In terms of radon, what does the term 'permeability' refer to?

The term 'permeability' in the context of radon specifically refers to the ability of soil to allow gases, such as radon, to flow through it. This characteristic is critical when assessing radon levels in a given area, as the permeability influences how easily radon can move from the ground into structures. High permeability in soil indicates that gases can readily pass through, which may lead to higher indoor radon concentrations as the gas seeps into homes and buildings. Understanding permeability helps in evaluating the potential radon exposure risk and determining appropriate mitigation strategies if necessary. The other options focus on aspects that do not directly relate to the movement of gases, such as moisture retention, soil density, or temperature, thus making them irrelevant to the definition of permeability in this context.