Health Risks of Radon and Other Internally Deposited Alpha-Emitters BEIR IV (1988) / Chapter Skim
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From page 1... ...
The largest part of the report deals with health ejects of exposure to radon and its progeny, primarily because of a need to characterize the lung-cancer risk associated with exposure to radon and its short-lived daughters in indoor domestic environments. The report also addresses health effects of exposure to other groups of radionuclides and their progeny that Ernst alpha particles the isotopes of polonium, radium, thorium, uranium, and the transuranic elements.
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From page 2... ...
Among the natural sources, inhaled radon and radon decay products indoors are the largest contributors to population exposure and might be responsible for a large number of lung-cancer deaths each year.3 That has led to recommendations, now being implemented, for national studies to assess the magnitude of the problem, for adopting remedial action levels of radon progeny in the indoor environment, and for introducing Instigation procedures to take effect at or below such levels to reduce population exposures from this source.8 9 For estimation of risks associated with exposure to the alphaemitting radionuclides, the most ~rnportant human populations examined are the underground miners who are exposed to widely differing concentrations of radon-222 progeny,3 the American radium-dial workers who ingested various amounts of long-lived radium-226 and radium-22B,6 the German patients who received injections of shortlived radium-224~i with different activities, and the German patients who received injections of graded volumes of Thorotrast (colloidal thorium-232 dioxide) .~° Human data on cancer induction by alphaparticle irradiation are sparse, but preliminary risk estimates have been calculated for some sites and tissues lung, bone, head sinus and mastoid, and liver.
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Even when human data were available, the committee has tried to rely on its own studies using newly developed methods for the analysis of occupational cohort data rather than relying solely on published information. The committee has also used novel statistical methods to analyze interspecies comparisons of the risks associated with different radionuclides when human data were insufficient.
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As in earlier reports from the Committee on the Biological Effects of ionizing Radiations, the so-called BEIR reports, the committee cautions that the risk estimates derived from epidemiological and experimental animal data should not be considered precise. They are derived from analyses of incomplete data and involve numerous uncertainties.
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From page 5... ...
Numerous studies of underground miners exposed to radon daughters in the air of nones have shown an increased risk of lung cancer in comparison with nonexposed populations. Laboratory animals exposed to radon daughters also develop lung cancer.
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The dosimetric approach used by other investigators and committees provides an estimate of Jung-cancer risk related to radon-daughter exposure that Is based specifically on modeling of the dose to target cells. The various dosimetric models ah require assumptions, some of which are not subject to direct verification, as to breathing rates; the deposition of radon daughters in the respiratory tract; and the type, nature, and location of the target cells for cancer induction.
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Several assumptions are required to transfer risk estimates from an occupational setting to the indoor domestic environment. Accordingly, the committee assumed that the epidemiological findings in the underground miners could be extended across the entire life span, that cigarette smoking and exposure to radon daughters interact multiplicatively, that exposure to radon progeny increases the risk of lung cancer in proportion to the sex-specific ambient risk of lung cancer associated with other causes, ant!
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The committee concluded that more complete specifications of aerosol characteristics in mines and homes and the relevant physiological parameters are needed to permit quantitative assessment of the comparative dosimetry of radon daughters in the occupational and environmental settings. On the basis of the estimates of excess relative risks per W[M of exposure to radon progeny derived from analysis of the four miner cohorts examined and the assumptions outlined above, the committee projected Jung-cancer risks for U.S.
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Unlike the modified relative-risk mode! developed by this committee, risk estimates by the 1980 BEIR ITI committees were based on the assumption of an additive risk of lung-cancer mortality due to exposure to radon progeny that increased with age.
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The committee concluded that estimates of lung-cancer risks based on studies on miners can be used to estimate the potential lung-cancer risk associated with increased concentrations of indoor radon; however, the estimates derived are imprecise. The committee recognizes that the differences between risks in mining and domestic environments and the interaction between smoking and exposure to radon progeny remain incompletely resolved.
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Depending on which isotope of radium is being considered, a variety of dose-response relationships are consistent with the human data linear, dose squared, linear with correction for dose protraction, dose-squared exponential, linearquadratic exponential, 1 minus an exponential, and threshold.6 In the dose range in which bone tumors have occurred, the lifetime risk associated with radium-224 is estimated to be about 2 x 10-2 excess bone sarcomas per person Gy (200 per million person-red) when a linear function is assumed and an apparent increase in risk with dose protraction is taken into account.
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The cells at risk of bone cancer induction appear to be proliferating osteogenic cells or their precursors at bone surfaces. Identification of cell type and location is complicated by the diversity of ceils that lie within the range of alpha particles emitted from bone surfaces.
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Risk estimates for thorium-232-induced liver cancer, bone cancer, and leukemia have been calculated on the basis of Thorotrast patients who received injections of colloidal [232Thithorium dioxide
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, where the alpha radiation dose ~ to the liver. For bone sarcomas, the lifetime risk is estimated to be about (0.55-1.2)
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Observations on animals exposed to high-specific-activity uranium suggest that a small excess of bone sarcomas in human populations could result from naturally occurring uranium, but that the magnitude of the excess depends on which mathematical mode} is chosen. If the dose-response relationship is quadratic, virtually no effect ~ expected at environmental natural uranium concentration~.
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In the absence of sufficient human surveys to calculate risk estimates for cancer induction, the animal data, together with data on radium-224 and radium-226 in humans, provide a basis for cancer risk estimation. Human exposures occur primarily among occupationally exposed workers in nuclear facilities.
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From page 17... ...
The experimental animal data are quite extensive, and the committee has applied Bayesian components of variance models to 15 data sets for bone sarcoma induction in humans and laboratory animals. The analysis yields, for plutonium deposition in human bone, a lifetime risk estimate of 3 x 10-2 per person-Gy (300 excess bone-cancer deaths per million person-red)
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. The committee recommends continued epidemiological study, with parallel multivariate analysis, of the temporal expression of lung cancer in underground miners exposed to radon progeny.
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Further studies of dosimetric modeling in the indoor environment and in mines are necessary to determine the comparability of risk per W[M in domestic environments and underground mines. ~ The committee recommends continuation of epidem~ological studies of lung cancer and other health outcomes resulting from indoor radon exposure; such studies must have sufficient statistical power to quantify any significant differences between the risks in environmental and occupational settings.
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THO RIUM . The carcinogenic risk estimates related to thorium-232 depend primarily on studies of patients who received Thorotrast.
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The radiation effects depend on the physical properties of the emitted radiation and on the physical and chemical characteristics of the radionuclide and its aggregation, movement, and deposition. ~ The committee recommends further study of the dosimetry of thorium radioisotopes at the cellular level in the target organs or tissues; these processes are central to an understanding of the biological effects, notably in liver and bone.
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Such studies have the potential for detecting potential harmful biological effects at low radiation doses, identifying persons of special risk to radiation injury, determining whether certain diseases are attributable to transuranic exposures, and directing therapeutic measures to sites of injury. ~ The committee recommends that the Bayesian methods for interspecies extrapolation be developed further and applied to the determination of other risk factors in humans.
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From page 23... ...
1983. Dose-response relationships for radium-induced bone sarcomas.
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