THE CURRENT STATUS OF RESIDENTIAL RADON EPIDEMIOLOGY

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1 THE CURRENT STATUS OF RESIDENTIAL RADON EPIDEMIOLOGY RW Field * Department of Epidemiology, N222 Oakdale Hall, University of Iowa, Iowa City, IA USA ABSTRACT Lung cancer is the major cause of cancer death in the United States. Because of the magnitude of lung cancer incidence and its poor survival rate, even secondary causes of lung cancer, such as prolonged residential radon exposure, are important. The primary objective of this paper is to provide a brief overview of both the major occupational radon studies and previously published residential radon case-control studies with a focus on the past, current and future residential radon epidemiologic activities in the United States. Extrapolations from occupationally radon-exposed miners suggest that residential radon exposure is responsible for approximately 18,600 lung cancer deaths per year in the United States. Direct evidence from case-control radon epidemiologic investigations generally support the extrapolations from miners. Findings from case-control studies incorporating new radon gas and radon progeny exposure techniques suggest that earlier case-control studies may have underestimated the risk posed by residential radon exposure. INDEX TERMS Radon, Epidemiologic, Lung Cancer, Residential, Case-Control INTRODUCTION Lung cancer is responsible for 31% and 25% of all cancer deaths in males and females, respectively, in the United States (U.S.)(Greenlee et al. 2001). The majority (85%) of the lung cancer deaths are directly attributable to tobacco usage. However, because of the magnitude of lung cancer incidence and its poor survival rate, even secondary causes of lung cancer are of great public health consequence. Radon is a known human carcinogen (IARC 1998, ATSDR 1990). In fact, the occupational data set linking radon progeny (radon) exposure and lung cancer is almost unparalleled in comparison to other occupational carcinogens (NRC 1998). Based on comparative risk analyses, numerous state and the federal agencies have ranked residential radon exposure as one of the most serious environmental hazards facing the U.S. (Johnson 2000). While it is has been well established that prolonged radon exposure at high concentrations causes lung cancer, the risk posed by residential radon exposure has not been as well characterized. This paper provides a brief overview of both the major occupational radon studies and previously published residential radon case-control studies with a focus on the past, current and future activities in the field of residential radon epidemiology within the United States. * Contact author bill-field@uiowa.edu 617

2 UNDERGROUND MINER EPIDEMIOLOGIC STUDIES Findings from approximately 20 underground miner studies have served as the foundation for the current risk projections for residential radon exposure (NRC 1998). Although, the methodologies differed between the various miner studies, there was a surprising degree of consistency in the findings. There studies showed excess lung cancer mortality, as compared to lung cancer mortality from the general population, with an increasing risk associated with increased radon progeny exposure. A National Cancer Institute (NCI) epidemiologic pooling of 11 of these studies encompassing 65,000 miners from uranium mines in the United States (Colorado, New Mexico), France, Australia, the Czech Republic, Canada (Ontario, Port Radium, Beaverlodge), metal miners in Sweden, fluorspar miners in Canada and tin miners in China found a consistent linear relationship between cumulative radon progeny exposure and lung cancer deaths within the range of miners exposures (Lubin et al. 1995). From these findings, the NCI researchers inferred that residential radon exposure would also carry some risk. To further examine risks at lower exposures, Lubin and colleagues (1997) performed a restricted analysis of miners whose radon progeny exposures extended into the levels experienced by some homeowners. Again a generally linear response was noted with a statistically significant increasing trend of lung cancer incidence with increasing exposure. The National Research Council s (NRC s) Committee on Health Risks of Exposure to Radon estimated, from studies of underground miners, that residential radon exposure accounts for approximately 18,600 lung cancer deaths each year in the United States. Unfortunately, there are significant uncertainties in the risk estimate because of the difficulty of assessing radon exposure in the mines, differences between the mines and residential settings (dust, silica, progeny equilibrium, etc), differences in behaviors between the miners and homeowners (breathing rates, volume of air inspired, etc.) and uncertainties related to the extrapolations. Partially because of these uncertainties, the BEIR VI Committee pointed out that a more direct way to derive risk estimates for residential radon exposure is to compare residential radon exposure among people in the general population who have lung cancer versus the exposure received by people in the population who have not developed lung cancer. RESIDENTIAL RADON EPIDEMIOLOGIC STUDIES The two major types of epidemiologic study designs that have been performed to date include the descriptive ecologic study design and the observational case-control design. The majority of ecologic studies have correlated geographically based lung cancer rates with a mean radon concentration obtained from short term radon measurements conducted in a specified geographic area (Neuberger 1991, Stidley and Samet 1993, Neuberger et al. 1994). The ecologic study design has major limitations for assessing risk and does not utilize an individual's current or retrospective radon exposure. Because of these limitations, the 1989 Study Design Group of the International Workshop on Residential Radon Epidemiology concluded that future ecologic studies are not endorsed for the assessment of residential radon risk (U.S. DOE 1989). The case-control study design overcomes many of the limitations inherent in ecologic studies as data are collected at the level of the individual, and people with lung cancer (cases) are compared to people without lung cancer (controls). Because case-control studies collect information at the individual level, the study design is inherently subject to less bias than the ecologic study design. The residential case-control studies do not require 618

