Necro thread, but no one has pointed out yet that Radon is an inert gas. It doesn't react chemically. It's not absorbed by your body. If you inhale one or two atoms, you exhale those one or two atoms.
You neglected the fact that it is a high-energy alpha emitter. Not only that, but once it decays, it leaves a nice long chain of alpha-emitting daughters behind. As radon is a gas, it is inhaled, and the lungs are very radiation sensitive organs.
ICRP rate alpha-particles 20x as dangerous as beta/gamma on an equivalent (Gray for Gray) basis, so internal alpha irradiation carries a surprisingly high "biologically effective dose".
At the EPA limit of 4 pCi/l, you would receive a radiation dose of about 1 µSv/h, or 8 mSv per year. That's about 4x the normal natural background radiation, and represents an estimated 1 in 35 excess early cancer death during a life time (based on current knowledge of radiation biology). Obviously, this depends on the amount of time you stay in the at-risk area (which is mainly the basement, but in some cases, it can leak into the ground floor).
It's quite easy to see this. Your lungs contain, on average about 2 litres of air. If the Rn concentration is 4 pCi/l, that means you get 20 disintegrations per minute in your lungs, and as they are alpha, almost every disintegration will result in a hit of biological tissue (lung tissue is sponge like, and alpha particles are immediately absorbed by solid tissue).