Key characteristics of 86 agents known to cause cancer in humans

Since the inception of the International Agency for Research on Cancer (IARC) in the early 1970s, the IARC Monographs Programme has evaluated more than 1000 agents with respect to carcinogenic hazard; of these, up to and including Volume 119 of the IARC Monographs, 120 agents met the criteria for classification as carcinogenic to humans (Group 1). Volume 100 of the IARC Monographs provided a review and update of Group 1 carcinogens. These agents were divided into six broad categories: (I) pharmaceuticals; (II) biological agents; (III) arsenic, metals, fibers, and dusts; (IV) radiation; (V) personal habits and indoor combustions; and (VI) chemical agents and related occupations. Data on biological mechanisms of action (MOA) were extracted from the Monographs to assemble a database on the basis of ten key characteristics attributed to human carcinogens. After some grouping of similar agents, the characteristic profiles were examined for 86 Group 1 agents for which mechanistic information was available in the IARC Monographs up to and including Volume 106, based upon data derived from human in vivo, human in vitro, animal in vivo, and animal in vitro studies. The most prevalent key characteristic was "is genotoxic", followed by "alters cell proliferation, cell death, or nutrient supply" and "induces oxidative stress". Most agents exhibited several of the ten key characteristics, with an average of four characteristics per agent, a finding consistent with the notion that cancer development in humans involves multiple pathways. Information on the key characteristics was often available from multiple sources, with many agents demonstrating concordance between human and animal sources, particularly with respect to genotoxicity. Although a detailed comparison of the characteristics of different types of agents was not attempted here, the overall characteristic profiles for pharmaceutical agents and for chemical agents and related occupations appeared similar. Further in-depth analyses of this rich database of characteristics of human carcinogens are expected to provide additional insights into the MOA of human cancer development.

Development of a database on tumors and tumor sites in humans and in experimental animals for 'Group 1 agents identified through volume 109 of the IARC Monographs

Volume 100 in the series of IARC Monographs on the Evaluation of Carcinogenic Risks to Humans comprises an update and review of relevant information on all agents determined to induce cancer in humans. These Group 1 agents are categorized in 6 Monographs (Volumes 100A-F) published in 2012. This paper describes the methodology and stringent criteria used in the creation of a comprehensive database on tumors noted in animals and humans for the carcinogens reviewed in Volume 100, and for additional Group 1 agents that were identified in subsequent Monographs through Volume 109. The development of this database involved the systematic collection of relevant data on tumors detected in humans and experimental animals identified by the Working Groups that conducted evaluations reported in the IARC Monographs. The database includes all human tumor sites identified by the Working Groups, along with all tumor sites for which there was sufficient evidence in experimental animals. This database provides a basis for assessing the degree of concordance between tumor sites observed in humans and experimental animals for Group 1 agents identified through Volume 109.

Development of a database on key characteristics of human carcinogens

A database on mechanistic characteristics of human carcinogenic agents was developed by collecting mechanistic information on agents identified as human carcinogens (Group 1) by the International Agency for Research on Cancer (IARC) in the IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. A two-phase process is described for the construction of the database according to 24 toxicological endpoints, derived from appropriate test systems that were acquired from data obtained from the mechanisms sections of the IARC Monographs (Section 4) and a supplementary PubMed search. These endpoints were then aligned with 10 key characteristics of human carcinogens that reflect the broader attributes of these agents relating to the development of cancer in humans. The considerations involved in linking of toxicological endpoints to key characteristics are described and specific examples of the determination of key characteristics for six specific agents (tamoxifen, hepatitis B virus, arsenic, ultraviolet and solar radiation, tobacco smoking, and dioxin) are provided. Data for humans and animals were tabulated separately, as were results for in-vivo and for in-vitro sources of information. The database was constructed to support a separate analysis of the expression of these endpoints by 86 Group 1 carcinogens, in-vivo and in-vitro along with an analysis of the key characteristics of these agents.

Concordance between sites of tumor development in humans and in experimental animals for 111 agents that are carcinogenic to humans

