Characteristics of the German uranium miners cohort study

To evaluate the risk of cancer associated with low and high levels of radon exposure one of the largest single cohort studies on uranium miners is being conducted in Germany including 58,721 men who were employed for at least 6 mo between 1946 and 1989 at the former Wismut uranium company in Eastern Germany. Information on job history, smoking, dust, and arsenic was collected from the original payrolls and the medical records. Exposure to radon and its progeny was estimated by using a detailed job-exposure matrix. The first mortality follow-up determining the vital status as of 31 December 1998 has been started. The cohort includes 49,342 exposed miners who have worked underground or in processing/milling facilities and 9,379 never-exposed workers. Miners who had been exposed for the first time between 1946 and 1954 (n = 19,865), the years with the poorest working conditions, show higher mean cumulative radon exposures (709 working level months, WLM) and a longer duration of exposure (mean = 13 y) than those with the first exposure between 1955 to 1970 (121 WLM and 11 y, n = 14,155) or after 1970 (10 WLM and 6 y, n = 15,322), respectively. Information on smoking is available for 38% of the cohort, demonstrating that most miners were heavy smokers. In the first mortality follow-up a total of about 15,000 deceased men including about 2,200 lung cancer deaths are expected. The main strengths of the study are its size and the large group of workers having received low exposures over relatively long periods of time.

Experimental determination of the absorption rate of unattached radon progeny from respiratory tract to blood

An exposure methodology was developed for the determination of the absorption rate of unattached radon progeny deposited in the human respiratory tract to blood. Twenty-one volunteers were exposed in a radon chamber during well-controlled aerosol and radon progeny conditions, with predominantly unattached radon daughters. Special efforts were made to restrict the dose to the volunteers to an absolute maximum of 0.08 mSv. Measurements of radon gas and radon progeny in blood samples of these volunteers indicated absorption half times of 20 min to 60 min. Former determinations, mainly performed with much larger aerosol particles of diameters between 100 nm and 1,000 nm, implied absorption half times around 10 h. This indicates that the absorption of radon decay products from ciliated airways into blood is dependent upon particle size and particle composition.

Distributions of specific energy in sensitive layers of the human respiratory tract

The purpose of the present work was to calculate the specific energy distribution in sensitive cells of the human lung. Specific energy distributions were calculated by applying Monte Carlo methods and the ICRP 66 model of the human respiratory tract. Specific energies were calculated at various depths in the epithelium and combined according to the relative cell abundance. Distributions are given for various combinations of sources, alpha-particle energies, targets and regions of the lung. The chord length does not follow the triangular distribution when the particle range is comparable to the diameter of the target. The notion of "effective volume" is introduced and defined, which is needed for estimation of hit frequency in some particular targets. It has been shown that basal cells are subjected to a larger proportion of alpha-particle hits with small energy transfer than secretory cells. Small energy transfer events that lead to minor damage of the DNA are more efficient in cancer induction than are hits with large energy deposition that lead to cell killing.

Modelling lung tumour risk in radon-exposed uranium miners using generalizations of the two-mutation model of Moolgavkar, Venzon and Knudson

PURPOSES

To model radon-induced lung cancer in uranium miners using a quasi-biological model of carcinogenesis.

MATERIALS AND METHODS

Fitting of generalizations of the stochastic two-mutation carcinogenesis model of Moolgavkar, Venzon and Knudson to a case-control dataset nested within the cohort and to the full cohort of lung cancer mortality in the Colorado Plateau uranium miners, taking account of exposure to cigarette smoke and to radon daughters.

RESULTS

Models with three mutations gave adequate descriptions of the time and age patterns of radon-daughter-induced excess lung tumour mortality. The overall fit of the two-mutation model to the case-control data was somewhat worse than that of the three-mutation model. For both the optimal two- and three-mutation models radon daughters and cigarette smoke were assumed to act on the first mutation rate. In the optimal two-mutation model, radon daughters also modified the intermediate cell death or differentiation rate. In the optimal three-mutation model, radon daughters modified the second mutation rate. In all models, the action of radon daughters and cigarette smoke was markedly non-linear, particularly in their action on the mutation rates. The optimal two- and three-mutation models fitted to the cohort data were of slightly different form to those fitted to the case-control data. The model fits to the cohort data are preferred to those to the case-control data on grounds of plausibility.

CONCLUSIONS

Quasi-biological carcinogenesis models with three mutations give adequate descriptions of the time and age patterns of radon-daughter-induced excess lung tumour mortality. The overall fit of the two-mutation model is somewhat worse than that of the three-mutation model.

Microdosimetric calculation of absorption fraction and the resulting dose conversion factor for radon progeny

It is an established fact that radon progeny can induce lung cancers. However, there is a well-known discrepancy between the epidemiologically derived dose conversion factor for radon progeny (4 mSv/WLM) and the dosimetrically derived value (15 mSv/WLM) (mSv is a unit of the dose while WLM is a unit of exposure to radon progeny). Up to now there is no satisfactory explanation to this. In the present study we propose that microdosimetry will help reduce the discrepancy significantly. The ICRP Human Respiratory Tract Model (HRTM) has been applied to calculate the effective dose conversion factor. All parameters have been kept at their best estimates. Modifications were made in the calculation of the absorbed fractions of alpha particles. In contrast to the ICRP approach where the energy has been considered to be deposited in the layer containing the sensitive cells, we used a microdosimetric approach in which the alpha particles deposit their energy only in the nuclei of sensitive cells. This modification alone has lowered the dose conversion factor by about one-third (from 15 mSv/WLM down to approximately 10 mSv/WLM).

