Daniel J. Steck

Residential radon and risk of lung cancer : A combined analysis of 7 north american case-control studies

Background: Underground miners exposed to high levels of radon have an excess risk of lung cancer. Residential exposure to radon is at much lower levels, and the risk of lung cancer with residential exposure is less clear. We conducted a systematic analysis of pooled data from all North American residential radon studies. Methods: The pooling project included original data from 7 North American case–control studies, all of which used long-term α-track detectors to assess residential radon concentrations. A total of 3662 cases and 4966 controls were retained for the analysis. We used conditional likelihood regression to estimate the excess risk of lung cancer. Results: Odds ratios (ORs) for lung cancer increased with residential radon concentration. The estimated OR after exposure to radon at a concentration of 100 Bq/m3 in the exposure time window 5 to 30 years before the index date was 1.11 (95% confidence interval = 1.00–1.28). This estimate is compatible with the estimate of 1.12 (1.02–1.25) predicted by downward extrapolation of the miner data. There was no evidence of heterogeneity of radon effects across studies. There was no apparent heterogeneity in the association by sex, educational level, type of respondent (proxy or self), or cigarette smoking, although there was some evidence of a decreasing radon-associated lung cancer risk with age. Analyses restricted to subsets of the data with presumed more accurate radon dosimetry resulted in increased estimates of risk. Conclusions: These results provide direct evidence of an association between residential radon and lung cancer risk, a finding predicted using miner data and consistent with results from animal and in vitro studies.

A Combined Analysis of North American Case-Control Studies of Residential Radon and Lung Cancer

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 α-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 α-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 α-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. E. G. Létourneau and J. B. Schoenberg have retired; J. A. Stolwijk holds an emeritus position. We acknowledge the helpful input of the following individuals who served on the International Steering Committee for the North American combined analysis: Ken Chadwick (CEC Radiation Protection Program), Susan Conrath (U.S. Environmental Protection Agency), Sarah Darby (Oxford University), Evan Douple (U.S. National Academy of Sciences), Colin Muirhead (UK National Radiation Protection Board), and Susan Rose (U.S. Department of Energy). Salary support for Drs. Field, Lynch, and Steck was provided in part by grant numbers R01 ES05653 and P30 ES05605 from the National Institute of Environmental Health Sciences, NIH and grant number R01 CA85942 from the National Cancer Institute, NIH. This research was supported by grants from the Canadian Institutes of Health Research (formerly the Medical Research Council of Canada) and the Natural Sciences and Engineering Research Council of Canada to D. Krewski, who currently holds the NSERC/SSHRC/McLaughlin Chair in Population Health Risk Assessment at the University of Ottawa. Financial support for the meetings of the Analysis Team and the Steering Committee was also provided by Health Canada and the U.S. Department of Energy. We are grateful to Dr. Huixia Jiang for assistance with the combined analysis, and to Jackie Monaghan for technical assistance in preparing this report.

An Overview of the North American Residential Radon and Lung Cancer Case-Control Studies

Lung cancer has held the distinction as the most common cancer type worldwide since 1985 (Parkin et al., 1993). Recent estimates suggest that lung cancer accounted for 1.2 million deaths worldwide in 2002, which represents 17.6% of the global cancer deaths (Parkin et al., 2005). During 2002, the highest lung cancer rates for men worldwide reportedly occurred in North America and Eastern Europe, whereas the highest rates in females occurred in North America and Northern Europe (Parkin et al., 2005). While tobacco smoking is the leading risk factor for lung cancer, because of the magnitude of lung cancer mortality, even secondary causes of lung cancer present a major public health concern (Field, 2001). Extrapolations from epidemiologic studies of radon-exposed miners project that approximately 18,600 lung cancer deaths per year (range 3000 to 41,000) in the United States alone are attributable to residential radon progeny exposure (National Research Council, 1999). Because of differences between the mines and the home environment, as well as differences (such as breathing rates) between miners and the general public, there was a need to directly evaluate effects of radon in homes. Seven major residential case-control radon studies have been conducted in North America to directly examine the association between prolonged radon progeny (radon) exposure and lung cancer. Six of the studies were performed in the United States including studies in New Jersey, Missouri (two studies), Iowa, and the combined states study (Connecticut, Utah, and southern Idaho). The seventh study was performed in Winnipeg, Manitoba, Canada. The residential case-control studies performed in the United States were previously reviewed elsewhere (Field, 2001). The goal of this review is to provide additional details regarding the methodologies and findings for the individual studies. Radon concentration units presented in this review adhere to the types (pCi/L or Bq/m3) presented in the individual studies. One picocurie per liter is equivalent to 37 Bq/m3. Because the Iowa study calculated actual measures of exposure (concentration × time), its exposures estimates are presented in the form WLM5–19 (Field et al., 2000a). WLM5–19 represents the working level months for exposures that occurred 5–19 yr prior to diagnosis for cases or time of interview for control. Eleven WLM5–19 is approximately equivalent to an average residential radon exposure of 4 pCi/L for 15 yr, assuming a 70% home occupancy. Ernest G. Létourneau and Janet B. Schoenberg are retired; Jan A. Stolwijk has emeritus status. Salary support for Drs. Steck and Field was provided in part by grant numbers R01 ES05653 and P30 ES05605 from the National Institute of Environmental Health Sciences, NIH and grant number R01 CA85942 from the National Cancer Institute, NIH.

