term behavior of houses built with radon-resistance features. One house in Florida built and tested as part of the FRRP was revisited as part of an examination of the durability of active radon-mitigation systems. At the time of construction, provision was made for installation of an active system because the soil-gas concentrations were about 60,000 Bq m-3. The postconstruction indoor radon concentration was 60 Bq m-3, so an active system was never installed. The revisit, only 16 mo after construction, found essentially the same indoor radon concentration (70 Bq m-3), on the basis of the results of two long-term alpha-track measurements over periods of 4 and 5 mo (Dehmel and others 1993).
Essentially no studies have explicitly examined whether there might be an upper limit to the efficacy of purely radon-resistant construction. The research conducted in the FRRP gave some indication that there could be an upper limit. For example, the postconstruction indoor radon concentration was 460 Bq m-3 in a house where the soil-gas radon concentration was 290,000 Bq m-3 (Najafi and others 1995). In another study, the author concluded that radon-resistant techniques could be used in Florida for soil-gas concentrations up to 310,000 Bq m-3, as long as indoor air-exchange rates were kept above about 0.3 h-1 (Hintenlang and others 1994). The modeling done in support of the development of a radon potential map identified areas where the soil radon-potential was high enough that the reduction factors used for radon resistance were not sufficient to ensure that indoor radon concentrations would remain below 150 Bq m-3. These regions were mapped as needing the use of an ASD system, in addition to the radon-resistance features (which often enhance the performance of active systems by limiting air flow between the interior of the house and the depressurized region below the floor slab) (Nielson and others 1994; Rogers and Nielson 1994).
In a few radon-resistant houses examined in the various studies described earlier, indoor radon concentrations exceeded 150 Bq m-3 (based on short-term testing). Most of these houses also had soil-gas concentrations exceeding 40,000 Bq m-3 (where such measurements were done). The degree to which radon-resistance construction standards and guidelines were followed was highly variable in these studies, so it is difficult to determine whether the resulting indoor (basement) radon concentrations above 150 Bq m-3 were due to inadequate construction techniques, low air-exchange rates (in some cases, postconstruction radon testing was done before occupancy), or inherent limits to the principle of radon-resistant construction.
All the radon or radon-progeny control systems that have been described have failure modes that can reduce or eliminate the effectiveness of mitigation.