Since the inception of the IARC Monographs Programme in the early 1970s, this Programme has developed 119 Monograph Volumes on more than 1000 agents for which there exists some evidence of cancer risk to humans. Of these, 120 agents were found to meet the criteria for classification as carcinogenic to humans (Group 1). Volume 100 of the IARC Monographs, compiled in 2008-2009 and published in 2012, provided a review and update of the 107 Group 1 agents identified as of 2009. These agents were divided into six broad categories: (I) pharmaceuticals; (II) biological agents; (III) arsenic, metals, fibers and dusts; (IV) radiation; (V) personal habits and indoor combustions; and (VI) chemical agents and related occupations. The Group I agents reviewed in Volume 100, as well as five additional Group 1 agents defined in subsequent Volumes of the Monographs, were used to assess the degree of concordance between sites where tumors originate in humans and experimental animals including mice, rats, hamsters, dogs, and non-human primates using an anatomically based tumor nomenclature system, representing 39 tumor sites and 14 organ and tissue systems. This evaluation identified 91 Group 1 agents with sufficient evidence (82 agents) or limited evidence (9 agents) of carcinogenicity in animals. The most common tumors observed in both humans and animals were those of the respiratory system including larynx, lung, and lower respiratory tract. In humans, respiratory system tumors were noted for 31 of the 111 distinct Group 1 carcinogens identified up to and including Volume 109 of the IARC Monographs, comprising predominantly 14 chemical agents and related occupations in category VI; seven arsenic, metals, fibers, and dusts in category III, and five personal habits and indoor combustions in category V. Subsequent to respiratory system tumors, those in lymphoid and hematopoietic tissues (26 agents), the urothelium (18 agents), and the upper aerodigestive tract (16 agents) were most often seen in humans, while tumors in digestive organs (19 agents), skin (18 agents), and connective tissues (17 agents) were frequently seen in animals. Exposures to radiation, particularly X- and γ-radiation, and tobacco smoke were associated with tumors at multiple sites in humans. Although the IARC Monographs did not emphasize tumor site concordance between animals and humans, substantial concordance was detected for several organ and tissue systems, even under the stringent criteria for sufficient evidence of carcinogenicity used by IARC. Of the 60 agents for which at least one tumor site was identified in both humans and animals, 52 (87%) exhibited tumors in at least one of the same organ and tissue systems in humans and animals. It should be noted that some caution is needed in interpreting concordance at sites where sample size is particularly small. Although perfect (100%) concordance was noted for agents that induce tumors of the mesothelium, only two Group 1 agents that met the criteria for inclusion in the concordance analysis caused tumors at this site. Although the present analysis demonstrates good concordance between animals and humans for many, but not all, tumor sites, limitations of available data may result in underestimation of concordance.

A comparative risk assessment of burden of disease and injury attributable to 67 risk factors and risk factor clusters in 21 regions, 1990-2010: a systematic analysis for the Global Burden of Disease Study 2010

BACKGROUND

Quantification of the disease burden caused by different risks informs prevention by providing an account of health loss different to that provided by a disease-by-disease analysis. No complete revision of global disease burden caused by risk factors has been done since a comparative risk assessment in 2000, and no previous analysis has assessed changes in burden attributable to risk factors over time.

METHODS

We estimated deaths and disability-adjusted life years (DALYs; sum of years lived with disability [YLD] and years of life lost [YLL]) attributable to the independent effects of 67 risk factors and clusters of risk factors for 21 regions in 1990 and 2010. We estimated exposure distributions for each year, region, sex, and age group, and relative risks per unit of exposure by systematically reviewing and synthesising published and unpublished data. We used these estimates, together with estimates of cause-specific deaths and DALYs from the Global Burden of Disease Study 2010, to calculate the burden attributable to each risk factor exposure compared with the theoretical-minimum-risk exposure. We incorporated uncertainty in disease burden, relative risks, and exposures into our estimates of attributable burden.

FINDINGS

In 2010, the three leading risk factors for global disease burden were high blood pressure (7·0% [95% uncertainty interval 6·2-7·7] of global DALYs), tobacco smoking including second-hand smoke (6·3% [5·5-7·0]), and alcohol use (5·5% [5·0-5·9]). In 1990, the leading risks were childhood underweight (7·9% [6·8-9·4]), household air pollution from solid fuels (HAP; 7·0% [5·6-8·3]), and tobacco smoking including second-hand smoke (6·1% [5·4-6·8]). Dietary risk factors and physical inactivity collectively accounted for 10·0% (95% UI 9·2-10·8) of global DALYs in 2010, with the most prominent dietary risks being diets low in fruits and those high in sodium. Several risks that primarily affect childhood communicable diseases, including unimproved water and sanitation and childhood micronutrient deficiencies, fell in rank between 1990 and 2010, with unimproved water and sanitation accounting for 0·9% (0·4-1·6) of global DALYs in 2010. However, in most of sub-Saharan Africa childhood underweight, HAP, and non-exclusive and discontinued breastfeeding were the leading risks in 2010, while HAP was the leading risk in south Asia. The leading risk factor in Eastern Europe, most of Latin America, and southern sub-Saharan Africa in 2010 was alcohol use; in most of Asia, North Africa and Middle East, and central Europe it was high blood pressure. Despite declines, tobacco smoking including second-hand smoke remained the leading risk in high-income north America and western Europe. High body-mass index has increased globally and it is the leading risk in Australasia and southern Latin America, and also ranks high in other high-income regions, North Africa and Middle East, and Oceania.

INTERPRETATION

Worldwide, the contribution of different risk factors to disease burden has changed substantially, with a shift away from risks for communicable diseases in children towards those for non-communicable diseases in adults. These changes are related to the ageing population, decreased mortality among children younger than 5 years, changes in cause-of-death composition, and changes in risk factor exposures. New evidence has led to changes in the magnitude of key risks including unimproved water and sanitation, vitamin A and zinc deficiencies, and ambient particulate matter pollution. The extent to which the epidemiological shift has occurred and what the leading risks currently are varies greatly across regions. In much of sub-Saharan Africa, the leading risks are still those associated with poverty and those that affect children.