[Radon exposure and risks for public health]

Radon is a radioactive gas of natural origin. Its principal isotope is radon-222, stemming from uranium-238 present at various concentrations in all soils. Radon is found everywhere in the earth's atmosphere but has low reactivity by itself. Radon particulate daughters are however responsible for alpha irradiation of the bronchial epithelium. Epidemiologic studies on miners have shown long ago that radon exposure causes an increased risk of lung cancer in these workers. More recently, its risk associated with radon exposure was addressed in public dwellings. It is generally agreed that the risk of lung cancer is plausible in this setting. In particular, radon exposure in some dwellings may be comparable to that undergone by several uranium miner populations. However, simple and low-cost steps can be taken to significantly reduce radon exposure in dwellings.

Theoretical study of the relation between radon and its long-lived progeny in a room

We present a theoretical study of the complex relation between radon and its long-lived progeny implanted in glass surfaces. The well known (extended) Jacobi room model, which is normally used to describe radon and its progeny in a room, was transformed into a two-parameter model revealing a linear correlation between long term radon exposure and surface activity due to implanted radon decay products. Furthermore, this new approach made integration into a Monte Carlo simulation possible so that the large variation of different room model parameters could be taken into account. This allowed the calculation of a probability distribution for radon exposure from the measurement of the implanted 210Po activity. The availability of a 95% confidence interval for the radon exposure is valuable in the application of retrospective radon assessment in epidemiological studies.

Radon progeny exposure and lung cancer risk among non-smoking uranium miners

Studies of miners provide the basis for public health efforts to reduce residential radon progeny exposure. Because the preponderance of households do not have members who smoke indoors, studies of non-smoking miners contribute essential data for risk assessments for residential radon progeny exposure. We studied a cohort of 2,209 never-smokers who were underground uranium miners employed in the western U.S. from 1956 to the early 1990's and who participated in a screening program for lung cancer conducted by Saccomanno and colleagues. After determining the vital status and cause of death in the cohort, we conducted a nested case-control study of 55 lung cancer deaths in males and 3 age-matched controls for each case. The relative risk of lung cancer was 29.2 (95% CI 5.1, 167.2) for miners with greater than 1,450 WLM compared with those exposed to less than 80 WLM. Temporal factors affected risk, including average dose rate, which was inversely associated with lung cancer risk, and the length of time since last exposure, which was directly associated with decreased risk. As in studies of non-smokers and smokers combined, the exposure response relationship in never-smokers was consistent with a decreased slope at higher WLM, which resulted, in part, from an inverse dose rate effect.

Monte Carlo calculations of LR115 detector response to 222Rn in the presence of 220Rn

The sensitivities (in m) of bare LR115 detectors and detectors in diffusion chambers to 222Rn and 220Rn chains are calculated by the Monte Carlo method. The partial sensitivities of bare detectors to the 222Rn chain are larger than those to the 220Rn chain, which is due to the higher energies of alpha particles in the 220Rn chain and the upper energy limit for detection for the LR115 detector. However, the total sensitivities are approximately equal because 220Rn is always in equilibrium with its first progeny, which is not the case for the 222Rn chain. The total sensitivity of bare LR115 detectors to 222Rn chain depends linearly on the equilibrium factor. The overestimation in 222Rn measurements with bare detectors caused by 220Rn in air can reach 10% in normal environmental conditions. An analytical relationship between the equilibrium factor and the ratio between track densities on the bare detector and the detector enclosed in chamber is given in the last part of the paper. This ratio is also affected by 220Rn, which can disturb the determination of the equilibrium factor.

Lung exposure from inhalation of radon progeny: calculated from in vivo measurements of 210Pb in the skull

To calculate the radiation dose to the lungs from the inhalation of radon and its short-lived progeny, an accurate estimate of cumulative exposure is necessary. In this preliminary study, the content of 210Pb in the skeleton is used to obtain a measure of integrated exposure to the lungs of people living in homes with above average concentrations of radon. Measurements of skeletal 210Pb made in vivo allow the exposed individuals to become, in effect, their own "samplers" and "dosimeters" through the normal physical and physiological processes of inhalation, deposition, and retention. 210Pb measurements have been made on 40 subjects whose homes have above average levels of radon. These data are used to obtain their cumulative lung exposures, defined as RLM (Respiratory Level Months). RLM is calculated from the numbers of atoms of RaA, RaB, and RaC,C' deposited in their respiratory systems over the time periods lived in the surveyed homes. The RLM values obtained are not significantly different than conventional WLM exposures calculated for the same time periods.

Radiation dose to the human respiratory tract from inhalation of radon-222 and its progeny

In this article the regional dose equivalent for exposure to 1 working level month (WLM), the annual effective dose equivalent and the dose equivalent conversion factor due to the inhalation of short-lived radon progeny (218Po, 214Pb and 214Bi/214Po) are calculated with a dosimetric model. Experimental results of aerosol size characteristics, unattached activity fractions, equilibrium factors, and activity size distributions of the short-lived radon progeny in indoor and outdoor atmosphere are summarized. Dose calculations based on these results show that the changes in dose rate values are influenced by the different model assumptions concerning deposition probabilities of aerosol particles in the respiratory system during inhalation, the sensitivity of different cells (epithelial, basal) to radiation and assuming nose or mouth breathing. The inverse relationship between the unattached fraction of potential alpha-particle energy in room air and the radioactive equilibrium between radon gas and its progeny indicates that dose equivalent is closely related to the concentration of radon gas averaged over a period of exposure.

