Radon gas

Is exposure to radon gas a problem when building with earth bags? What could be done to prevent it?

I don't see how it would be, but don't have any specific scientific study to back that up. Radon is a naturally occurring gas the comes from decomposition of uranium (?) deep within the earth. It does not generate from the soils that would be used to fill earth bags. If radon is present in the soil, it will be there whatever for or building technique used. Earthbags have no influence over that. You will have radon percolate up from the deep no matter what structure you put on the surface, if it is there to start.

The critical factor in any structure, natural or industrial, is how well ventilated the structure is. Will the radon build up or dissipate? That is a function of design and not material.

As Jack said, Radon is a gas that is formed from the radioactive decay of Uranium.

This gas must have a pathway for migration into your home. These are faults and fractures above a uranium source. If you choose a site that is not on or around a fault, then you should be fine even if you are above a uranium source. All this remediation stuff is absurd. I have to laugh when I hear that someone would pull radon from the ground and spray it out above their home, when Radon is heavier than air and falls right down on you and your garden or whatever. Use a good flooring that limits air exchange from the ground and don't build over a fault.