FUNDING

Bill & Melinda Gates Foundation.

A meta-analysis of evidence for hormesis in animal radiation carcinogenesis, including a discussion of potential pitfalls in statistical analyses to detect hormesis

A database containing 800 datasets on the incidence of specific tumor types from 262 radiation carcinogenicity experiments identified in a comprehensive literature search through September 2000 was analyzed for evidence of hormesis. This database includes lifetime studies of tumorigenic responses in mice, rats, and dogs to exposures to alpha, beta, gamma, neutron, or x-ray radiation. A J-shaped dose response, in the form of a significant decreased response at some low dose followed by a significant increased response at a higher dose, was found in only four datasets from three experiments. Three of these datasets involved the same control animals and two also shared dosed animals; the J shape in the fourth dataset appeared to be the result of an outlier within an otherwise monotonic dose response. A meta-analysis was conducted to determine whether there was an excess of dose groups with decreases in tumor response below that in controls at doses below no-observed-effect levels (NOELs) in individual datasets. Because the probability of a decreased response is generally not equal to the probability of an increased response even in the null case, the meta-analysis focused on comparing the number of statistically significant diminished responses to the number expected, assuming no dose effect below the NOEL. Only 54 dose groups out of the total of 2579 in the database had doses below the dataset-specific NOEL and that satisfied an a priori criterion for sufficient power to detect a reduced response. Among these 54, a liberal criterion for defining a significant decreases identified 15 such decreases, versus 54 × 0.2 = 10.8 expected. The excess in significant reductions was accounted for almost entirely by the excess from neutron experiments (10 observed, 6.2 expected). Nine of these 10 dose groups involved only 2 distinct control groups, and 2 pairs from the 10 even shared dosed animals. Given this high degree of overlap, this small excess did not appear remarkable, although the overlap prevented a formal statistical analysis. A comprehensive post hoc evaluation using a range of NOEL definitions and alternative ways of restricting the data entering the analysis did not produce materially different results. A second meta-analysis found that, in every possible low dose range ([0, d] for every dose, d) of each of the radiation types, the number of dose groups with significantly increased tumorigenic responses was either close to or exceeded the number showing significantly reduced responses. This meta-analysis was considered to be the more definitive one. Not only did it take dose into account by looking for consistent evidence of hormesis throughout defined low-dose ranges, it was also potentially less susceptible to limitations in experimental protocols that would cause individual animals to respond in a non-independent fashion. Overall, this study found little evidence in a comprehensive animal radiation database to support the hormesis hypothesis. However, the ability of the database to detect a hormetic effect was limited both by the small number of dose groups with doses below the range where positive effects have been found in epidemiological studies (≤ 0.1 Gy) and by the limited power of many of these dose groups for detecting a decrease in response.

Database of radiogenic cancer in experimental animals exposed to low doses of ionizing radiation

For decades, there have been debates regarding the nature of the relationship between exposure to low doses of ionizing radiation and cancer risk. Under the linear no-threshold hypothesis, which serves as a theoretical basis for current radiation protection standards, the risk of cancer at low levels of exposure is presumed to be directly proportional to dose. Opponents of this hypothesis claim that there are threshold doses for radiation carcinogenesis, or even a reduction in cancer risk at low doses (a phenomenon referred to as "radiation hormesis"). Epidemiological, animal, molecular, and cellular studies were conducted to resolve this controversy, although each of these study types has its strengths and limitations. Although the results of animal experiments are not directly applicable to humans, data can substantially add to our knowledge on the form of relationship between radiation dose and cancer risk in a wide range of doses. Laboratory animals are a homogeneous population with little biological variability; animal experiments are conducted under controlled conditions with good estimates of radiation doses. In order to address the question of whether or not the dose-response curve for radiation carcinogens is linear at low doses, a comprehensive database of animal carcinogenesis experiments was assembled involving exposure to different types of ionizing gradation. The database includes virtually all publicly accessible data on the induction of radiogenic cancer in laboratory mammals. This review provides a descriptive overview of the experiments included in the database, along with a qualitative assessment of the shape of the dose-response relationship for radiation carcinogenesis at low doses in experimental animals.

Uranium in drinking water: renal effects of long-term ingestion by an aboriginal community

The authors conducted a study of an aboriginal community to determine if kidney func-tion had been affected by the chronic ingestion of uranium in drinking water from the community's drilled wells. Uranium concentrations in drinking water varied from < 1 to 845 ppb. This nonin-vasive study relied on the measurement of a combination of urinary indicators of kidney function and markers for cell toxicity. In all, 54 individuals (12-73 years old) participated in the study. Correlation of uranium excreted in urine with bio-indicators at p <or=.05 indicated interference with the kidney's reabsorptive function. Because of the community's concerns regarding cancer incidence, the authors also calculated cumulative radiation doses using uranium intake in drinking water over the preceding 15-year period. The highest total uranium intake over this period was 1,761 mg. The risk of cancer from the highest dose, 2.1 mSv, is 13 in 100,000, which would be difficult to detect in the community studied (population size = 1,480). This study indicates that at the observed levels of uranium intake, chemical toxicity would be a greater health concern than would radiation dose.