Spatial and Temporal Indoor Radon Variations

This paper examines the ability of standard radon measurement protocols to predict long-term radon concentrations in houses located in the upper Midwest. It was observed that: (1) significant radon variations can occur on a spatial scale as small as a single floor; (2) radon measurements that integrate for periods less than 3 mo are reliable only to within a factor of 2 or more; and (3) contemporary, short-term measurements within existing structures may not accurately reflect past radon concentrations. Two-hundred forty-three occupied houses located in 40 towns were monitored for at least 1 y using alpha-track detectors. If lifetime radon exposure estimates need to be determined accurately, then long-term, integrating radon detectors should be placed in several rooms of each house. In radon atmospheres that may not be stable for long periods of time, it is suggested that multiple, year-long measurements or surface alpha activity measurements in combination with year-long alpha-track measurements are needed for an accurate lifetime radon assessment.

Long-term radon concentrations estimated from 210Po embedded in glass.

Measured surface-alpha activity on glass exposed in radon chambers and houses has a linear correlation to the integrated radon exposure. Experimental results in chambers and houses have been obtained on glass exposed to radon concentrations between 100 Bq m-3 and 9 MBq m-3 for periods of a few days to several years. Theoretical calculations support the experimental results through a model that predicts the fractions of airborne activity that deposit and become embedded or adsorbed. The combination of measured activity and calculated embedded fraction for a given deposition environment can be applied to most indoor areas and produces a better estimate for lifetime radon exposure than estimates based on short-term indoor radon measurements.

Residential radon exposure and lung cancer: Variation in risk estimates using alternative exposure scenarios

The most direct way to derive risk estimates for residential radon progeny exposure is through epidemiologic studies that examine the association between residential radon exposure and lung cancer. However, the National Research Council concluded that the inconsistency among prior residential radon case-control studies was largely a consequence of errors in radon dosimetry. This paper examines the impact of applying various epidemiologic dosimetry models for radon exposure assessment using a common data set from the Iowa Radon Lung Cancer Study (IRLCS). The IRLCS uniquely combined enhanced dosimetric techniques, individual mobility assessment, and expert histologic review to examine the relationship between cumulative radon exposure, smoking, and lung cancer. The a priori defined IRLCS radon-exposure model produced higher odds ratios than those methodologies that did not link the subjects retrospective mobility with multiple, spatially diverse radon concentrations. In addition, the smallest measurement errors were noted for the IRLCS exposure model. Risk estimates based solely on basement radon measurements generally exhibited the lowest risk estimates and the greatest measurement error. The findings indicate that the power of an epidemiologic study to detect an excess risk from residential radon exposure is enhanced by linking spatially disparate radon concentrations with the subjects retrospective mobility.

210Po implanted in glass surfaces by long term exposure to indoor radon.

