Denis L. Henshaw

Radon as a causative factor in induction of myeloid leukaemia and other cancers

The international incidence of myeloid leukaemia, cancer of the kidney, melanoma, and certain childhood cancers all show significant correlation with radon exposure in the home. For myeloid leukaemia, analysis suggests that in the UK 6-12% of incidence may be attributed to radon. In Cornwall, where radon levels are higher, the range is 23-43%. For the world average radon exposure of 50 Bq.m-3, 13-25% of myeloid leukaemia at all ages may be caused by radon.

High resolution alpha-particle spectroscopy using CR-39 plastic track detector☆

Abstract We present a technique for high resolution alpha particle spectroscopy using CR-39 plastic. An energy resolution, δE, close to the range straggling limit is reported on particles of energy ≳3 MeV under typical etch conditions. For 6 MeV α-particles δE is ∼35 keV on individual tracks. For groups of particles δE∼20 keV can be achieved using certain data selection criteria. The technique is based on the determination of track length in the plastic from measurement of the etch track parameters. The etch track data can be analysed by two methods. Method 1 employs all five parameters of the etched track: track depth, Z; major axis Mj; minor axis, Mi overall track length, X and the diameter of the etched out end, m, which is equal to zero in the case of a non-etched out track. Method 2 employs only the horizontal measurements of the projected parameters X, Mi and m. The experimental data presented include (1) VT-range curves at etch temperatures of 50, 75 and 100°C in 6.25N NaOH for CR-39 available commercially and for material manufactured to specific temperature-time curing profiles. (2) The variation of etch induction time with etch temperature and α-particle energy. The theoretical analysis includes (1) a full discussion of α-particle track structure in CR-39 as a function of particle energy, dip angle and degree of etching including a comparative simulation of etched tracks, (2) a description of the methods of track analysis. The energy resolution attainable using the methods of analysis described is discussed fully. As an example of the best resolution achievable using selection criteria on a group of tracks the energy distribution of α-particles from a 252Cf source is considered. The 6.076 and 6.119 MeV α-particles are clearly resolved: δE=±20 keV.

Corona ions from powerlines and increased exposure to pollutant aerosols

PURPOSE To investigate corona ions emitted from high-voltage powerlines and assess their effects on exposure to environmental pollutant aerosols. MATERIALS AND METHODS The charge density of ions in air required to produce a given change in the observed DC field at ground level was calculated. A DC field mill meter was used to map the DC fields near 132, 275 and 400 kV powerlines. RESULTS The Earths natural DC field of around approximately 100 Vm(-1) was significantly modified near powerlines in 8 out of 14 cases. Typically, downwind of the powerlines the field direction was reversed giving values to -340 V m(-1). In one case, the effect extended more than 500 m from a 275 kV line. DISCUSSION These results can be analysed in terms of the charge density present. This analysis suggests that typically 2000 excess negative charges per cm3 are required to match the measured DC fields. Such space charge will result in unipolar aerosol charging in excess of the normal bipolar steady state charge distribution of pollutant aerosols. This may lead to increased lung deposition on inhalation.

Possible association between mutant frequency in peripheral lymphocytes and domestic radon concentrations.

To investigate whether previously found geographical correlations between leukaemia incidence and exposure to radon are reflected in a detectable mutagenic effect on individuals, the frequency of mutations in the hypoxanthine guanine phosphoribosyl transferase gene (hprt) in peripheral blood T lymphocytes was measured in subjects with known domestic radon concentrations. These concentrations were measured in December, 1989, in houses in Street, Somerset, UK, by passive alpha-track radon detectors. 20 non-smoking subjects aged 36-55 years were selected from the patient list at the local health centre on the basis of the radon concentrations in their homes--the range selected varied by a factor of ten. Blood samples for preparation of T lymphocytes were taken in July, 1990. There was a significant association between the log mutant frequency and radon concentration (t = 3.47, p less than 0.01). A second analysis of a further set of radon measurements (October, 1990, to January, 1991), in both living rooms and bedrooms, and repeated mutant frequency determinations also showed a significant relation, which remained significant even after exclusion of the highest frequency and adjustment for subjects age and cloning efficiency. These data must be regarded as preliminary and further more extensive studies should be done to determine whether the observed association is causal.

A method of producing thin CR-39 plastic nuclear track detectors and their application in nuclear science and technology

Abstract As part of a systematic investigation into the track response of CR-39 plastic manufactured in specifically controlled temperature-time curing cycles and the development of the manufacture of thin samples of this material, 5–70 μm uniform sheets were cast between glass plates following specially calculated cycles designed to give polymerisation at a constant rate. Repeated investigations into the track recording and physical properties of such CR-39 specimens for different initiator concentrations and cure conditions led to the choice of an optimal value of 7% by weight of CHPC initiator in the monomer. Irradiated stacks of thin CR-39 were etched to produce cylinders along particle tracks. These showed a maximum 3% random variation in sensitivity, conforming the excellent potential uniformity of this material. Evidence emerged that the track response varied as the amount of initiator that had been added, even though saturation of bulk etch rate had been achieved. The applications of CR-39 where the results of this work are particularly relevant are discussed.