A preliminary radon map for Canada according to health region

The recent publications of the combined analyses of residential radon studies in Europe and North America have shown that there is a significant risk of lung cancer at residential radon levels. In order to assess the population risk due to radon, the knowledge of the spatial distribution of indoor radon levels is essential. Here a preliminary radon map for Canada is presented, based on historical radon measurements collected in 6016 locations across Canada with the health region as the basic geographic units.

Determining the sample size required for a community radon survey

Radon measurements in homes and other buildings have been included in various community health surveys often dealing with only a few hundred randomly sampled households. It would be interesting to know whether such a small sample size can adequately represent the radon distribution in a large community. An analysis of radon measurement data obtained from the Winnipeg case-control study with randomly sampled subsets of different sizes has showed that a sample size of one to several hundred can serve the survey purpose well.

Case-control study of radon and lung cancer in New Jersey

Radon is known to cause lung cancer in humans; however, there remain uncertainties about the effects associated with residential exposures. This case-control study of residential radon and lung cancer was conducted in five counties in New Jersey and involved 561 cases and 740 controls. A year long alpha-track detector measurement of radon was completed for approximately 93% of all residences lived in at the time of interview (a total of 2,063). While the odds ratios (ORs) for whole data were suggestive of an increased risk for exposures >75 Bq m(-3), these associations were not statistically significant. The adjusted excess OR (EOR) per 100 Bq m(-3) was -0.13 (95% CI: -0.30 to 0.44) for males, 0.29 (95% CI: -0.12 to 1.70) for females and 0.05 (95% CI: -0.14 to 0.56) for all subjects combined. An analysis of radon effects by histological type of lung cancer showed that the OR was strongest for small/oat cell carcinomas in both males and females. There was no statistical heterogeneity of radon effects by demographic factors (age at disease occurrence, education level and type of respondent). Analysis by categories of smoking status, frequency or duration did not modify the risk estimates of radon on lung cancer. The findings of this study are consistent with an earlier population-based study of radon and lung cancer among New Jersey women, and with the North American pooling of case control radon seven studies, including the previous New Jersey study. Several uncertainties regarding radon measurements and assumptions of exposure history may have resulted in underestimation of a true exposure-response relationship.

Cigarette smoking and cancer risk: modeling total exposure and intensity

A recent analysis showed that the excess odds ratio (EOR) for lung cancer due to smoking can be modeled by a function which is linear in total pack-years and exponential in the logarithm of smoking intensity and its square. Below 15-20 cigarettes per day, the EOR/pack-year increased with intensity (direct exposure rate or enhanced potency effect), suggesting greater risk for a total exposure delivered at higher intensity (for a shorter duration) than for an equivalent exposure delivered at lower intensity. Above 20 cigarettes per day, the EOR/pack-year decreased with increasing intensity (inverse exposure rate or reduced potency effect), suggesting greater risk for a total exposure delivered at lower intensity (for a longer duration) than for an equivalent exposure delivered at higher intensity. The authors applied this model to data from 10 case-control studies of cancer, including cancers of the lung, bladder, oral cavity, pancreas, and esophagus. At lower intensities, there was enhanced potency for several cancer sites, but narrow ranges for pack-years increased uncertainty, precluding definitive conclusions. At higher intensities, there was a consistent reduced potency effect across studies. The intensity effects were statistically homogeneous, indicating that after accounting for risk from total pack-years, intensity patterns were comparable across the diverse cancer sites.

Biologically based analysis of lung cancer incidence in a large Canadian occupational cohort with low-dose ionizing radiation exposure, and comparison with Japanese atomic bomb survivors

Lung cancer incidence is analyzed in a large Canadian National Dose Registry (CNDR) cohort with individual annual dosimetry for low-dose occupational exposure to gamma and tritium radiation using the two-stage clonal expansion model (TSCE) and extensions of the model with up to 10 initiation steps. Models with clonal expansion turned off provide very poor fits and are rejected. Characteristic and distinct temporal patterns of excess relative risk (ERR) are found for dose response affecting early, middle, or late stages of carcinogenesis, that is, initiation with one or more stages, clonal expansion, or malignant conversion. Both fixed lag and lag distributions are used to model time from first malignant cell to incidence. Background rates are adjusted for gender and birth cohort. Lacking individual smoking data, surrogate annual smoking doses based on U.S. annual per capita cigarette consumption appear to account for much of the birth cohort effect, leaving radiation dose response relatively unchanged. The mean cumulative exposure for males receiving nonzero cumulative doses of gamma and tritium radiation was 18.2 mSv. The males have a significant dose response with 33 out of a total of 322 lung cancer cases attributable to radiation. There were 78 incident lung cancer among females, (with mean cumulative exposure of 3.8 mSv among females with nonzero exposure). The dose response for females appears smaller than for males but does not differ significantly from zero or from the male dose response. Findings for males include significant dose-response relationships for promotion and malignant conversion, but not initiation, and a protraction effect (sometimes called an inverse-dose-rate effect, where risk increases with protraction of a given dose). The dose response predicted by our analysis appears consistent with the risk for lung cancer incidence in the Japanese atomic bomb survivors cohort, provided that proper adjustments are made for duration of exposure and differences in background rate parameters.