Alpha-particle dosimetry for the basal layer of the skin and the radon progeny 218-Po and 214-Po

The population average dose to the basal layer of the skin is evaluated, in terms of dose per alpha-particle decay per unit area, for radon progeny 218-Po and 214-Po attached to the skin surface. Account is taken of the variation in basal layer depth between different individuals, and results are presented for several anatomical regions. On the face, for example, the population average dose to the basal layer from 218-Po is of the order of 0.5 microSv/decay/cm2 while that from 214-Po is of the order of 1 microSv/decay/cm2. Radiation induced stochastic effects in the skin are generally assumed to originate from target cells located in the basal layer. Provided this assumption is correct, these results suggest that both 218-Po and 214-Po could be regarded as potential carcinogenic agents for the induction of skin cancer.

A new mechanism for DNA alterations induced by alpha particles such as those emitted by radon and radon progeny

The mechanism(s) by which alpha (alpha) particles like those emitted from inhaled radon and radon progeny cause their carcinogenic effects in the lung remains unclear. Although direct nuclear traversals by alpha-particles may be involved in mediating these outcomes, increasing evidence indicates that a particles can cause alterations in DNA in the absence of direct hits to cell nuclei. Using the occurrence of excessive sister chromatid exchanges (SCE) as an index of DNA damage in human lung fibroblasts, we investigated the hypothesis that alpha-particles may induce DNA damage through the generation of extracellular factors. We have found that a relatively low dose of alpha-particles can result in the generation of extracellular factors, which, upon transfer to unexposed normal human cells, can cause excessive SCE to an extent equivalent to that observed when the cells are directly irradiated with the same irradiation dose. A short-lived, SCE-inducing factor(s) is generated in alpha-irradiated culture medium containing serum in the absence of cells. A more persistent SCE-inducing factor(s), which can survive freeze-thaw and is heat labile is produced by fibroblasts after exposure to the alpha-particles. These results indicate that the initiating target for alpha-particle-induced genetic changes can be larger than a cell's nucleus or even a whole cell. How transmissible factors like those observed here in vitro may extend to the in vivo condition in the context of a-particle-induced carcinogenesis in the respiratory tract remains to be determined.

Influence of air flow on the behavior of thoron and its progeny in a traditional Japanese house

Air flow influence on the spatial distribution of thoron (220Rn) concentration in a typical Japanese traditional house was investigated at various indoor air flow levels. The effect of air flow on the behavior of both thoron and radon progeny were examined simultaneously. Measurements were carried out by using two types of passive monitors, the radon-thoron discriminative monitor and the Radtrak monitor. Thoron and radon progeny were measured by filter grab sampling with ZnS scintillation counting. Under static condition, a horizontal distribution with greatly varied thoron concentrations was found as reported by previous studies. Under turbulent conditions, thoron concentrations in the middle of the room increased and the concentration gradient of thoron gas became lower. An obvious vertical distribution of thoron was also observed. Prominent diurnal variation of radon progeny concentrations was observed whereas that of thoron progeny concentrations was not. Concentration of thoron progeny changed little at different air flow levels, although the thoron gas level at the middle of the room varied significantly. The influence of air flows on detection efficiencies of the two types of thoron monitors were also checked. The mechanism of behavioral change of thoron and its progeny in turbulent atmosphere is discussed.

Radon daughter deposition on surfaces carrying alternating electric fields

A theoretical model is proposed to explain the radon daughter deposition enhancements on surfaces subject to power frequency AC fields, recently reported by Henshaw et al. (1996). The effect of oscillatory contacts with the surface combined with turbulent diffusion is investigated within the framework of the convective-diffusion equation, through an approximation of the dividing surface. The fine and the course activity fractions are treated separately for flat-plate and wire-plane configurations. The results indicate maximum enhancement factors for 218Po in the range 1.7-3.3 for flat-plate and 5.2-15.5 for wire-plane configurations under typical indoor environmental conditions. While these compare favourably with the corresponding observed values, further analysis appears necessary to explain the greater enhancements observed in the case of 214Po.

Radon in the workplace--a study of occupational exposure in BT underground structures

During the period August 1993-October 1994 a study was undertaken throughout British Telecommunications plc to assess occupational exposure to radon. This paper is concerned only with that portion of the work concerned with underground structures. The results show that radon can build up to very high concentrations in manholes and implies a significant risk to those who need to work in them. For various reasons, which are explained, exposures are much less than predicted and in all but a very few cases the annual predicted radiation dose due to radon is expected to be below 5 milliSieverts (mSv). A safe system of work is described which seeks to ensure that no BT people receive an annual radiation dose of greater than 5 mSv as a result of occupational exposure to radon.

