Pilot study of the Urals population by tooth electron paramagnetic resonance dosimetry

During 1949–1956, about 76 × 10⁶ m³ of radioactive liquid waste containing a total activity of 10¹⁷ Bq was discharged into the Techa River by the first Russian industrial nuclear facility Mayak. As a consequence, the population living in the river valley received considerable internal and external radiation doses. The results of a first application of electron paramagnetic resonance (EPR) of tooth enamel for a retrospective individual dose evaluation of the residents of the Techa riverside are presented. Three main contributions to the dose absorbed in tooth enamel have been considered: external exposure mainly from the Techa River sediments, internal exposure mainly due to ⁹⁰Sr; and background radiation including all other sources of exposure except the Techa River. The teeth of 86 inhabitants of the town Kamensk-Uralskii were analysed to determine the age-dependent contribution of the background radiation to the enamel dose. For 22 residents of the middle and lower Techa riverside, measurements of the ⁹⁰Sr whole-body content and EPR measurements of the absorbed dose in enamel were used to establish a correlation between these two quantities. Finally, absorbed doses in the enamel of five residents of the upper Techa riverside were determined by the EPR method. Contributions of the background radiation and the internal ⁹⁰Sr contamination were subtracted to determine the external exposure of the residents.     

Concentrations of 90Sr in the tooth tissues 60 years after intake: results of TL measurements and applications for Techa River dosimetry

This article focuses on the study of ⁹⁰Sr in the tooth tissues of Techa riverside residents 60 years after intake. The Techa River was contaminated by radioactive wastes in the 1950s. Contamination of the river system, including water, bottom sediment, floodplain soil, and grass, depended on the distance from the source of releases. Therefore, the average ⁹⁰Sr intake was different in different settlements located downstream the river. An additional factor influencing ⁹⁰Sr accumulation in the teeth is the rate of tissue mineralization at the time of intake which depended on the donor’s age at the time of releases. Measurements of ⁹⁰Sr concentration in various dental tissues (enamel, crown, and root dentin) of 166 teeth were performed about 60 years after the main intake using the method of thermoluminescence passive beta detection. The paper presents the current levels of tooth tissue contamination, and the tooth-to-tooth variability of ⁹⁰Sr concentration in tooth tissues was assessed for the tissues which were matured at the time of massive liquid radioactive waste releases into the Techa River. A model describing the expected levels of ⁹⁰Sr in matured dental tissues depending on age and intake has been elaborated for the population under study. The results obtained will be used for calculation of internal dose in enamel and for interpretation of tooth doses measured by means of the electron paramagnetic resonance method, among the population of the Techa River region.     

Does the cortical bone resorption rate change due to <Superscript>90</Superscript>Sr-radiation exposure? Analysis of data from Techa Riverside residents

The Mayak Production Association released large amounts of ⁹⁰Sr into the Techa River (Southern Urals, Russia) with peak amounts in 1950–1951. Techa Riverside residents ingested an average of about 3,000 kBq of ⁹⁰Sr. The ⁹⁰Sr-body burden of approximately 15,000 individuals has been measured in the Urals Research Center for Radiation Medicine in 1974–1997 with use of a special whole-body counter (WBC). Strontium-90 had mainly deposited in the cortical part of the skeleton by 25 years following intake, and ⁹⁰Sr elimination occurs as a result of cortical bone resorption. The effect of ⁹⁰Sr-radiation exposure on the rate of cortical bone resorption was studied. Data on 2,022 WBC measurements were selected for 207 adult persons, who were measured three or more times before they were 50–55 years old. The individual-resorption rates were calculated with the rate of strontium recirculation evaluated as 0.0018 year⁻¹. Individual absorbed doses in red bone marrow (RBM) and bone surface (BS) were also calculated. Statistically significant negative relationships of cortical bone resorption rate were discovered related to ⁹⁰Sr-body burden and dose absorbed in the RBM or the BS. The response appears to have a threshold of about 1.5-Gy RBM dose. The radiation-induced decrease in bone resorption rate may not be significant in terms of health. However, a decrease in bone remodeling rate can be among several causes of an increased level of degenerative dystrophic bone pathology in exposed persons.     