Recent epidemiologic investigations of the relationship between residential radon gas exposure and lung cancer relied on contemporary radon gas measurements to estimate past radon gas exposures. Significant uncertainties in these exposure estimates can arise from year-to-year variation of indoor radon concentrations and subject mobility. Surface implanted 210Po has shown potential for improving retrospective radon gas exposure estimates. However, in previous studies, the ability of implanted 210Po activity to reconstruct cumulative radon gas exposure was not tested because glass was not available from homes with known radon-gas concentration histories. In this study, we tested the validity of the retrospective radon gas reconstruction using implanted 210Po surface activity by measuring glass surfaces from homes whose annual-average radon gas concentrations had been measured almost every year during two decades. Regression analysis showed a higher correlation between measured surface activity and cumulative radon gas exposure in these homes (R2>0.8) than was observed in homes where only contemporary radon gas measurements were available. The regression slope (0.57 ky m−1) was consistent with our earlier retrospective results. Surface activity measurements were as reliable for retrospective radon gas exposure reconstruction as yearlong gas measurements. Both methods produced estimates that were within 25% of the long-term average radon gas concentrations in a home. Surface measurements can be used for home screening tests because they can provide rapid, reliable estimates of past radon gas concentrations. Implanted 210Po measurements are also useful in retrospective epidemiologic studies that include participants who may have been exposed to highly variable radon concentrations in previously occupied or structurally modified homes.

Spatial variation of residential radon concentrations: The Iowa radon lung cancer study

Homeowners and researchers frequently estimate the radon concentrations in various areas of the home from a single radon measurement often performed in the homes basement. This study describes the spatial variation of radon concentrations both between floors and between rooms on the same floor. The geometric mean basement and first floor radon concentrations for one-story homes were 13.8% and 9.0% higher, respectively, as compared to their counterparts in two-story homes. The median first floor/basement ratio of radon concentrations for one-story homes was 0.60. The median ratios between first floor/basement and second floor/basement for two-story homes were 0.51 and 0.62, respectively. The mean coefficient of variation for detectors placed on the same floor was 9.5%, which was only 2.6% higher than the mean coefficient of variation found for collocated (duplicate) quality control detectors. The wide individual variations noted in radon concentrations serve as a reminder of the importance of performing multiple radon measurements in various parts of the home when estimating home radon concentrations.

Annual average indoor radon variations over two decades.

Long-term exposure to elevated radon (222Rn) concentrations has been linked to increased lung cancer risk. Year-long measurements of contemporary radon concentrations have been the “gold standard” for epidemiologists trying to reconstruct past radon exposures and for homeowners trying to estimate future radon exposure. Random variations and persistent temporal trends can affect remedial action decisions and risk coefficients derived from epidemiological studies. Temporal fluctuations are possible when changes occur in a home’s structure, climate, environment, or occupants. The annual-average temporal radon behavior was studied at 196 sites in 98 Minnesota houses. Seventeen hundred year-long indoor radon measurements were made from 1983 to 2000 to determine year-to-year radon fluctuations and long-term temporal trends. Ten year-long measurements over a span of 13 years were made at the typical site. The median radon concentration was 120 Bq m−3. The median radon concentration of the group of houses showed little year-to-year variation and no persistent temporal trends. At individual sites, year-to-year radon variations ranged from 3 to 110%. The median variation was 26%. Climate, exposure to wind, and radon concentration affected year-to-year variation, but house age, construction, or measurement floor did not. Some individual sites showed significantly larger radon changes when modifications were made to the house structure and heating-ventilation systems. Year-long radon measurements on the first floor provided better estimates of cumulative radon exposure than screening measurements. The radon variations observed in this study provide uncertainty estimates for year-long measurements that could help improve remediation decision protocols and refine risk estimates from epidemiological studies.

Variation in yearly residential radon concentrations in the upper midwest.

It is well known that inhalation of 222Rn and 222Rn decay products increases the risk of lung cancer. While the occurrences of high radon areas in the United States are generally known, studies examining the temporal yearly radon variation in homes across different regions are lacking. This information is essential to assess the ability of a year-long radon measurement to predict the future radon concentration in a home or reconstruct the retrospective residential radon concentration. The purpose of this study is to help fill this gap by examining the temporal variation of residential radon concentrations in homes over several years as well as to explore factors that affect the yearly temporal variability of residential radon concentrations. The coefficient of variation was used as a measure of relative variation between multiple measurements performed across homes over several years. Generalized linear model analyses were applied to investigate factors affecting the coefficient of variation. The median coefficient of variation between the first and second test period was 12%, while a median coefficient of variation of 19% was found between the first and third test period. Factors impacting the coefficients of variation were found to vary for different types of homes and by floors of a home. This study provides important insights into the uncertainty of residential radon gas concentrations that can be incorporated into the sensitivity analyses for the risk estimates of both the North American and global pooling of residential radon studies to improve risk estimates.