resistance is effective will require a careful sampling design to ensure that the comparison can be established with statistical validity.
It is well known that radon concentrations in a house vary from day to day, season to season, and to some extent year to year. Some of the variations are driven by the weather and some by the behavior of the occupants indoors, such as window-opening and door-opening, and furnace and exhaust-fan operation. Thus, such comparisons should be conducted over periods long enough to average out the effects of behavior. Minimum measurement times would be two seasons, but care would have to be taken not to compare data on different houses that were taken during different seasons.
Changes due to the settling and aging of a building substructure over time can create—or extend openings through which soil gas can enter a building. Most radon-resistance approaches use a high-permeability zone just below the slab, created by either a gravel layer or a drainage mat system. The purpose of this zone is to ensure adequate lateral extension of the pressure field created by the passive stack. However, recent theoretical and experimental research has shown that such high permeability zones can substantially enhance radon entry, compared with construction in which the subslab region is not altered (Robinson and Sextro 1995; Gadgil and others 1994; Revzan and Fisk 1992). Not only might the radon-entry rate increase, because this high-permeability zone acts as a uniform-pressure plenum, but the effect of crack size (area) and location is reduced. Even cracks with small total area can transmit as much radon as openings with areas 10—15 times larger (Robinson and Sextro 1995). Thus, it is extremely important to evaluate radon resistance as buildings age to ensure that the radon resistance features are not compromised.
Research and data are needed that would permit reliable estimates of the benefits that might accrue through encouraging the use of radon-resistance construction practices in new houses as an alternative means of reducing radon-related risks. Specific research objectives are discussed in chapter 10.
One approach to meeting the requirements of the Safe Drinking Water Act Amendments of 1996, with respect to lowering the risk associated with radon, is to treat the water directly. Several studies have evaluated water-treatment technologies for their ability to lower the radon concentration in water. Drago (1998) reported the removal efficiency, flow range, and construction cost of 34 mitigation systems now being used in small and large communities to remove radon from drinking water (table 8.1). The purpose of this section is to present an overview of existing and emerging technologies for removing radon from drinking water.