Analysis of lung tumor risks in rats exposed to radon

Using data on 3117 rats exposed by inhalation to radon, radon progeny and uranium ore dust, the hazard function (or age-specific risk) for lung tumor incidence was modeled as a function of exposure, exposure rate and other factors. The overall estimate of lifetime risk was 237 cases per 10(6) rats per WLM (237 per 10(6) WLM), reasonably comparable to estimates obtained from data for humans. The data below 1000 WLM (20-640 WLM) were consistent with linearity with positive excess risks at all levels; however, evidence of statistically significant excess risk was limited to exposures of 80 WLM or greater. Evidence for an inverse exposure-rate effect was limited primarily to cumulative exposures exceeding 1000 WLM (1280-10,240 WLM) and to comparison of results at 100 and 1000 WL. Even at these levels, the possibility that the effect might be explained by time since last exposure or by heterogeneity across experiments could not be entirely excluded. The inverse exposure-rate effect was strongest for epidermoid and adenosquamous tumors, and the only indication of such an effect at exposures below 1000 WLM was modest evidence (P=0.024) in analyses limited to these tumors. When all lung tumors, or all malignant lung tumors, were included, there was no evidence of such an effect below 1000 WLM. These data support the viewpoint that the inverse exposure-rate effect is primarily a high-dose phenomenon.

[A health status study of the population subjected to combined exposures to ionizing and nonionizing radiation factors]

The values of the dose effective extrinsic and intrinsic radiation of humans in the regions near to urane mines and urane plants in Bulgaria are pointed out. Some aspects of investigation of the health and risk evaluation of the population and of the former workers are reviewed.

Estimation of the dose of radon progeny to the peripheral lung and the effect of exposure to radon progeny on the alveolar macrophage

Groups of rats were exposed to radon progeny attached to a vector aerosol of cigarette smoke particles in a closed-loop exposure system. The total exposures ranged from 70 to 1020 WLM over a 3-5-h period. Alveolar macrophages were lavaged from exposed and control rats (exposed to the vector aerosol alone). There were no differences between phagocytic capabilities of the macrophages lavaged from the controls and any group of exposed rats. The increases in the numbers of binucleated/multinucleated and micronucleated macrophages were enumerated using cytological preparations of lavaged alveolar macrophages after exposure compared to the numbers in the control rats. The peak response in numbers of binucleated/multinucleated and micronucleated macrophages occurred at 21 days after exposure, beyond which the numbers decreased. This peak response showed a significant exposure-dependent relationship. The number of micronuclei provided a sensitive indicator of exposure. The lowest exposure level used (70 WLM) resulted in a significant difference in the number of micronucleated macrophages between control and exposed rats. The alpha-particle dose to the peripheral lung was estimated by comparing formation of micronuclei in cultured macrophages exposed to alpha particles from 238Pu (where the dose was known) to micronuclei in lavaged macrophages from animals exposed to radon progeny (where the exposure concentration was known, but the dose was not known). The dose estimated in this way for the peripheral lung was 9.8 +/- 1.2 mGy/WLM. This value is three to five times higher than predicted from mathematical models currently available.

High indoor radon concentrations in an Alpine region of western Tyrol

In a village in western Tyrol, Austria (Umhausen, 2,600 inhabitants), unusually high indoor radon concentrations were measured, and the lung cancer mortality rate was found to be higher than that of the total population of Tyrol (620,000 inhabitants). Annual means of radon concentrations were found to be particularly high in the area between the two rivers Otztaler Ache and Hairlachbach, geologically an alluvial fan of a giant rock slide of granitic gneisses (area A, median of annual means on the ground floors: 1,868 Bq m-3); radon concentrations were comparatively low in the rest of the village (area B, median of annual means on the ground floors: 182 Bq m-3). On the basis of these medians, the annual exposures were calculated according to the ICRP model (area A: 58.8 x 10(5) Bq h m-3; area B: 5.7 x 10(5) Bq h m-3). Data taken from the Cancer Registry of Tyrol were used to determine the age- and sex-standardized lung cancer mortality rate (area A: 6.17; area B: 1.43).

Changes in the rat lung after exposure to radon and its progeny: effects on incorporation of bromodeoxyuridine in epithelial cells and on the incidence of nuclear aberrations in alveolar macrophages

The aim of this study was to investigate some responses of cells in the rat respiratory tract as a function of time after inhalation exposure to various levels of radon and its progeny. Rats were exposed to a constant concentration of radon and its progeny to give cumulative exposure levels of 120, 225, 440 and 990 working level months (WLM). An additional unexposed group of rats served as controls. The end points selected for investigation were (a) the incorporation of bromodeoxyuridine (BrdU) in epithelial cells of the conducting airways and of the alveolar region of the respiratory tract and (b) the incidence of alveolar macrophages with nuclear aberrations. After exposure, the incidence of epithelial cells incorporating BrdU--the labeling index--increased in all regions of the respiratory tract examined, but the increase occurred later in alveolar than in airway epithelial cells. The highest labeling index was found in bronchial epithelial cells, which probably received the highest radiation dose. After an initial induction period, the incidence of alveolar macrophages with nuclear aberrations also increased. The possibility of using the labeling index of alveolar and airway epithelial cells, and/or the incidence of nuclear aberrations in alveolar macrophages, to estimate the radiation dose to various regions of the respiratory tract after exposure of rats to radon and its progeny is discussed.

