The series of radioactive elements, or daughters, that come from the decay of uranium and thorium include radon-222 and radon-220, which contribute 37% of an individual's total radiation exposure. Radon-222 and radon-220, referred to collectively as radon, seeps out of the earth all over the world. 

The doses from radon, however, are contributed by the radon decay products (daughters) rather than by the gas itself. The decay products of the gas are solid, and they attach themselves to dust particles in the air which when inhaled, irradiate the lung.

Radon contributes, on average, about 200 mrem/year (2 mSv/year), which is one-half of the dose received by individuals from all natural and man-made sources.