codes and guidelines for new buildings. In addition, some builders in various parts of the United States have voluntarily adopted construction practices that they believe will limit radon entry into new homes (Spears and Nowak 1988). In 1991, Washington state adopted a radon provision as part of the Washington State Ventilation and Indoor Air Quality Code (WSBCC 1991). It provides specific details for houses built with crawlspaces for the entire state and specifies radon-resistance features for eight counties thought to have potential for high indoor radon concentrations. The specifics include use of aggregate and a membrane below the floor slab, sealing of all floor penetrations and joints, and the use of a passive stack extending from the subslab through the heated portion of the house and exhausting through the roof. At the time of its inception, the code also required provision of a long-term radon monitor in each new home; this part of the code is no longer in force.

In March 1994, EPA published a set of standards and techniques for construction of radon-resistant residential buildings (EPA 1994a). These provisions, along with EPA's county-by-county radon zone map of the United States, were incorporated as a recommendation in the Council of American Building Officials residential building code and have been adopted, with modifications in some cases, by various local building-code authorities around the country.

In 1989, Florida initiated the Florida Radon Research Program (FRRP), which was designed to be a comprehensive program of research to examine many of the details involved in typical residential construction practice and how they might be modified to provide resistance to radon entry (Sanchez and others 1990). Many of the features are directed toward reducing radon entry in the elevated slab-on-grade construction method used widely in that state. Particular attention has been paid to attempting to ensure the integrity of the floor slab and to provide a subslab membrane that is sealed at all floor penetrations (for example, plumbing pipes) and at the edge of the slab.

The FRRP, conducted in cooperation with EPA, has produced the most extensive research on radon-resistant new construction to date. Most of the features in the proposed code have been evaluated, but in only a small number of houses (see, for example Fowler and others 1994; Hintenlang and others 1994; Najafi and others 1993; Najafi and others 1995). One part of the FRRP was the development of a radon-potential map of the state, delineating regions where no special radon controls are needed, regions where only passive (radon-resistance) features are required, and regions where both radon resistance and ASD are needed (Rogers and Nielson 1994).

Results of several house-evaluation studies conducted as part of the FRRP have shown that most of the houses built in conformity with the proposed standard appear to have short-term indoor radon concentrations below 150 Bq m-3. However, there are a number of important limitations regarding these results, the most important of which might be the timing and duration of the indoor radon testing. With few exceptions, the indoor radon was measured after construction

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