exposure in those environments are calculated using dosimetric methods.Comparing with the results in urban indoor environment,it shows that the thoron progeny size distributions of rural indoor environments(AMAD: 76.5 nm; GSD: 2.7) are much smaller than those of urban(AMAD: 115 nm; GSD: 2.0),which make the dose conversion factors of thoron in rural environments(307.4 nSv/(Bq·m-3·h-1)) are much higher than those in urban indoor environments(113.4 nSv/(Bq·m-3·h-1)).The annual average effective dose of thoron is influenced by dose conversion factor as well as thoron equilibrium equivalent concentration,and the value in mud house of Yangjiang is 10.12 mSv·a-1 according to our survey,which need much attention.