Increase in accumulation of strontium-90 in the maternal skeleton during pregnancy and lactation: analysis of the Techa River data

The unique contamination of the Techa River (Southern Urals, Russia) in the 1950s by long-lived ⁹⁰Sr allows investigation of the accumulation of bone-seeking elements in humans. This study is based on information compiled at the Urals Research Center for Radiation Medicine (Chelyabinsk, Russia) over a long period of time. It includes the results of in vivo measurements of ⁹⁰Sr-body burden with a whole body counter (WBC), data on personal medical examinations and residence and family histories. Data on 185 women from two Techa riverside villages Muslyumovo and Brodokalmak were selected. The settlements differ in terms of ⁹⁰Sr diet intake (higher in Muslyumovo than in Brodokalmak) and ethnicity (residents were mainly Slavs in Brodokalmak and Turkic in Muslyumovo). Results of a total of 555 WBC measurements performed in 1974–1997 were available for the women studied; maximum measured values reached 40 kBq/body. The women from each settlement were subdivided into three groups according to their childbearing history: pregnancy and lactation occurred (1) during the period of maximal ⁹⁰Sr intake (1950–1951); (2) after the period of maximal intake and (3) before this period or women who were childless. An increase was found in accumulation of ⁹⁰Sr in maternal skeleton during pregnancy and lactation (group 1) by a factor of 1.5–2 in comparison with non-pregnant, non-lactating women. This result was found in both Muslyumovo and Brodokalmak samples. An increase in accumulation of toxic elements in pregnant/lactating women is associated with increased radiation/toxic doses and risk for the women’s health.     

Reconstruction of radionuclide intakes for the residents of East Urals Radioactive Trace (1957–2011)

The East Urals Radioactive Trace (EURT) was formed after a chemical explosion in the radioactive waste-storage facility of the Mayak Production Association in 1957 (Southern Urals, Russia) and resulted in an activity dispersion of 7.4?×?10¹⁶ Bq into the atmosphere. Internal exposure due to ingestion of radionuclides with local foodstuffs was the main factor of public exposure at the EURT. The EURT cohort, combining residents of most contaminated settlements, was formed for epidemiological study at the Urals Research Center for Radiation Medicine, Russia (URCRM). For the purpose of improvement of radionuclide intake estimates for cohort members, the following data sets collected in URCRM were used: (1) Total β-activity and radiochemical measurements of ⁹⁰Sr in local foodstuffs over all of the period of interest (1958–2011; n?=?2200), which were used for relative ⁹⁰Sr intake estimations. (2) ⁹⁰Sr measurements in human bones and whole body (n?=?338); these data were used for average ⁹⁰Sr intake derivations using an age- and gender-dependent Sr-biokinetic model. Non-strontium radionuclide intakes were evaluated on the basis of ⁹⁰Sr intake data and the radionuclide composition of contaminated foodstuffs. Validation of radionuclide intakes during the first years after the accident was first carried out using measurements of the feces β-activity of EURT residents (n?=?148). The comparison of experimental and reconstructed values of feces β-activity shows good agreement. ⁹⁰Sr intakes for residents of settlements evacuated 7–14 days after the accident were also obtained from ⁹⁰Sr measurements in human bone and whole body. The results of radionuclide intake reconstruction will be used to estimate the internal doses for the members of the EURT cohort.     

