Until very recently, radiation from natural sources was regarded as unremarkable and unalterable—a background phenomenon.

However, radon decay products are now recognized to be one of the largest sources of natural radiation and it is fairly easy to reduce levels in existing homes.

For all intents and purposes, exposure to the other natural sources cannot be controlled, and comprises a basic "background" dose from radiation. Individuals are exposed to this non life-threatening background radiation on a continuing basis.

Man-made radiation sources, on the other hand, are more susceptible to control than natural sources. Potential doses from man-made sources can be minimized by controlling their manufacture and distribution.