Area Of Effect
Radiation is an emanation poison, meaning that a victim only needs to enter an area suffused with radiation to be affected by it. Radiation suffuses a spherical area of effect that can extend into solid objects. The closer one gets to the center of an area of radiation, the stronger the radiation effect becomes. Radiation entries list the maximum level of radiation in an area, as well as the radius out to which this radiation level applies. The radiation continues to suffuse each increment out to an equal length beyond that radius, its strength degraded by one level per increment. For example, a spherical area of high radiation with a radius of 20 feet creates a zone of medium radiation spanning 20 feet to 40 feet from the center in all directions, and a similar zone of low radiation spanning 40 to 60 feet from the center.
Source
Core Rulebook pg. 403