Comparisons of various indoor air treatment methods have shown that while flow-through air-cleaning methods, such as filtration and electrostatic precipitation, were effective in reducing total potential alpha energy concentrations (PAECs), they caused a greater percentage the radon decay products (subsequently formed through decay of the remaining radon gas) to be unattached to particles. Estimates show that this results in a substantial increase in the dose to the bronchial tissues of people breathing the treated air. The optimal form of air treatment appears to be a combination of nonuniform space charge generated by an ion generator and enhanced convection using a fan. Laboratory studies showed that this combination provided reductions in PAECs ranging up to 95%; reductions in the mean dose to the bronchial tissues ranged up to 87%. Tests of a portable fan-ion generator unit in homes in Massachusetts and Connecticut showed PAEC removals in the range of 75% to 90%.