Increased exposure to pollutant aerosols under high voltage power lines

PURPOSE To assess increased exposure to airborne pollutants near power lines by investigating theoretically and experimentally the behaviour of 222Rn decay product marker aerosols in the 50 Hz electric field under power lines. MATERIALS AND METHODS The behaviour of aerosols in outdoor air including those carrying 222Rn decay products was modelled theoretically in the presence of an AC field. TASTRAK alpha-particle spectroscopy was used to characterize 218Po and 214Po aerosols outdoors. Sampling points were chosen along a line at right angles up to 200 m from a number of high voltage power (transmission) lines. Each sampling point comprised an arrangement of mutually orthogonal TASTRAK detectors. Exposures were carried out at different power line locations in various weather conditions. RESULTS The model predicts a two- to three-fold increase in deposition of aerosols on spherical surfaces mimicking the human head under high voltage power lines. Experimental measurements using detectors mounted on grounded metal spheres showed an enhanced deposition of both 218Po and 214Po aerosols. Enhanced 218Po deposition on 400 kV lines ranged from 1.96+/-0.15 to 2.86+/-0.32. Enhanced 214Po deposition on 275 kV and 132 kV lines were 1.43+/-0.07 and 1.11+/-0.21, respectively, where the latter value was not significant. CONCLUSIONS The observations demonstrate a mode of increased exposure to pollutant aerosols under high voltage power lines by increased deposition on the body. The total (indoor + outdoor) 218Po and 214Po dose to the basal layer of facial skin is estimated to be increased by between 1.2 and 2.0 for 10% of time spent outdoors under high voltage power lines.

Enhanced deposition of radon daughter nuclei in the vicinity of power frequency electromagnetic fields

We report the attraction of radon daughter nuclei in normal domestic room air to everyday sources of power frequency electromagnetic fields. TASTRAK plastic track detectors were used to measure alpha-emission from the radon (222Rn) daughters 218Po and 214Po close to 50-Hz mains frequency power cables and domestic appliance leads. We observed that wires carrying mains frequency potential attract radon daughter nuclei in a manner visually similar to the attraction of iron filings to a magnet. Increased deposition of up to a factor of 18 was observed. Further experiments designed to simulate the effect of overhead power lines at ground level showed large scale variations in local plateout and airborne concentration of radon daughters. The effects appear to be due to interactions of the electric component of the EM-field with both the ultrafine and attached fraction of radon daughter aerosols. Three mechanisms have been examined. First, aerosols instantaneously polarize in sympathy with the electric component of an applied mains frequency EM-field and as a result may migrate up strong E-field gradients. This effect is however quite weak and may only affect larger aerosols in strong field gradients. Second, charged aerosols oscillate in a 50-Hz field and in the case of the ultrafine fraction this may lead to significantly increased plateout. Third, there is the possibility that oscillation of charged aerosols induces fluctuations in the magnitude of the instantaneous charges carried and this results in a diffusive-like motion along the field lines. The effect on dose to body organs is considered. Enhanced plateout on the skin is likely, increasing the dose to the basal layer. The dose to internal organs is governed initially by inhalation and lung deposition of radon daughters. Therefore increased local concentration in room air could lead to increased dose by inhalation. E-fields are heavily attenuated by the human body and body values in tissue of no more than 10(-4) of the external field strength have been suggested. Nevertheless calculations suggest that under exposure to strong external fields this may still be sufficient to alter the deposition pattern of the charged component of the ultrafine fraction in the bronchial region. In contrast it is unlikely that internal electric fields could significantly affect the attached radon daughters. In this context the lack of measurements of residual E-fields in the cavities of the mouth, pharyngeal and bronchial regions is of considerable importance and should be addressed. We believe that the observations may have implications for the apparent enigma that there is no persuasive biological evidence to show that power frequency electromagnetic fields can influence any of the accepted stages in carcinogenesis. On the contrary, the observations show that EM-fields can concentrate in their vicinity a cocktail of radon daughter nuclei, a known carcinogen, and presumably other potentially harmful agents.

Lack of evidence for an association between the frequency of mutants or translocations in circulating lymphocytes and exposure to radon gas in the home

Radon measurements in the living room and main bedroom of 41 houses in the town of Street, Somerset, England have been made. Exposure levels, weighted using the formula of the UK National Radiological Protection Board, of 19-484 Bq m-3 (about half > 100 Bq m-3) were found. Blood samples were obtained from a total of 66 occupants in these homes, and the frequency of genetic alterations in lymphocytes was estimated using two different end points. Gene mutations at the hypoxanthine guanine phosphoribosyl transferase locus were determined in T lymphocytes for 65 subjects using a clonal assay, and the frequency of the BCL-2 t(14;18) translocation, a chromosomal event associated with leukemia/lymphoma, was estimated in lymphocytes using a polymerase chain reaction-based technique for 64 subjects. In neither case was a significant correlation with radon levels in the home found, in contrast to our earlier observation with a smaller series.

Alpha-particle autoradiography in CR-39: a technique for quantitative assessment of alpha-emitters in biological tissue

The techniques for alpha-particle autoradiography based on the plastic nuclear track detector CR-39, previously reported, have been developed considerably. The techniques are applied to alpha-autoradiography of human lung tissue in particular but are applicable to any biological tissue. The most important developments are: (i) Improvements in the manufacture and pre-etching of the plastic. These allow activities as low as approximately 10(-15) Ci g-1 to be determined. (ii) High resolution alpha-particle spectroscopy in CR-39 plastic based on the analysis of the structure of the etched track. This enables the energy of individual alpha-particles to be determined to approximately 35 keV. (iii) Calculation of the effective thickness of tissue sampled by the plastic. This relates the tissue activity to the track density on the plastic. (iv) A deconvolution analysis which takes the distributions of track length and dip angle in the plastic and determines the alpha-particle range spectrum and distribution of tissue activity with height above the plastic surface. This enables both the absolute abundance and the microdistribution of alpha-active nuclides present to be determined. (v) The analysis of radon diffusion in tissue to determine the mean radon diffusion distance in tissue and plastic.

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.