Cohort studies have consistently shown underground miners exposed to high levels of
radon to be at excess risk of
lung cancer, and extrapolations based on those results indicate that residential
radon may be responsible for nearly 10-15% of all
lung cancer deaths per year in the United States. However, case-control studies of residential
radon and
lung cancer have provided ambiguous evidence of
radon lung cancer risks. Regardless, alpha-particle emissions from the short-lived radioactive
radon decay products can damage cellular
DNA. The possibility that a demonstrated lung
carcinogen may be present in large numbers of homes raises a serious public health concern. Thus, a systematic analysis of pooled data from all North American residential
radon studies was undertaken to provide a more direct characterization of the public health risk posed by prolonged
radon exposure. To evaluate the risk associated with prolonged residential
radon exposure, a combined analysis of the primary data from seven large scale case-control studies of residential
radon and
lung cancer risk was conducted. The combined data set included a total of 4081 cases and 5281 controls, representing the largest aggregation of data on residential
radon and
lung cancer conducted to date. Residential
radon concentrations were determined primarily by a-track detectors placed in the living areas of homes of the study subjects in order to obtain an integrated 1-yr average
radon concentration in indoor air. Conditional likelihood regression was used to estimate the excess risk of
lung cancer due to residential
radon exposure, with adjustment for attained age, sex, study, smoking factors, residential mobility, and completeness of
radon measurements. Although the main analyses were based on the combined data set as a whole, we also considered subsets of the data considered to have more accurate
radon dosimetry. This included a subset of the data involving 3662 cases and 4966 controls with a-track
radon measurements within the exposure time window (ETW) 5-30 yr prior to the index date considered previously by Krewski et al. (2005). Additional restrictions focused on subjects for which a greater proportion of the ETW was covered by measured rather than imputed
radon concentrations, and on subjects who occupied at most two residences. The estimated odds ratio (OR) of
lung cancer generally increased with
radon concentration. The OR trend was consistent with linearity (p = .10), and the excess OR (EOR) was 0.10 per Bq/m3 with 95% confidence limits (-0.01, 0.26). For the subset of the data considered previously by Krewski et al. (2005), the EOR was 0.11 (0.00, 0.28). Further limiting subjects based on our criteria (residential stability and completeness of
radon monitoring) expected to improve
radon dosimetry led to increased estimates of the EOR. For example, for subjects who had resided in only one or two houses in the 5-30 ETW and who had a-track
radon measurements for at least 20 yr of this 25-yr period, the EOR was 0.18 (0.02, 0.43) per 100 Bq/m3. Both estimates are compatible with the EOR of 0.12 (0.02, 0.25) per 100 Bq/m3 predicted by downward extrapolation of the miner data. Collectively, these results provide direct evidence of an association between residential
radon and
lung cancer risk, a finding predicted by extrapolation of results from occupational studies of
radon-exposed underground miners.