term) and the dynamic (short-term) components of the exposure. The potentially most important source of short-term exposure is the release of radon from water during showering and the subsequent inhalation of its decay products. The steady-state component has been studied in considerable detail (Fitzgerald and others 1997; Bernhardt and Hess 1996) and is described in chapter 3. Only recently have there been studies of the time-varying exposure. The exposure assessment of waterborne radon includes both its contributions to long-term average indoor radon concentrations and the short-term, perturbed conditions that exist as a result of showering.

It is necessary to provide the radon-progeny activity size distribution as a function of time during and after showering. There have been two recent studies of the increments in exposure and dose that arise from showering with radon-laden water (Fitzgerald and others 1997; Bernhardt and Hess 1996).

Exposure conditions are quite different during showering because radon is transferred from water to air with little or no direct release of decay products from the water. As a result, there might be a high local concentration of ²²²Rn, but it takes time for the decay products to grow into equilibrium concentrations. Because the ingrowth of the activity will occur with an effective half-life of about 30 min, the highest concentrations of decay-product activity occur after the person