Neutralization of thoron progeny in gases

This paper reports charge neutralization phenomena of 212Pb particles in nitrogen or oxygen atmospheres with trace amounts of other gases. Newly produced thoron or radon progeny are positively charged, stable molecular clusters that are subsequently neutralized by several mechanisms. The charged clusters have a smaller diffusion coefficient than neutral clusters of the same size due to the interaction of the charge with the surrounding gas molecules. In this study, we have found that the diffusion coefficients of 212Pb in O2, N2, NH3/O2, NH3/N2, and C6H12/N2 (IPs between 15.58 and 9.8 eV) ranged between 0.015 and 0.030 cm2 s-1. In the case of C6H12/O2, NO2/O2, NO/O2, and dimethylamine/O2 (ionization potential between 9.8 and 8.23 eV), the diffusion coefficients have increased to between 0.046 and 0.69 cm2 s-1. These results are consistent with previous results of 218Po, indicating that charged progeny are neutralized by electron transfer from a gas molecule with a lower ionization potential than lead oxide. We estimate the ionization potential of lead oxide to range between 9.8 and 10.2 eV. 212Pb was also neutralized by an electron scavenging mechanism in NO2/nitrogen.

Discrimination of airborne radioactivity from radon progeny

Naturally occurring radon and thoron progeny are the most interfering nuclides in the aerosol monitoring system. The high background and fluctuation of natural radioactivity on the filter can cause an error message to the aerosol monitor. A theoretical model was applied in the simulation of radon and thoron progeny behavior in the environment and on the filter. Results show that even a small amount of airborne nuclides on the filter could be discriminated by using the beta:alpha activity ratio instead of gross beta or alpha counting. This method can increase the sensitivity and reliability of real-time aerosol monitoring.

Silicosis and lung cancer risk in underground uranium miners

The presence of radiographic silicosis as a risk factor for lung cancer was assessed in a case-control study conducted within a study cohort of New Mexico underground uranium miners. Chest radiographs were interpreted for the presence of silicosis for 65 lung cancer cases and 216 controls. The presence of silicosis on the chest radiograph taken closest to the start of employment or on the latest radiograph available was not associated with lung cancer risk after adjustment for cumulative exposure to radon progeny. The odds ratio associated with the presence of any type of opacity indicative of pneumoconiosis on the chest x ray closest to the start of employment was 1.33 (95% confidence interval, 0.31-5.72). For the most recent available chest x ray, the corresponding odds ratio was 1.16 (95% confidence interval, 0.35-3.84). Although the findings are limited by the relatively small number of subjects, the lack of association of silicosis with lung cancer suggests that silica exposure should not be regarded as a major uncertainty in extrapolating radon risk estimates from miners to the general population.

Intercomparison of activity size distributions of thoron progeny by alpha- and gamma-counting methods

It is difficult to calibrate sampling devices using radon or thoron progeny or particles measuring 1-4 nm; therefore, an interlaboratory comparison is important to verify the performance of graded diffusion batteries for the activity size distributions of the "unattached" progeny. This paper describes the results of an interlaboratory comparison of 220Rn progeny size distributions using graded diffusion batteries by alpha- and gamma-counting methods with different data inversion schemes. Graded diffusion batteries designed at the Inhalation Toxicology Research Institute and at the Environmental Measurement Laboratory were used in the study. Screens and backup filters from the Environmental Measurement Laboratory-graded diffusion batteries were counted simultaneously in alpha counters for total alpha activities, and those of the Inhalation Toxicology Research Institute-graded diffusion batteries were counted in a gamma detector for gamma activities from 212Pb. Because of the different counting methods and data analysis procedures used, this interlaboratory study of 220Rn progeny allows a more rigorous way of testing instrument performance. 212Pb particles generated in well-controlled environments of oxygen, nitrogen, or oxygen with 1 ppm of nitrogen oxide were measured. In general, good agreement in activity size distributions was obtained from these two methods. Some differences observed in individual size spectra were attributable to the data inversion programs used in each laboratory. When the data were analyzed by the same computer program, most differences disappeared.

Measurement of the equilibrium factor for 222Rn progeny in Bangladesh dwellings

Time-integrated solid-state nuclear track detectors were employed for the measurement of radon concentration and potential alpha-energy exposure level. From these values, the equilibrium factor F between radon and its progeny was estimated for each house. An average value of 0.40 was obtained. The experimentally and theoretically determined F values were compared.

Video display terminals and radon

Recent reports indicate that video display terminals (VDTs) can collect radon progeny from the air. This occurs especially when they are turned off and may have negative electric fields which attract positively charged radioactive dust. We have studied this phenomenon in detail and evaluated various techniques for removing the gettered radioactivity while the video display terminal is both off and on. We have evaluated what happens when the video display terminal is switched, thereby reversing the electric field near the screen. We have also studied possible inhalation effects by a video display terminal operator during field reversal and find that although some radioactivity may be released during the cycle, room air currents redistribute it into the room with no detectable levels being inhaled by users.

Determination of calibration coefficient for radon measurements using a track detector

The calibration coefficient for radon measurements using a LR-115-II detector in a diffusion chamber was determined. Deposition of radon decay products on the inner chamber walls influenced the track density distribution. It was assumed that 214Po was 100% deposited before decay; however, part of 218Po atoms decay before they can be deposited. The radial track density distribution depends on the partially deposited 218Po. This dependence was assumed in order to estimate the amount of deposited 218Po so that more realistic calibration coefficients for radon measurements could be determined.