Fetal dose assessment for the offspring of the Techa Riverside residents

Massive releases of fission products from the plutonium facility Mayak (Southern Urals, Russia) resulted in the contamination of the Techa river and its floodlands (1949–1956). The results of long-term studies of exposure populations have been used for different purposes of retrospective dosimetry. The riverside residents were exposed via various pathways and their progeny were exposed in utero both to external radiation and to internal radiation from radionuclides ingested by the mothers prior to conception and during pregnancy. Fetal doses due to ⁹⁰Sr from the maternal skeleton were estimated and compared with doses from other pathways. Individual red bone marrow (RBM) doses to the late fetuses were calculated on the basis of ⁹⁰Sr contents measured in the maternal skeleton and the ⁹⁰Sr transfer coefficients (TC) to the fetal skeleton were determined on the basis of the Techa river data. Two values of TC were assumed depending on the mothers’ maturation status in the period of maximum releases: TC=0.2 for group 1 (adulthood of mothers in 1950) and TC=0.02 for group 2 (mothers not yet in menarche age in 1950). Fetal doses in both groups that resulted from the different TC values varied by a factor of about 5–8. Furthermore, the fetal RBM doses due to ⁹⁰Sr from the maternal skeleton were found to depend on the distances from the site of release and the time after major ⁹⁰Sr intake. The average fetal RBM doses in the population of the upper, middle and lower Techa riverside regions were close to the ratio 3:2:1 and 20 years after the onset of contamination, the fetal doses have decreased by a factor of about 3–5. The distance from the site of release determined the relative contribution of different pathways to the fetal dose. For the settlements that are located closest to the site of release, the external exposure was of major concern up to the date of evacuation. For the non-evacuated settlements, the contribution of internal doses was higher and after 1956, external exposure is assumed to be negligible. From 1956 on, ⁹⁰Sr that has been transferred from the maternal skeleton was practically the single source of in utero exposure. Received: 8 December 2000 / Accepted: 1 July 2001     

Direct beta measurements to determine in vivo whole-body activities of 90Sr in residents of the Southern Urals: description of a new method

For 44 individuals living in areas of the Southern Urals with historical ⁹⁰Sr contamination, whole-body activities of this radionuclide were investigated using a new mobile detection system. β-particles from ⁹⁰Sr/⁹⁰Y decay were measured in vivo via two proportional counters mounted in front of the forehead and above the head, respectively. In order to correct for absorption by the skin, scalp thickness was measured using ultrasonic techniques. Corrections are given with respect to self-absorption by the bone matrix and absorption by hair. A procedure is described to extrapolate from measured ⁹⁰Sr activity of the skull bone to total ⁹⁰Sr skeleton burden. As a result, ⁹⁰Sr whole-body activities of up to 50 kBq were recorded in the selected cohort. For the same individuals, ⁹⁰Sr was measured via the detection of bremsstrahlung using an established whole-body counting device. The overall results of both systems agree within 15%, but differences exceeding a factor of 2 were observed in some cases.     

Monte Carlo-based calculation of imaging plate response to <Superscript>90</Superscript>Sr in teeth: experimental validation of the required correction on sample thickness

Recently, a numerical method was proposed to correct the imaging plate (IP) response to ⁹⁰Sr concentration in tooth samples, depending on the sample thickness. This is important to quantify any ⁹⁰Sr concentration in teeth, which in turn is necessary to determine any ⁹⁰Sr incorporation of a person retrospectively. Although the final goal will be to evaluate the (inhomogeneous) spatial distribution of ⁹⁰Sr inside tooth samples precisely, the present study was restricted—as a first step—to the evaluation of ⁹⁰Sr in teeth assuming a uniform ⁹⁰Sr distribution. A numerical method proposed earlier was validated experimentally in the present study by measuring the IP response to standard sources of various thicknesses and ⁹⁰Sr concentrations. For comparison, the energy deposition of the β-rays emitted by ⁹⁰Sr in the IP—which is considered to be proportional to the IP luminescence signal—was calculated for the various sample thicknesses involved, by means of the MCNP-4C code. As a result, the measured IP response could be reproduced by the calculations within the uncertainties, depending on the thickness of the standard sources. Thus, the validity of the proposed numerical method to correct the IP response for sample thickness has successfully been demonstrated.     

Retrospective dosimetry using OSL of tooth enamel and dental repair materials irradiated under wet and dry conditions

Following a radiological or nuclear emergency event, there is a need for quick and reliable dose estimations of potentially exposed people. In situations where dosimeters are not readily available, the dose estimations must be carried out using alternative methods. In the present study, the optically stimulated luminescence (OSL) properties of tooth enamel and different dental repair materials have been examined. Specimens of the materials were exposed to gamma and beta radiation in different types of liquid environments to mimic the actual irradiation situation in the mouth. Measurements were taken using a Ris? TL/OSL reader, and irradiations were made using a ⁹⁰Sr/⁹⁰Y source and a linear accelerator (6 MV photons). Results show that the OSL signal from tooth enamel decreases substantially when the enamel is kept in a wet environment. Thus, tooth enamel is not reliable for retrospective dose assessment without further studies of the phenomenon. Dental repair materials, on the other hand, do not exhibit the same effect when exposed to liquids. In addition, dose–response and fading measurements of the dental repair materials show promising results, making these materials highly interesting for retrospective dosimetry. The minimum detectable dose for the dental repair materials has been estimated to be 20–185?mGy.     