Assessment and management of residential radon health risks: a report from the health Canada radon workshop

Epidemiologic studies of uranium miners and other underground miners have consistently shown miners exposed to high levels of radon to be at increased risk of lung cancer. More recently, concern has arisen about lung cancer risks among people exposed to lower levels of radon in homes. The current Canadian guideline for residential radon exposure was set in 1988 at 800 Bq/m(3). Because of the accumulation of a considerable body of new scientific evidence on radon lung cancer risks since that time, Health Canada sponsored a workshop to review the current state-of-the-science on radon health risks. The specific objectives of the workshop were (1) to collect and assess scientific information relevant to setting national radon policy in Canada, and (2) to gather information on social, political, and operational considerations in setting national policy. The workshop, held on 3-4 March 2004, was attended by 38 invited scientists, regulators, and other stakeholders from Canada and the United States. The presentations on the first day dealt primarily with scientific issues. The combined analysis of North American residential radon and lung cancer studies was reviewed. The analysis confirmed a small but detectable increase in lung cancer risk at residential exposure levels. Current estimates suggest that radon in homes is responsible for approximately 10% of all lung cancer deaths in Canada, making radon the second leading cause of lung cancer after tobacco smoking. This was followed by a perspective from an UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) working group on radon. There were two presentations on occupational exposures to radon and two presentations considered the possibility of radon as a causative factor for cardiovascular disease and for cancer in other organs besides the lung. The possible contribution of environmental tobacco smoke to lung cancers in nonsmokers was also considered. Areas for future research were identified. The second day was devoted to policy and operational issues. The presentations began with a perspective from the U.S. Environmental Protection Agency, followed by a history of radon policy development in Canada. Subsequent presentations dealt with the cost-effectiveness of radon mitigation, Canadian building codes and radon, and a summary of radon standards from around the world. Provincial representatives and a private consultant were given opportunities to present their viewpoints. A number of strategies for reducing residential radon exposure in Canada were recognized, including testing and mitigation of existing homes (on either a widespread or targeted basis) and changing the building code to require that radon mitigation devices be installed at the time a new home is constructed. The various elements of a comprehensive national radon policy were set forth.

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 x time), its exposures estimates are presented in the form WLM(5-19) (Field et al., 2000a). WLM(5-19) represents the working level months for exposures that occurred 5-19 yr prior to diagnosis for cases or time of interview for control. Eleven WLM(5-19) is approximately equivalent to an average residential radon exposure of 4 pCi/L for 15 yr, assuming a 70% home occupancy.

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 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.

World Health Organization's International Radon Project

Following initial in vitro and in vivo studies and important studies of uranium miners, scientists have now completed impressive case-control studies of lung cancer risk from exposure to residential radon. Researchers have pooled these studies, in which all the information from the individual studies was reanalyzed. These pooled analyzes confirm that in the context of residential exposure, radon is now an established risk factor for lung cancer. Many of the initial uncertainties have been reduced, and health risk assessors are now confident that radon may contribute to as much as 10% of the total burden of lung cancer--that is, 2% of all cancers in the population, worldwide. To reduce residential radon lung cancer risk, national authorities must have methods and tools based on solid scientific evidence and sound public health policies. To meet these needs, the World Health Organization (WHO) has initiated the WHO International Radon Project. This three year project, to be implemented during the period 2005-2008, will include (1) a worldwide database on national residential radon levels, radon action levels, regulations, research institutions, and authorities; (2) public health guidance for awareness-raising and mitigation; and (3) an estimation of the global burden of disease (GDB) associated with radon exposure.

The effect of censoring on cancer risk estimates based on the Canadian National Dose Registry of occupational radiation exposure

Cohort studies represent an important epidemiological tool for exploring the potential adverse health effects of low-dose exposure to ionizing radiation in the workplace. Analyses of data from the National Dose Registry of Canada have suggested that occupational radiation exposure leads to increased risk of several specific types of cancer, as well as increased overall risk of cancer. An important aspect of such studies is the censoring in recorded exposures induced by dosimetry detection limits. Such a censoring effect can lead to significant underestimation of cumulative doses which, in turn, can result in overestimation of the excess cancer risk associated with occupational radiation exposure. In this article, we present analytic results, supported by a simulation study, on the magnitude of overestimation of risk based on the additive relative risk model used in the analysis of the NDR data that can occur due to censoring. Our results indicate that overestimation of risk is modest, being less than 20% in all situations considered here. Because censoring also results in ovestimation of the precision of the risk estimates, the significance levels of Wald-type statistical tests for increased risk based on the ratio of the estimate to its standard error are virtually unaffected by censoring. These results suggest that although the application of the additive excess relative risk model in the presence of censoring may lead to some overestimation of risk, the model does not lead to invalid conclusions regarding the association between occupational radiation exposure and cancer risk based on data from the NDR.