Single-cell gel technique supports hit probability calculations

We have used the alkaline single-cell gel technique to provide a biological estimate of the percentage of cell nuclei "hit" by alpha particles during in vitro radon exposure. The single-cell gel electrophoretic technique measures DNA strand breaks as increased migration of the DNA out of lysed cells embedded in the middle layer of a three-layer gel formed on a microscope slide. Two of the advantages of this system are that individual cells of an exposed population can be evaluated and that histograms can be constructed to estimate the population response. Chinese hamster ovary and AL cells were each exposed to 0.39 Gy of radon, a dose at which our dosimetry model predicts that 63 and 73%, respectively, of the cell nuclei will be traversed by an alpha particle. The difference in the percentages at similar doses is mainly due to the larger nucleus volume in AL cells. A 1.5-Gy x-ray response was also evaluated as a low-LET control. As expected, the x-ray profile of DNA damage was shifted from the nonirradiated profile in the direction of greater DNA migration and approximated a normal distribution. The profile of the radon-exposed cells was biphasic, with one distribution corresponding to the control (nonirradiated) response and the other profile showing increased DNA migration. We interpret the second profile in the biphasic profile as representing cell nuclei that had received an alpha "hit." The percentages of cell nuclei in the "hit" category (approximately 51 and 45% for CHO and AL, respectively), as judged by the single-cell gel technique, were 81 and 62% of the calculated values.

Integrated measurements of short-lived 222Rn progeny by rotating filters

The dependence of the risk from inhalation of radon progeny on their disequilibrium suggests that the measurements of the time-integrated concentrations of each of the short-lived radon progeny are necessary for complete risk estimations. This paper presents a method that, in principle, allows the determination of the integrated specific volume activities in air of each of the radionuclides 218Po, 214Pb, 214Bi, 212Pb, and 212Bi. The method employs thermoluminescence detectors positioned around uniformly rotating filters. Two prototypes that are suitable for practical applications are described and mathematical expressions for data processing are given. Experiments with these "rotating filter dosimeters" were conducted in atmospheres radiologically dominated by 222Rn progeny. The comparison between the results obtained by the proposed method and those given by simultaneously conducted series of instantaneous grab-sampling measurements support the conclusion that the method works for 222Rn progeny. The method can be experimentally extended for 220Rn progeny as well as for unattached fractions.

The biological effectiveness of radon daughter alpha particles. I. Radon, cigarette smoke and oncogenic transformation

Epidemiological data from uranium miners have not been able to provide a definitive model depicting the interaction between radiation exposure and smoking in the etiology of lung cancers among miners who also smoke. Using the well-characterized C3H 10T1/2 cell system, the toxicity and oncogenic transforming incidence by either gamma-rays or He4 ions, with or without concurrent exposure to cigarette smoke condensate (CSC), were examined. He4 ions had the same energy spectrum as radon. CSC, extracted from 2R1 cigarettes, induced a dose-dependent toxicity and oncogenic transformation incidence in 10T1/2 cells. CSC, in combination with a 2 Gy dose of gamma-rays, induced a toxicity and transforming response that was largely additive in nature. Similar additive modes of interaction were also observed when CSC was combined with He4 ions at a dose that was equivalent in cell killing to that used for gamma-rays. These results provide evidence that radiation and smoking have an additive effect in oncogenic transforming potential and may provide an experimental handle towards an epidemiological uncertainty.

Radon progeny dosimetry in the rat lung

Deposition, mucociliary clearance, and dosimetry for the inhalation of radon progeny in the rat lung have been simulated for a variety of inhalation conditions. Computations indicate that the exposure-dose conversion factor for the rat lung is approximately twice as high as the corresponding value for the human lung for the same exposure conditions. However, if typical aerosol characteristics are used for animal inhalation experiments and human indoor exposures, the resulting exposure-dose conversion factors are comparable, thereby suggesting similar lung cancer risks per unit exposure. The predicted relative effects of radon progeny disequilibrium and unattached fractions on bronchial doses agree with results from inhalation experiments with laboratory rats.

Rapid determination of radon daughters and of artificial radionuclides in air by online gamma-ray spectrometry

For the determination of airborne radionuclide concentrations in real time, a fixed filter device was constructed which fits directly onto a germanium detector with standard nuclear electronics and a multichannel analyzer buffer connected via a data line to a personal computer for remote control and on-line spectrum evaluation. The on-line gamma-ray spectrometer was applied to the study of radon decay product concentrations in ground-level air and to the rapid detection of any contamination of the environmental air by artificial radionuclides. At Munich-Neuherberg, depending on the meterological conditions, the measured air concentrations of 214Pb, the first gamma-ray-emitting member of the 222Rn decay series, varied from about 1 to 50 Bq m-3. For the artificial radionuclides 60Co, 131I and 137Cs the detection limits were determined as a function of the varying natural radon daughter concentrations at sampling and counting times of 1 h or 1 day. For these radionuclides minimum detectable air activity concentrations of 0.3 or 0.001 Bq m-3, respectively, were obtained at low radon daughter levels. At high radon daughter levels the respective detection limits were found to be higher by a factor of only about 2.