Childhood lead exposure in an enslaved African community in Barbados: Implications for birthplace and health status

Lead was ubiquitous on Caribbean sugar plantations, where it was used extensively in the production of sugar and rum. Previous studies suggest that skeletal lead contents can be used to identify African‐born individuals (as opposed to Creoles) among slave burials found in the New World. To test this hypothesis, we measured lead concentrations in enamel samples from 26 individuals from the Newton Plantation cemetery in Barbados, which was in use from around 1660 to 1820, and compared the results with enamel ⁸⁷Sr/⁸⁶Sr measurements that had been previously obtained for the same population. Results show a clear association between low (i.e., below 1 ppm) enamel lead concentrations and higher enamel ⁸⁷Sr/⁸⁶Sr ratios which have previously been interpreted as being indicative of African birth, suggesting that individuals with low enamel lead levels were indeed born in Africa as opposed to the New World. Based on these results, we propose that enamel lead measurements provide an effective and inexpensive way to determine African birth from skeletal remains. Furthermore, the lead measurements can provide useful insights into the health status and childhood environment of enslaved Africans during the colonial period. Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc.     

9OSr +90Y bremsstrahlung efficiency predicted for humans

Summary Bremsstrahlung x-rays are produced when energetic-particles are emitted and absorbed. Measurements of total-body bremsstrahlung efficiency (x-ray photon output per Ci⁹⁰Sr in the body, relative to that in water) have been made in the intact mouse, rat, rabbit, and dog sacrificed 2 weeks after the injection of⁹⁰Sr +⁸⁵Sr. Efficiencies were determined by a comparison of the bremsstrahlung output from⁹⁰Sr + its daughter⁹⁰Y and the gamma-ray emission of⁸⁵Sr. Results were checked by a-assay of the ashed animals. Bremsstrahlung efficiencies averaged 1.10 in a 0.04 kg mouse, 1.14 in a 0.13 kg rat, 1.23 in a 2.6 kg rabbit, and 1.32 in an 8.5 kg beagle. Extrapolating to a 70 kg human, a relative bremsstrahlung efficiency of about 1.4 is predicted. An estimate was made of the⁹⁰Sr body content in a former dial painter based on in vivo counting and a bremsstrahlung efficiency of 1.39 predicted for a 55 kg human female by these animal data. Our value of 1.42 ± 0.08 Ci⁹⁰Sr was in good agreement with corresponding results reported for this subject by 8 other laboratories.Supported by USERDA Contract E (11-1)-119     

Fish otoliths as biological dosimeter: internal dose calculation

Otoliths are organs used by fish for hearing and keeping balance. They consist of biogenic crystals of hydroxyapatite and do not contain any living cells. Upon exposure to ionizing radiation, otolith hydroxyapatite accumulates radiation-induced stable CO₂^? radicals whose amount is proportional to absorbed dose. In electron paramagnetic resonance (EPR) dosimetry, carbonate ions are registered and, hence, the total accumulated dose in the fish otolith can be quantified. Therefore, otoliths can be used as individual fish dosimeters to support radiobiological and radioecological studies. An important aspect of otolith-based EPR dosimetry on fish from contaminated water bodies is the potential presence of bone-seeking ⁹⁰Sr. Consequently, cumulative absorbed doses measured with EPR in otoliths may reflect the superposition of internal exposure to ⁹⁰Sr/⁹⁰Y and external exposure due to radionuclides circulating in soft tissue of the fish as well as due to environmental contamination. The objective of the present study was to develop a method that allows for an assessment of the contribution of ⁹⁰Sr to the total dose in otolith. The method has been tested using otoliths from seven fish taken from reservoirs located in the Southern Urals contaminated with radionuclides including ⁹⁰Sr. It has been shown that dose to otoliths is largely determined by ⁹⁰Sr in the hydroxyapatite. The internal dose component can be calculated using activity concentration-to-dose conversion factors, which vary slightly in the range of 2.0–2.8?×?10^–3 Gy year^?1 per Bq g^?1 depending on fish species and age. Internal doses to fish from water bodies with different levels of ⁹⁰Sr contamination were calculated in the range from 2 mGy to?~?200 Gy. External dose contribution was derived for two fish only to be about 100 and 40 Gy. It is concluded that EPR dosimetry on fish otoliths is a promising tool when external exposure prevails or is comparable to internal exposure due to ⁹⁰Sr.

