The committee also noted that its quantitative estimates of risks posed by residential radon exposure depend strongly on the assumption of a linear relationship, without a threshold, between low-dose exposure to radon and risk. As reviewed in chapter 2, that assumption is based on our current understanding of the mechanisms of radon-induced lung-cancer, although it is recognized that this understanding is incomplete. The committee did not attempt to quantitatively address uncertainty due to the linear, no-threshold assumption, because specific mechanistically plausible alternative dose-effect relationships were not identified in the committee's review.

Despite the uncertainty, the committee concluded that the weight of evidence of the available data supports a finding that residential-radon exposure increases lung-cancer risk. The best estimate of risk that can be obtained at this time is based on the committee's analysis of the combined updated data on the 11 cohorts of underground miners. The committee noted that this estimate is consistent with that derived from a recent meta-analysis of summary relative risks from the 8 residential case-control studies conducted to date.

The committee questioned whether further residential studies are likely to clarify the uncertainty surrounding residential-radon lung-cancer risks. The case-control studies conducted to date have been somewhat inconsistent. While most are compatible with the hypothesis of an elevated cancer risk, their results could also be interpreted as compatible with the hypothesis of no increase in risk in light of inherent uncertainties in data. Further, the residential studies offer only very limited information on never-smokers. Clear evidence of an increased lung-cancer risk in miners, many of whom were exposed to levels of radon only about two-fold higher than associated with residence in some homes, played an important role in supporting the committee's conclusions about the likelihood of an increased lung-cancer risk due to residential radon exposures. The committee agreed with the recommendations of workshops conducted by the Department of Energy and the Commission of European Communities: further studies should not be initiated until studies now in progress are completed and the data are pooled from these studies and studies already completed are pooled.

The committee examined the effect of reductions in radon levels in U.S. homes on lung-cancer risk, assuming different scenarios of the efficiency of reduction. On the basis of the committee's categorical risk models, reducing radon concentration in all homes that are above 148 Bqm^-3 (4 pCiL^-1) to below 148 Bqm^-3 (4 pCiL^-1) is estimated to result in the avoidance of about 3 to 4% of lung-cancers.