Measurement of radon/thoron and its daughter nuclides in room air

Pumping air through a soft tissue which acts as a membrane is a relatively easy and quick method to collect and measure radon/thoron and its daughter nuclides in air. Analysis of the activity of the radionuclides can be calculated using an alpha counter which has been calibrated. In this method the activity of radon/thoron cannot be separated from the activity of radionuclides already present in the aerosol or dust particles.

Radon and thoron associated dose to the basal layer of the skin

The radon related alpha-particle annual dose equivalent to the basal layer of the epidermis has been calculated theoretically and is estimated as 2.5 (range 1.7 to 17) mSv y-1, for the exposed, uncovered skin of the face and neck, at the UK average domestic radon exposure of 20 Bq m-3. The thoron-related annual dose equivalent is estimated as 0.3 (range 0 to 1.9) mSv y-1 at the equilibrium equivalent concentration of 0.3 Bq m-3 estimated for UK exposures. Considerably lower dose equivalents are received by regions of the skin which are habitually covered in clothing. The wide range in the dose estimates reflects the wide range in quoted plateout rates for radon and thoron daughters, and uncertainties in the magnitude of the electrostatic charge of the individual. To improve the dose estimates experimental measurements are needed of radon and thoron daughter deposition on the skin surface in situations corresponding to domestic exposure.

Dose to red bone marrow from natural radon and thoron exposure

The age-dependent radiation dose to the haematopoietic tissue of bone marrow has been calculated for exposure to radon, thoron and their daughter products. The component of dose due to pure radon is dependent on the fat content of the marrow, since the solubility of radon in fat is about 16 times that in tissue. The mean dose equivalent muSv to the total active marrow is estimated for a range of fat cell diameters from 25 to 200 microns, taking account of the percentage cellularity and distribution of active marrow as a function of age. Similarly, the dose due to the inhalation of short-lived radon daughters was estimated, based on measurements in blood and marrow, modified to allow for the greater deposition of daughter products expected in children. An estimate of the age-dependent dose from long-lived radon daughters was made from uranium miner and natural exposure data. Dose estimates were made for the average UK indoor exposure to radon gas of 20 Bq/m3 and an equilibrium equivalent thoron concentration of 0.3 Bq/m3. The annual radon and thoron derived dose to the active marrow of the newborn was calculated as 30 and 40 muSv, respectively. For a 10-year-old child, the radon and thoron derived annual dose are 70 and 40 muSv, and for a 40-year-old adult 90 and 30 muSv, respectively. The above values exhibit wide range limits due principally to uncertainties in the accumulation of 210Pb in bone, and 210Po in marrow. These data indicate that at the average UK exposure, the alpha-particle dose to active marrow is dominated by that derived from inhaled radon and thoron compared with dietary intake. In infants the dose is dominated by thoron daughters. At the UK radon Action Limit of 200 Bq/m3, the radon and associated thoron derived dose is similar to that from all low LET sources. This work shows that the dose to red bone marrow from radon and thoron is significant, and that the possibility of leukaemia induced by these radiation sources warrants further investigation.

[Sources of ionizing radiation in dwellings]

According to recent surveys made by the International Commission on Radiological Protection (ICRP) and the World Health Organization (WHO), it has been estimated that in temperate regions people spend only 20% of their time outdoors and spend the remaining 80% indoors (homes, schools, other buildings). It is therefore important to establish whether radiation sources exist inside buildings in order to assess risk for the population. The 238U and 232Th radioactive chains are of particular importance because of Radon and Radon daughter production and 40K as component of building materials. It has been estimated that about 4.5% of lung cancers observed in the population are associated with exposure to Radon daughters. In order to comply with international regulations it is estimated that 1.5% of the existing dwellings in temperate regions need to be improved.

Lung dosimetry of thorotrast patients

Deposits of intravascularly injected Thorotrast in the reticulo-endothelial system of Thorotrast patients are a continuous source of 220Rn. In this study, we modeled the transport of 220Rn from these deposits through the body into the lungs, exhalation of 220Rn from the lungs, production of 220Rn progeny in the lungs and their exhalation, and mucociliary clearance of 220Rn progeny deposited on airway surfaces. The injection of 1 mL Thorotrast produces annual doses of 0.48 mGy y-1 to the bronchial epithelium and 0.95 mGy y-1 to pulmonary tissue. Based on a mean injected volume of about 25 mL and an average exposure time of 30 y, German Thorotrast patients received a mean bronchial lifetime dose of 357 mGy. Despite these relatively high doses, comparable to exposure in uranium miners, no excess lung cancers could be observed in the epidemiologic follow-up study. This apparent discrepancy between predicted and observed bronchial tumors may have important implications for lung dosimetry and risk assessment of inhaled 222Rn progeny.

Dynamic spectrum of airborne gamma-rays

This note describes a method of direct measurement of airborne gamma-rays primarily from 222Rn daughters using a NaI(Tl) scintillation spectrometer with lead shields. This method has the advantage of being able to maintain the system easily compared to other usual systems including a pump. The pulse-height distributions are successively fed to a floppy disk in a personal computer every unit time. The gain shifts can be corrected automatically by a computer program. This technique would be applicable to the estimation of 222Rn daughters concentration and to examination of disequilibrium between 214Pb(RaB) and 214Bi(RaC) and of those height distribution up to about 200 m. The accuracy for estimating the concentration is as good as that of the filter method.

