be drawn. With the new dosimetric models that more accurately reflect nasal and oral deposition of ultrafine particles, it is extremely unlikely that an air cleaner can reduce exposure and increase dose as suggested by Maher and others (1987), Sextro and others (1986), and Rudnick and others (1983). Thus, there is no reasonable likelihood that the use of an air cleaner will increase the hazards posed by indoor radon.

In studies of different types of air-cleaning devices, reductions in exposure have always exceeded reductions in dose. However, cases have been observed in which there has effectively been no reduction in dose. Thus, for many air cleaners, the clean-air delivery rate is insufficient to provide substantial protection from the radon decay-product hazard. The air cleaner might be effective in removing other contaminants—including cigarette smoke, dust, pollen, and spores—and thus provide a considerable benefit to an occupant without lowering the radon-progeny risk substantially and, more important, without raising that risk at all.

The experiments with the newest systems suggest that the combination of substantial air movement and ionization could provide sufficient reduction in exposure and dose to be effective in reducing the radon-progeny risk at radon concentrations up to around 400 Bq m-3. If it is desirable to reduce the risk to that equivalent to the average dwelling in the United States, then such units would be useful only for lower 222Rn concentrations. There would also need to be multiple units in a home to provide complete room-to-room reduction.

Mitigation Methods for New Construction

Radon-mitigation methods for new buildings can be incorporated directly into the construction process and both enhance the performance of the system and reduce the cost of installation, compared with the cost of retrofit mitigation methods. Systems for controlling radon concentrations described earlier have essentially the same applicability whether their use is in existing or new buildings, and they will not be discussed further in this section. Some cost savings might be associated with installation of systems like an HRV during the construction process or with integrating such a system into the space-conditioning system of the building.

In the following sections, the application of "existing-house" techniques for radon-entry control is discussed briefly and then a more systemic approach for making buildings radon-resistant as part of the construction process is discussed.

Application of Existing-House Radon-Entry Control Methods

As described earlier, one of the most widely used radon-mitigation techniques is ASD. Key to the successful implementation of these systems is reversal of the pressure gradient at all the major soil-gas entry points. Typically, this

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