Is diffusion, thermodiffusion, or advection a primary mechanism of indoor radon entry?

Revision of the current theory of indoor radon entry is given. It is shown that there is insufficient evidence to accept the pressure-driven mechanism as the dominant mechanism of radon infiltration in homes. The criticism of the US Environmental Protection Agency's policy is presented. The similitude theory of gas transfer in Knudsen's and transition zones is developed. Comparison of the theory with experimental data confirms the validity of the assumptions made. Application of the theory to air flow in clay is shown. This theory allows the average pore radius in the clay sample to be estimated. That radius was found to be of the order of the mean free path of air molecules at atmospheric pressure. It is shown that thermodiffusion gas flow in clay and concrete can greatly exceed the pressure-driven flow. The necessity of including thermodiffusion gas flow in a general model of radon indoor entry is substantiated.