concentrations below the EPA guideline of 150 Bq m-3. Followup testing appears to be very rare. If the building is in a region of the country where radon testing is done as part of real-estate transactions, retesting might occur at that time.

In the case of new construction, particularly where radon-resistance building-construction codes are in place, there is usually no requirement for post-construction testing. Because indoor radon concentrations can be heavily influenced by the operation of a building, such as the use of a heating system (which creates the stack effect), and by occupant behavior, it is essential that radon be measured when the building is occupied. An occupant of a new home built in compliance with a radon-resistance construction code is not likely to have a strong incentive to conduct followup testing. In the case of new construction, the period between completion of construction and occupancy can be several months, and this reduces the likelihood that the construction contractor will have radon testing done.

If control of radon concentrations in indoor air is to be used as an alternative means of reducing radon (decay-product) exposures of the customers of a water-supply system, periodic testing of indoor radon concentrations will be necessary to ensure continued performance of the radon-control methods used. Because these alternatives—reduction of radon concentrations in the drinking water and reduction of radon in indoor air—can be compared only on the basis of health risks (not just indoor radon concentrations), long-term airborne-radon measurements are essential, in that they are the only basis for assessing the health risks associated with airborne radon.

Improved Estimation of the Effect of Radon-Resistant Construction

No rigorous test of the effect of radon-resistance construction practices has been done outside of work done for the FRRP. To be sure, there is evidence that radon-resistance systems can reduce the rate of radon entry into buildings in some cases, but it is not possible to determine the quantitative extent of the reduction on the basis of available data. The data are sparse, both in terms of the numbers of houses examined (for example, the variability in building types and construction practices has not been examined in detail) and in the actual followup testing procedures implemented. It is important to be able to make comparisons between similar houses built in the same area with and without radon-resistance techniques. There are a number of reasons for doing so. Aside from establishing baseline conditions with which to compare the effects of building homes radon-resistant, it will also ensure that the effects can be solidly established. Even though the current EPA building guidelines are applicable mainly in EPA zone 1 areas (the region thought to have the highest radon potential), the vast majority of homes in this region (about 86%) have annual average living-area concentrations below the 150-Bq m -3 guideline. That means that establishing whether radon



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