It is often assumed that the unattached fraction of the exposure to suspended radon progeny activity can be predicted from particle concentration measurements. While this may be true in many circumstances, there are important exceptions to this rule. In particular, in an atmosphere far from radioactive equilibrium, such as a working area in a mine close to a source of radon and supplied with fresh air, the unattached fraction can be a factor of ten or more greater than indicated by a simple particle concentration measurement. Recent measurements made in a diesel powered uranium mine are presented to illustrate this effect.