     

Transfer of 85Sr and 134Cs from diet to reindeer foetuses and milk

Sr-85 and ¹³⁴Cs in aqueous solution of the chlorides were administered daily to four pregnant reindeer during the last part of gestation. Radionuclide concentrations were determined in calves sacrificed at birth, and secretion of the nuclides was measured in milk. Although the gastrointestinal absorption of ⁸⁵Sr was low, an apparently higher transfer of the absorbed fraction of ⁸⁵Sr than ¹³⁴Cs from the mother to the foetus led to similar accumulation of ⁸⁵Sr and ¹³⁴Cs in foetuses. At birth 1.4–1.6 and 1.5–2.5% of the total administered activities of ⁸⁵Sr and ¹³⁴Cs, respectively, were present in the calves‘ bodies. Transfer coefficients (F _m) for ⁸⁵Sr and ¹³⁴Cs from feed to milk were estimated at 0.0218±0.0026 and 0.185±0.025 day kg⁻¹, respectively, and the observed ratio (OR_milk-diet) for ⁸⁵Sr was 0.124±0.037. Transfer of radiostrontium to reindeer milk was in agreement with previously reported relationships between Ca intake and radiostrontium transfer in ruminants. These relationships suggest that the transfer of radiostrontium to foetuses and milk of free-ranging reindeer can be 3–4 times higher than observed in this experiment (due to lower Ca intake with natural forage), but the transfer to milk will not be as high as that of ionic ¹³⁴Cs. The concentrations of ⁸⁵Sr in milk suggested that the does mobilized skeletal stores of Ca and ⁸⁵Sr for milk production, although the diet appeared to satisfy the Ca requirements. In reindeer with radiostrontium intake during the whole year, radiostrontium concentrations in milk will therefore be higher than indicated by the F _m value observed in our study. No differences in half-times for ⁸⁵Sr and ¹³⁴Cs secretion in milk were detected. Both nuclides were secreted with short- and long-term half-times of 1–2 and 12–19 days, respectively. For ⁸⁵Sr, 80–90% of the activity was excreted with the short half-time, whereas the corresponding value for ¹³⁴Cs was 30–50%.     

Assessment of radiation exposure of murine rodents at the EURT territories

The study provides data on contemporary levels of radiation exposure of organs and tissues of murine rodents (several species of mice and voles) inhabiting the East-Ural Radioactive Trace. The estimation procedure involves the most advanced approach based on application of appropriate voxel phantom and biokinetic model. Input data for dose assessment are the results of measurements of skeletal ⁹⁰Sr activity concentration. Maximal internal dose to skeleton, accumulated during 45 days, is 303 mGy. Median internal dose rates on the last day before trapping were 0.83, 0.092 and 0.023 mGy/day for animals trapped at the sites with initial (1957) ⁹⁰Sr surface contamination >37 MBq/m², 18.5–37 MBq/m² and 0.074–18.5 MBq/m² respectively. Taking to account internal and external exposures, upper boundary of the ICRP Derived Consideration Reference Level (DCRL) is exceeded on the territory with maximal level of the initial ⁹⁰Sr surface contamination. On the territory with 18.5–37 MBq/m², whole body mean dose rates to murine rodents exceed the lower boundary of DCRL. On the areas with lower level of surface contamination, even the 90-th percentile of dose rate is below the DCRL.     

