Retention of 90Sr in human bone at different ages and the resulting radiation doses

A review of measurements of radionuclides in members of the public in the UK

This paper summarises a comprehensive review of radio-analytical data from autopsy, whole or partial body monitoring and the assay of teeth, foetuses and urine for non-occupationally exposed members of the public in the UK between 1957 and 2003. Most attention has been given to measurements of artificial radionuclides formed in the nuclear fuel cycle, uranium and thorium. The review concentrates on measurements on people in the UK who live or have lived in the vicinity of nuclear power sites. When UK data are unavailable, or for the purposes of comparison, information has been included from studies in other countries. Highlights of key findings of the document are listed: The concentrations of strontium-90 in bone and teeth have reflected changes in the amounts present in the environment due to fallout from nuclear testing. There are higher concentration levels of 239+240Pu in samples from West Cumbria compared with the rest of the UK. However, the levels are so low that any increase in risk of induced skeletal tumours (including leukaemia) would be very small compared with those arising from the intake of natural radionuclides. As expected there have been only a few published autopsy studies. Both tissue sample mass and radionuclide concentrations were low, leading to relatively large measurement uncertainties. Whole body measurements of 137Cs in residents in Berkshire and Oxfordshire clearly show the effect of atmospheric testing of nuclear weapons and of the Chernobyl accident. A survey of whole body 137Cs and 134Cs content following the Chernobyl accident showed that residents of Central Scotland, North-West England and North Wales had twice the radiocaesium content of residents in the rest of England and Wales. Measurements of 131I in the thyroid have been reported following the accidents at Windscale and Chernobyl for most regions of the UK. Few excretion studies have been reported although this does not diminish their importance. One study on the urinary excretion rate of 90Sr in adults and children living in the Dounreay area suggested that the results did not support this radionuclide as being the cause of increased childhood leukaemia. Similar conclusions were drawn from another study involving the assay of 239Pu. It is suggested that a national database of measurements made on members of the public should be initiated. The database would provide a means for identifying future trends.

Laboratory analyses: environmental and biological measurements

From its inception in 1951 to the present, the measurement of radioactive fallout from nuclear weapons testing and the many associated programs to establish global distribution and human health effects have contributed significantly to the understanding of worldwide dispersal of contamination. The original measurements of regional surface deposition of fallout nuclides were with duplicate gummed film collectors. Later, collectors were established in a worldwide network to measure total deposition and specific radionuclides such as 90Sr and 137Cs, which evolved into the first large-scale, global environmental monitoring network. Programs were set up to determine dietary intake and human and animal tissue distribution of 90Sr and 137Cs. Some of the first measurements of natural background dietary radium and body potassium were a response to identify analog elements. The impact of the environmental measurements made for fallout went far beyond any dosimetric consequences. For example, present day information on bone tissue turnover rates are derived mainly from radiochemical analysis of 90Sr measurements in human bone. The spin off from the enormous expenditure in effort to make these measurements and to determine the health consequences of global fallout laid a rich basic and applied scientific foundation in many disciplines, particularly in exposure pathways from ground deposition to dietary uptake and human organ biokinetics.

Dietary intake evaluation and dosimetric modelling for the Techa River residents based on in vivo measurements of strontium-90 in teeth and skeleton

Explanations are given for the models and algorithms that are utilised to reconstruct the doses due to the incorporation of 90Sr in the inhabitants of the Techa riverside villages. A model for the 90Sr accumulation and excretion in the tooth enamel is employed to derive the relative intakes of 90Sr in the years 1950-1959. The measurements of beta-ray activity on the surface of the front teeth of persons born between 1940-1959 are used to derive the relative intakes in different calendar years and the transfer functions to the tooth enamel as a function of age. This derivation is based on a non-linear optimisation algorithm, to fit the observed average count rates for different birth cohorts in a village. The data for the village of Muslyumovo are used as the reference data set. Absolute values of the strontium intake are derived from the relative values by a fit to the measurements of the 90Sr body burdens that were performed after 1974. A model has been developed for the age-dependent metabolism of strontium in the skeleton. The computations in terms of this model are in reliable agreement with experimental data on strontium in man.

The uptake and turnover of 90Sr in the human skeleton

In the United Kingdom, measurements have been made of the concentration of 90Sr in human bone from 1955 to 1970, and in human diet since 1958. A correlation of these two series of observations has enabled estimates to be made of (i) the fraction of the dietary intake of 90Sr that reaches the skeleton, (ii) the rate of turnover of 90Sr in the skeleton, and (iii) the way in which both these parameters vary with age. The results may be used to predict future levels of 90Sr in human bone from measurements of the radionuclide in diet, and also to calculate the radiation doses received by tissues in bone from intakes of 90Sr and 89Sr.

Dependence on age at intake of committed dose equivalents from radionuclides

The dependence of committed dose equivalents on age at intake is needed to assess the significance of exposures of young persons among the general public resulting from inhaled or ingested radionuclides. The committed dose equivalents, which are evaluated using ICRP principles, depend on the body dimensions of the young person at the time of intake of a radionuclide and on subsequent body growth. Representation of growth by a series of exponential segments facilitates the derivation of general expressions for the age dependence of committed dose equivalents if metabolic models do not change with age. The additional assumption that intakes of radionuclides in air or food are proportional to a person's energy expenditure (which implies age-independent dietary composition) enables the demonstration that the age of the most highly exposed 'critical groups' of the general public from these radionuclides is either about 1 year or 17 years. With the above assumptions the exposure of the critical group is less than three times the exposure of adult members of the general public. Approximate values of committed dose equivalents which avoid both underestimation and excessive overestimation are shown to be obtainable by simplified procedures. Modified procedures are suggested for use if metabolic models change with age.