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 alpha-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.

Modeling seasonal variation in indoor radon concentrations

Radon, a well-established risk factor for human lung cancer, is present at low concentrations in most homes. Consequently, many countries have established national guidelines for residential radon concentrations. In this article, we evaluate two models for describing seasonal variation in residential radon concentrations based on the data from a large case-control study conducted in Winnipeg, Canada. In this study, radon levels in homes were monitored during two successive 6-month periods, with integrated annual radon concentrations obtained using CR-39 alpha-track detectors. Significant differences were noted among measurements taken during different seasons of the year. Using the model introduced by Pinel et al. (1995) to describe temporal variation in residential radon levels in southwest England using seasonal adjustment factors, reasonable predictions of annual average radon concentrations were obtained from the 6-month integrated radon measurements. However, a simple multiplicative model was found to provide better predictions than the seasonal adjustment model. Although model coefficients vary somewhat from one geographic location to another, the concordance with respect discriminating between results above and below 150 Bq/m(3) in Winnipeg was in the range 85-90% using seasonal adjustment models with coefficients derived from data in either Winnipeg or southwest England.

Residential radon in Canada: an uncertainty analysis of population and individual lung cancer risk

Following a comprehensive evaluation of the health risks of radon, the U.S. National Research Council (US-NRC) concluded that the radon inside the homes of U.S. residents is an important cause of lung cancer. To assess lung cancer risks associated with radon exposure in Canadian homes, we apply the new (US-NRC) techniques, tailoring assumptions to the Canadian context. A two-dimensional uncertainty analysis is used to provide both population-based (population attributable risk, PAR; excess lifetime risk ratio, ELRR; and life-years lost, LYL) and individual-based (ELRR and LYL) estimates. Our primary results obtained for the Canadian population reveal mean estimates for ELRR, PAR, and LYL are 0.08, 8%, and 0.10 years, respectively. Results are also available and stratified by smoking status (ever versus never). Conveniently, the three indices (ELRR, PAR, and LYL) reveal similar output uncertainty (geometric standard deviation, GSD approximately 1.3), and in the case of ELRR and LYL, comparable variability and uncertainty combined (GSD approximately 4.2). Simplifying relationships are identified between ELRR, LYL, PAR, and the age-specific excess rate ratio (ERR), which suggest a way to scale results from one population to another. This insight is applied in scaling our baseline results to obtain gender-specific estimates, as well as in simplifying and illuminating sensitivity analysis.

Decreases in occupational exposure to ionizing radiation among Canadian dental workers

OBJECTIVE

To describe doses of ionizing radiation and their possible associations with mortality rates and cancer incidence among Canadian dental workers.

METHODS

The National Dose Registry (NDR) of Canada was used to assess occupational dose of ionizing radiation received by dental workers. The NDR cohort includes 42,175 people classified as dental workers. Subjects in the NDR were linked to both the Canadian Mortality Database and the Canadian Cancer Database to ascertain cause of death and cancer incidence, respectively.

RESULTS

The cohort consisted of 9,051 male and 33,124 female dental workers. A total of 656 incident cases of cancer and 558 deaths were observed. The standardized mortality ratio associated with all-cause mortality was 0.53 (90% confidence interval [CI] 0.49-0.57). The incidence of cancer among dental workers was lower than that for the Canadian population for all cancers except melanoma of the skin (for melanoma, the standardized incidence ratio was 1.46 [90% CI 1.14-1.85]). Occupational doses of ionizing radiation among dentists and dental workers have decreased markedly since the 1950s.

CONCLUSIONS

Dental workers receive very low doses of ionizing radiation, and these doses do not appear to be associated with any increase in cancer incidence; the increased incidence of melanoma is more likely related to other risk factors such as exposure to ultraviolet radiation from sunlight.

The use of biologically based cancer risk models in radiation epidemiology

Biologically based risk projection models for radiation carcinogenesis seek to describe the fundamental biological processes involved in neoplastic transformation of somatic cells into malignant cancer cells. A validated biologically based model, whose parameters have a direct biological interpretation, can also be used to extrapolate cancer risks to different exposure conditions with some confidence. In this article biologically based models for radiation carcinogenesis, including the two-stage clonal expansion (TSCE) model and its extensions, are reviewed. The biological and mathematical bases for such models are described, and the implications of key model parameters for cancer risk assessment examined. Specific applications of versions of the TSCE model to important epidemiological datasets are discussed, including the Colorado uranium miners' cohort; a cohort of Chinese tin miners; the lifespan cohort of atomic bomb survivors in Hiroshima and Nagasaki; and a cohort of over 200,000 workers included in the National Dose Registry (NDR) of Canada.