Ecotoxicological study of bottom sediments from reservoir 11 of the Techa River cascade

The ecotoxicological properties of sediments from reservoir R-11 of the Techa River cascade are studied in comparison to the Shershni reservoir. Radiochemical analysis of sediment and water samples from R-11 show that the radioactivity of the sediments ranges from 240 to 360 kBq/dm³ dry weight for ⁹⁰Sr and from 10 to 161 kBq/dm³ dry weight for ¹³⁷Cs. The absorbed doses for Oligochaeta from R-11 are calculated on the basis of radiochemical data. The hydrobiological studies include (1) study of the population density and diversity of Oligochaeta species in the reservoirs and (2) laboratory bioassay of sediments with Tubifex tubifex as a biomonitor species. The results indicate that the Oligochaeta population density in R-11 is less than in the Shershni reservoir. No significant effects of the absorbed dose rate on population density, survival rate, or fertility are found in a laboratory Tubificidae bioassay of R-11 sediments. However, a decrease in fertility is noted in experiments at a higher absorbed dose.     

Genotoxic and cytotoxic effects of <Superscript>60</Superscript>Co γ-rays and <Superscript>90</Superscript>Sr/<Superscript>90</Superscript>Y β-rays on Chinese hamster ovary cells (CHO-K1)

Among various types of ionizing radiation, the beta emitter radionuclides are involved in many sectors of human activity, such as nuclear medicine, nuclear industries and biomedicine, with a consequently increased risk of accidental, occupational or therapeutic exposure. Despite their recognized importance, there is little information about the effect of beta particles at the cellular level when compared to other types of ionizing radiation. Thus, the objective of the present study was to evaluate the genotoxic and cytotoxic effects of ⁹⁰Sr/⁹⁰Y—a pure, highly energetic beta source—on Chinese hamster ovary (CHO) cells and to compare them with data obtained with ⁶⁰Co. CHO cells irradiated with different doses of ⁶⁰Co (0.34 Gy min^–1) and ⁹⁰Sr/⁹⁰Y (0.23 Gy min^–1) were processed for analysis of clonogenic death, induction of micronuclei (MN) and interphase death. The survival curves obtained for both types of radiation were fitted by the exponential quadratic model and were found to be similar. Also, the cytogenetic results showed similar frequencies of radio-induced MN between gamma and beta radiations and the MN distribution pattern among cells did not follow the expected Poisson probability pattern. The relative variance values were significantly higher in cells irradiated with ⁹⁰Sr/⁹⁰Y than with ⁶⁰Co in all exposure doses. The irradiated cells showed more necrotic cells 72 h and 96 h after exposure to beta than to gamma radiation. In general, the ⁹⁰Sr/⁹⁰Y -radiation was more damaging than ⁶⁰Co -rays. The data obtained also demonstrated the need to use several parameters for a better estimate of cellular sensitivity to the action of genotoxic agents, which would be important in terms of radiobiology, oncology and therapeutics.     

The use of deciduous molars in EPR dose reconstruction

In the present study naturally loose deciduous molars were investigated. The feasibility of separating enamel from small size molars was analysed. EPR spectrum parameters of whole molars and separated enamel only were evaluated before and after laboratory irradiation. The Electron paramagnetic resonance (EPR) signal amplitudes of CO ₂ ⁻ and native signals were determined by spectrum deconvolution, as a function of radiation dose in the range 0.1–10 Gy. A detection threshold of absorbed dose from deciduous molars of 198 and 21 mGy is estimated for massive (that contained both enamel and dentine) and grainy samples (that contained enamel only), respectively. The elimination of the organic material from the massive deciduous samples reduced the mean bias dose for the native signal from 90±18 to 34±13 mGy. A decay of the background signal within 2 weeks after irradiation was found, while the dosimetric signal was stable before and after the irradiation process. The presented results suggest deciduous teeth to be suitable for retrospective dose assessment. To get reasonable dose estimates, however, any organic material must be eliminated, and the measurements should be performed 2 weeks after the chemical and mechanical preparation and the irradiation process are done.N. El-Faramawy: On leave from Department of Physics, Faculty of Science, Ain Shams University, 65511 Abbassia, Cario, Egypt.     