Radon-222, 222Rn progeny, and 220Rn progeny levels in 70 houses

A year-long, multipollutant, indoor air quality study involving 70 occupied houses in four states was completed in 1987. All of the houses included in the study had a partial or complete basement with a concrete slab floor and block walls. On an approximately quarterly schedule, integrating monitors for short-lived Rn progeny, nitrogen dioxide, formaldehyde, and water vapor were exposed for 1 wk in each house on both the basement and main floors. At the beginning of the study, a pair of alpha-track detectors were placed on top of the refrigerator in the kitchen (or some other sampling location on the main floor) and at a location in the basement. One detector at each location was left in place for a year while the other detector was retrieved and replaced once every 3-mo period. In addition, short-term measurements of Rn and 222Rn progeny were made at all sampling locations once per quarter. In this study, comparisons were made between: (1) seasonal and annual averages, (2) summer and winter averages, (3) living-area and basement results, (4) 222Rn and 220Rn progeny, and (5) short- and long-term measurements. The Rn and Rn progeny concentrations in houses near Huntsville, AL were found to be well above recommended action levels (150 Bq m-3). For houses near Birmingham, AL, summer Rn concentrations were found to exceed winter concentrations, whereas for the other houses in the study, winter concentrations exceeded summer concentrations. Potential alpha energy concentrations (PAEC) from 220Rn progeny were found to be generally less than PAEC from 222Rn.

A two-mutation model for radon-induced lung tumors in rats

The recessive oncogenesis model, according to which inactivation of both alleles of specific genes leads to cancer, has received much recent attention. A mathematical formulation of a two-mutation model for carcinogenesis, which includes the recessive oncogenesis model as a special case, was fitted to data from a large experimental study in which rats exposed to radon daughters developed malignant lung tumors. The model described the data well. The results indicate that fractionation of exposure increased the lifetime probability of tumor. Examination of the parameters of the model suggests that the effect of fractionation can be explained by the relative effects of radon daughters on the mutation rates and on the kinetics of growth of initiated cells. The first mutation rate is very strongly dependent upon the rate of exposure to radon daughters, the second mutation rate much less so, suggesting that the nature of the two mutational events is different. The model makes predictions which are testable in future experiments.

[Evaluation of the exposure levels to radioactivity in the hot-spring environment of the Island of Ischia during a year]

It is known that the decay products of the U-238 are findable in nature in certain materials (such as tufa, limestone, etc.), therefore some populations are naturally exposed to radiations originated by building materials, houses and isotopes present in drinking-waters. It is evident that, for example, the residents of Campania or Lazio will be involved, on such conditions and during their whole life, in the action of natural radiations of decay products of Uranium and consequently of Radon, which is originated and, as gas, also findable in the air of those indoor areas, at a concentration inversely proportional to the relevant ventilation factors. Furthermore, it is known that this isotope is always found in thermal waters too and sometimes at levels which make them classify as radioactive and with therapeutic properties still subjects of studies and researches. Actually, presence of Radon in warm waters is particularly evident in some basins (the Baths of Badgastein in Austria) and is also findable in some italian thermal towns such as Merano Laurisia, Abano, Baths of Lucca or the island of Ischia itself. All underground waters and especially the warm ones, including, those of Ischia basins, hold radioactive isotopes, since their composition is related to the nature of the underground, which they come from, and is strongly influenced by the kind of rocks present in that area, by the granulometry of the geological strata or by existent fractures, by flow speed and finally by solubilization capacity. On such conditions it is evident that all waters, during their scouring, will be enriched by Radon 222 (a gaseous, therefore more soluble, element); just because of its physical state, it will be more present in external environment in case of warm waters such as those ones now examined. Consequently the environmental state from a radioecological point of view of the utilized waters has to be considered, as well as any other thermal resort, fundamentally due to Radon (Rn) and to the relevant decay products. As a matter of fact, whereas from the family of Uranium come Radon (Rn 222), Lead (Pb 210) and Polonium (Po 210), these last two, respectively with halving-times of 22.3 years and 138.4 days, from the family of Thorium (Th 232) originates Radon (Rn 220 in past times defined "Toron" according to its "forebear") and Lead (Pb 212 with a halving-time of 10.6 hours).(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term tissue distribution and steady state activity ratios of 232Th and its daughters in rats after intravascular injection of Thorotrast

To estimate the absorbed dose in the critical organs of Thorotrast patients, it is necessary to know not only the distribution and concentration of 232Th but also its daughter nuclides in the body. The present investigation was undertaken in order to clarify the long-term 232Th tissue distribution and steady state activity ratios between subsequent daughters in the critical tissues using about 30 Wister male rats, as a basis for estimating absorbed doses. The tissue distribution of thorium was examined by means of an autoradiography of the whole body and/or the gamma-ray spectrometry at various times during 2 to 24 months following injection. The concentrations of daughter nuclides in tissues were determined by repetitive gamma examination over a period from 1 hr to 35 days after being sacrificed. The data indicate (1) that approximately 90% of injected Thorotrast is retained in the body for a prolonged period, but about 50% of radium and 10% of radon produced from thorium are eliminated from the body, (2) that the mean steady state activity ratios of 224Ra and 212Pb to 228Th for liver are 0.56 and 0.28, and 0.54 and 0.16 for spleen, 0.58 and 0.82 for lungs, respectively, and (3) that the parent 228Th is translocated to the bone.