Uranium gastrointestinal absorption: the f1 factor in humans

The present investigation was undertaken by the Department of Health, Canada, to determine the most appropriate value to use for uranium gastrointestinal absorption (f1) in setting the guideline for drinking water. Fifty participants, free from medical problems, were recruited from two communities: a rural area where drinking water, supplied from drilled wells, contained elevated levels of uranium and an urban area where the water supplied by the municipal water system contained < 1 microg l(-1). Uranium intake through food, drinking water and other beverages was monitored using the duplicate diet approach. Intake and excretion were measured by inductively coupled plasma-mass spectrometry (ICP-MS) in samples collected concurrently from the same individuals over a 3 d period. The range of f1 values was between 0.001 and 0.06, with a median of 0.009. These values were independent of gender, age, duration of exposure, daily total uranium intake and allocation of intake between food and water. Consistent with the recommendation of ICRP Publication 69, 78% were below 0.02.

Gastrointestinal absorption of uranium in humans

The gastrointestinal absorption factor (f1) for uranium in humans has been determined from a study of 50 volunteers, ingesting uranium at natural levels in drinking water and food. The purpose of the study was to find an appropriate f1 value for humans to use in deriving exposure guidelines for uranium. The participants ranged in age from 13 to 87 years. They were selected from two communities: New Ross, Nova Scotia with elevated uranium in drinking water, and Ottawa, Ontario with very low levels of uranium. Uranium intake and excretion were measured in samples collected concurrently from the same individuals over a three-day period. The duplicate diet method was used to monitor uranium intake in food and water. Uranium levels in all samples were measured by inductively coupled plasma mass spectrometry (ICP/MS). The distribution of f1 values obtained was non-Gaussian with a range of 0.001 to 0.06 and a median of 0.009. Seventy-eight percent of the subjects had values less than 0.02. These values are consistent with the recommendations of ICRP 69. The f1 values were not gender-sensitive and were independent of age at time of study, duration of exposure, and total uranium intake. The implications of these findings are discussed in terms of setting drinking water guidelines.

First analysis of cancer incidence and occupational radiation exposure based on the National Dose Registry of Canada

A cohort study was conducted to investigate the relation between cancer incidence and occupational exposure to ionizing radiation. Records containing dose information from 1951 to 1988 for 191,333 persons were extracted from the National Dose Registry of Canada. The records were linked to the Canadian Cancer Data Base, with incidence data from 1969 to 1988. Standardized incidence ratios were calculated using Canadian cancer incidence rates stratified by age, sex, and calendar year. Excess relative risks were obtained from internally based dose-response analyses. The following significant results were found for males and females combined: a deficit in the standardized incidence ratio for all cancers combined; elevated standardized incidence ratios for thyroid cancer and melanoma; and elevated excess relative risks for rectum, leukemia, lung, all cancers combined, all except lung, and all except leukemia. For males, cancers of the colon, pancreas, and testis also showed significantly elevated excess relative risks. The specific cancer types listed above have been implicated in previous studies on occupational exposure to ionizing radiation, except for testis, colon, and melanoma, while the findings on thyroid cancer from previous studies are inconclusive. The thyroid standardized incidence ratios in this study are highly significant, but further investigation is needed to assess the possibility of association with occupational radiation exposure.

Characterization of uncertainty and variability in residential radon cancer risks

Radon, a naturally occurring gas found at some level in most homes, is an established risk factor for human lung cancer. The U.S. National Research Council has recently completed a comprehensive evaluation of the health risks of residential exposure to radon and developed models for projecting radon lung cancer risks to the general population. This analysis suggests that radon may play a role in the etiology of 10-15% of all lung cancer cases in the United States, although these estimates are subject to considerable uncertainty. In this article, we present a detailed analysis of uncertainty and variability in estimates of lung cancer risk due to residential exposure to radon. We use a general framework for the analysis of uncertainty and variability that we developed previously. Specifically, we focus on estimates of the age-specific excess relative risk (ERR) and lifetime relative risk (LRR), both of which vary substantially among individuals. We also consider estimates of the population attributable risk (PAR), which reflects the proportion of the lung cancer burden attributable to radon. Variability in the ERR and LRR is largely determined by variability in residential exposure levels and in the dosimetric K-factor used to extrapolate from occupational to environmental settings. Uncertainty in the ERR and LRR is due to uncertainty in the model parameters, notably those reflecting the carcinogenic potency of radon and the modifying effect of attained age. Uncertainty in the PAR is determined by uncertainty about the values of the parameters in the risk models used to estimate the PAR. Uncertainty in radon levels in homes and the dosimetric K-factor contribute comparatively little to uncertainty in the PAR. These results suggest that reduction in uncertainty about the PAR for radon induced lung cancer can only be achieved if more reliable risk projection models can be developed.

First analysis of mortality and occupational radiation exposure based on the National Dose Registry of Canada

A cohort mortality study of occupational radiation exposure was conducted using the records of the National Dose Registry of Canada. The cohort consisted of 206,620 individuals monitored for radiation exposure between 1951 and 1983 with mortality follow-up through December 31, 1987. A total of 5,426 deaths were identified by computerized record linkage with the Canadian Mortality Data Base. The standardized mortality ratio for all causes of death was 0.61 for both sexes combined. However, trends of increasing mortality with cumulative exposure to whole body radiation were noted for all causes of death in both males and females. In males, cancer mortality appeared to increase with cumulative exposure to radiation, without any clear relation to specific cancers. Unexplained trends of increasing mortality due to cardiovascular diseases (males and females) and accidents (males only) were also noted. The excess relative risk for both sexes, estimated to be 3.0% per 10 mSv (90% confidence interval 1.1-4.8) for all cancers combined, is within the range of risk estimates previously reported in the literature.