3 projections from underground miner data. Major residential case-control studies have been performed in Canada (Letourneau et al. 1994), China (Blot et al. 1990), Finland (Auvinen et al. 1996, Ruosteenoja et al. 1996), Germany (Kreienbrock et al. 2001), Sweden (Pershagen et al. 1992, Pershagen et al. 1994), the United Kingdom (Darby et al. 1998), and the United States (Alavanja et al. 1994, Alavanja et al.1999, Field et al. 2000). A 1997 National Cancer Institute report directly examines the effects of residential radon exposure on lung cancer risks by using data from eight of the previously published largescale case-control studies that were performed in Canada (Letourneau et al. 1994), China (Blot et al. 1990), Finland (Auvinen et al. 1996, Ruosteenoja et al. 1996), Sweden (Pershagen et al. 1992, Pershagen et al. 1994), and the United States (Schoenberg et al. 1992, Alavanja et al. 1994). The results of the meta-analysis showed an increased summary excess risk of 14% at an average radon exposure of 148 Bq.m -3 (Lubin and Boice 1997). The findings from the more recent residential epidemiologic studies performed in Germany (Kreienbrock et al. 2001) and the United Kingdom (Darby et al. 1998) are in close agreement with the risk estimates obtained from the meta-analysis performed by the NCI. Five large-scale residential radon epidemiologic studies have been performed in the United States. The published findings include one study in both New Jersey (Schoenberg et al. 1990; Schoenberg et al. 1992), and Iowa (Field et al. 2000) and two studies in Missouri (Alavanja et al. 1994, Alavanja et al. 1999). A combined states study in Utah, Connecticut and Southern Idaho (Sandler et al. 1999) has been completed, but the results have not been published. Three out of the four published U.S. studies have suggested an increased risk for residential radon exposure (Schoenberg et al. 1992, Alavanja et al.1999, Field et al. 2000). The first U.S. study to report a risk from residential radon exposure was the New Jersey Study (Schoenberg et al. 1990, Schoenberg et al. 1992), which had a statistically significant odds ratio for the highest exposure category based on the 90% confidence interval. The authors also reported a statistically significant test for trend in odds ratios with increasing radon exposure. However, the authors urged cautious interpretation of their results because the upper exposure category contained a limited number of subjects. The only U.S. studies that failed to document an association between residential radon exposure and lung cancer were the Missouri-I study that focused on non-smoking women (Alavanja et al. 1994) and the gas measurement phase of the Missouri-II study (Alavanja et al. 1999). Alavanja has suggested that the inability to detect an association in these studies may have been due to poor retrospective radon gas exposure assessment (Alavanja et al. 2000). Historical radon gas exposure reconstruction is often difficult because many houses will no longer exist, others may have a current occupant who refuses entry, and still others may have current radon concentrations that differ from the concentrations that were typically found in the home when the study subject lived there. This missing data and radon variation reduces a study s power to detect an association. The glass-based measurement (Alavanja et al. 1999) phase of the Missouri-II study found a statistically significant upper exposure category and a statistically significant lung cancer trend with increasing radon concentrations. The excess lung cancer risk noted for average radon exposures exceeding 150 Bqm -3 was greater than 200%. The positive findings based on the glass-based dosimetry technique was possibly attributable to decreased exposure misclassification using the glass-based technique. The alternative glass-based 619

4 dosimetry approach (Field et al. 1999) measures the accumulation of a long-lived radon decay product, 210 Pb, in glass items through the alpha emission of a subsequent decay product, 210 Po. A specially designed detector that can be attached to a glass surface, such as a graduation picture, measures the alpha emission. The mean radon exposure of the glass, and by proxy its owner, over its placement period in the home can be calculated from the known decay properties of radon and the deposition of its progeny. This approach minimizes some of the retrospective measurement uncertainty due to missing data or residential radon variation over time. In fact, recent studies have shown good correlation between the historically measured cumulative radon gas exposure and the measured glass surface activity (Steck et al. 2002). The Iowa Study (Field et al. 2000) also found statistically significant increased categorical trends in lung cancer risk with increasing cumulative radon exposure. An excess lung cancer risk of approximately 50% was noted at average radon exposures of 150Bqm -3. The Iowa findings also suggested that improved retrospective radon estimates enhanced the study s ability to detect an association between radon exposure and lung cancer. Because the Iowa study limited enrollment to only women who lived in their current home for at least the previous 20 years, gaps in the radon measurement data were minimized. The study also optimized exposure assessment by linking information on where the subject spent time with numerous radon measurements. In fact, information on personal mobility was linked to the radon concentrations to determine a cumulative retrospective radon exposure. An error analysis recently performed by Field et al. (2002) indicated that alternative exposure methods used by other residential radon studies, which are less rigorous, likely produce lower risk estimates (Field et al. 2002). The Iowa Study also incorporated glass-based detectors, but the results have not been published yet. SUMMARY AND FUTURE ACTIVITIES IN RESIDENTIAL RADON EPIDEMIOLOGY This paper summarizes the findings of the residential radon studies. Additional detailed review of the individual residential radon studies performed in the United States is published elsewhere (Field 2001). The scientific foundation for the carcinogenicity of prolonged residential radon exposure will be further refined as the individual case-control studies are concluded, the various poolings are completed, and further research is performed on the cellular level. Further validation of the glass-based detectors is currently underway. Once glass-based results from Iowa (Field et al. 2000) are published, a pooling of glass-based detectors between the Missouri (Alavanja et al. 1999) and Iowa studies are planned. Also in progress, is a pooling of the North American residential studies that will include approximately 4,000 lung cancer cases and over 5,000 controls. Additional casecontrol residential epidemiologic studies are nearing completion in Europe, and a future pooling of these studies is planned. Eventually, a joint American and European pooling is anticipated. In summary, the general findings from the existing residential radon studies along with the extrapolations from underground miners support the contention that prolonged exposure to residential radon contributes to a significant increase in lung cancer risk. 620

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