Estimation of background radiation doses for the Peninsular Malaysia’s population by ESR dosimetry of tooth enamel

Background radiation dose is used in dosimetry for estimating occupational doses of radiation workers or determining radiation dose of an individual following accidental exposure. In the present study, the absorbed dose and the background radiation level are determined using the electron spin resonance (ESR) method on tooth samples. The effect of using different tooth surfaces and teeth exposed with single medical X-rays on the absorbed dose are also evaluated. A total of 48 molars of position 6–8 were collected from 13 district hospitals in Peninsular Malaysia. Thirty-six teeth had not been exposed to any excessive radiation, and 12 teeth had been directly exposed to a single X-ray dose during medical treatment prior to extraction. There was no significant effect of tooth surfaces and exposure with single X-rays on the measured absorbed dose of an individual. The mean measured absorbed dose of the population is 34 ± 6.2 mGy, with an average tooth enamel age of 39 years. From the slope of a regression line, the estimated annual background dose for Peninsular Malaysia is 0.6 ± 0.3 mGy y⁻¹. This value is slightly lower than the yearly background dose for Malaysia, and the radiation background dose is established by ESR tooth measurements on samples from India and Russia.     

Enamel thickness and the helicoidal occlusal plane

In the present study 38 unworn maxillary molars (M¹ = 16, M²= 12, M³ = 10) of modern humans from a Slavic necropolis were sectioned through the mesial cusps in a plane perpendicular to the cervical margin of the crown. Five slightly worn M¹s and one slightly worn M³ were also used thus increasing the total sample to 44, but measurements made on the worn areas were coded as missing values. Seven measurements of enamel thickness as well as the heights of the protocone and the paracone dentine horns were recorded in order to analyze whether changes in these dimensions in anteroposterior direction can be related to the helicoidal occlusal plane. Uni- and multivariate analyses revealed that the distribution of enamel thickness within and between maxillary molars corresponds to a helicoidal occlusal wear pattern. Enamel thickness along the occlusal basin increases from anterior to posterior, which may lead to rapid development of a reverse curve of Monson in first molars when compared to posterior teeth. However, although these overall differences together with the serial, especially delayed eruption pattern of human molars, contribute to the marked expression of the helicoidal occlusal plane in Homo, differences in enamel patterning between molars indicate that a helicoidal plane is a structural feature of the orofacial skeleton. In contrast to first upper molars, second and third molars show absolutely and relatively thicker enamel under the Phase I wear facet of the paracone, i. e., the lingual slope of the paracone, than under the Phase II facet of the protocone, i. e., the buccal slope of that cusp. These proportional differences are most pronounced in M³, as evidenced by uni- and multivariate statistics. It thus appears that the pattern of enamel thickness distribution from M¹ to M³ follows a trend towards providing additional tooth material in areas that are under greater functional demands, that is, corresponding to a lingual slope of wear anteriorly and to a flat or even buccal one posteriorly. In addition, the heights of the dentine horns in anteroposterior direction change in a way that lends support to the hypothesis that the axial inclination of teeth could be one of the most important factors for the development of the helicoidal occlusal plane. Finally, the changes in morphology and enamel thickness distribution from first to third upper molars found in this study suggest that molars could be “specialized” in their function, i. e., from performing proportionally more shearing anteriorly to increased crushing and grinding activities posteriorly. © 1994 Wiley-Liss, Inc.     

Removal of radioactive strontium from the rat by feeding stable strontium

The effect of administering the stable isotope of strontium (as phos-phate) at different dietary levels to adult rats (fed on a cereal and pulse-based diet containing 0.4% Ca) on the retention of radiostrontium (⁸⁹Sr) and radiocalcium (⁴⁵Ca) in the femur and the whole skeleton was studied for a period up to 6 weeks after an intraperitoneal injection of the two radioisotopes. The ability of strontium to remove⁸⁹Sr under the above dietary conditions was examined. Feeding Sr at 0.5% or 1% levels for 6 weeks had no effect on the skeletal content of⁸⁹Sr or⁴⁵Ca while a dietary regimen of 2% Sr (2000 times the normal content), significantly lowered the⁸⁹Sr and⁴⁵Ca content by about 30% in the femur but not in the whole skeleton. At this Sr level, the urinary excretion of the isotopes increased with a concomitant decrease in their excretion in the faeces. This study underscores the limitations of dietary Sr to mobilise⁸⁹Sr from the bones after it is incorporated in the bone mineral.