Chronic ingestion of uranium in drinking water: a study of kidney bioeffects in humans

A study was conducted of the chemical effects on the human kidney induced by the chronic ingestion of uranium in drinking water. Subjects were divided into two groups: The low-exposure group, whose drinking water was obtained from a municipal water system and contained < 1 microgram uranium/L, and the high-exposure group, whose drinking water was obtained from private drilled wells and contained uranium levels that varied from 2 to 781 micrograms/L. Years of residence varied from 1 to 33 years in the low-exposure group and from 3 to 59 years in the high-exposure group. The indicators of kidney function measured in this study included glucose, creatinine, protein, and beta 2-microglobulin (BMG). The markers for cell toxicity studied were alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), lactate dehydrogenase (LDH), and N-acetyl-beta-D-glucosaminidase (NAG). Urinary glucose was found to be significantly different and positively correlated with uranium intake for males, females, and pooled data. Increases in ALP and BMG were also observed to be correlated with uranium intake for pooled data. In contrast, the indicators for glomerular injury, creatinine and protein, were not significantly different between the two groups nor was their urinary excretion correlated to uranium intake. These results suggest that at the intakes observed in this study (0.004 microgram/kg to 9 micrograms/kg body wt), the chronic ingestion of uranium in drinking water affects kidney function and that the proximal tubule, rather than the glomerulus, is the site for this interference.

Radiocesium body burdens in residents of northern Canada from 1963-1990

Measurements of 137Cs body burdens in over 1,100 people from five northern Canadian communities were carried out with a portable whole body counting system during the winters of 1989 and 1990. These results are compared with over 3,000 similar measurements carried out during 1967-1969. Community mean body burdens and body concentrations had decreased by approximately a factor of 30 between the two survey periods. The dependence of body concentrations on the sex and age of the subjects has also changed significantly. This can be related to changes in the patterns of caribou consumption in the northern communities. Measurements of 137Cs in urine are also available for an earlier period (1963-1966) when world-wide fallout was at its highest level. A normalization procedure was developed to calculate the average radiocesium body concentration in each community from the concentrations in urine. From data spanning a period of nearly 30 y (1963-1990), lifetime radiation doses have been estimated for most communities in the Yukon and Northwest Territories. These cumulative doses vary from 0.3 to nearly 40 mSv, with an Arctic-wide average of about 12 mSv. No health effects would be expected at these levels.

Case-control study of residential radon and lung cancer in Winnipeg, Manitoba, Canada

A case-control study of lung cancer in relation to exposure to radon in homes in Winnipeg, Manitoba, Canada, was conducted during 1983-1990. In total, 738 individuals with histologically confirmed incident cases of lung cancer were interviewed, along with 738 controls matched on age (+/- 5 years) and sex. Radon dosimeters were placed in all residences in which the study subjects had reported living within the Winnipeg metropolitan area for at least 1 year. Radon dosimetry was done by means of integrated alpha-track measurements over a 1-year period. In the homes monitored, the average level of radon-222 was about 120 becquerels (Bq)/m3 in the bedroom area and 200 Bq/m3 in the basement. After adjusting for cigarette smoking and education, no increase in the relative risk for any of the histologic types of lung cancer observed among the cases was detected in relation to cumulative exposure to radon.

Radon, cigarette smoke, and lung cancer: a re-analysis of the Colorado Plateau uranium miners' data

Much of our knowledge regarding the interaction of radon and tobacco smoke in the etiology of human lung cancer derives from studies of uranium miners. In this article, we present a re-analysis of lung cancer mortality in the Colorado Plateau miners' cohort within the framework of the two-mutation clonal expansion model of carcinogenesis. This analysis takes into account the patterns of exposure to radon and cigarette smoke experienced by individuals in the cohort. A simultaneous re-analysis of the British doctors' cohort indicated that those model parameters relating to the effects of tobacco were comparable in the two data sets. We found no evidence of interaction between radon and tobacco smoke with respect to their joint effect on the first or second stage mutation rates or on the rate of proliferation of initiated cells. The age-specific relative risks associated with joint exposure to radon and cigarette smoke, however, were supra-additive but submultiplicative. The analysis also confirmed that fractionation of radon exposures leads to higher lung cancer risks. Finally, we present some estimates of lung cancer risk from environmental radon exposure for non-smokers and smokers.

On the estimation of the distribution of the latent period of multiple sclerosis

A widely accepted hypothesis is that the initiation of multiple sclerosis occurs many years before the clinical onset of disease. Thus far, attempts to describe the characteristics of the latent period have depended entirely on ad hoc methods relying heavily on the results of migrant studies. Here, by introducing a stochastic model to describe the initiation-onset process, the distribution of the latent period is estimated, and several important consequences for multiple sclerosis discussed.