NRPB - W17 The Availability of Soil-Associated Radionuclides for Uptake After Inadvertent Ingestion by Humans

Synopsis

A non-technical summary is also available.

Assessments of the radiological impact of routine and accidental releases of radionuclides into the environment generally take account of the effective dose from the ingestion of radionuclides associated with soil or sediment. The ingestion of soil by humans is relatively common and may be either inadvertent or purposeful. The results of a recent study on the potential incursion of marine sediment inland in northwest England indicated that inadvertent ingestion of radionuclides associated with deposited material could account for a substantial fraction of the estimated dose, especially for children.

A lack of relevant data has meant that radionuclides associated with soil or sediment are often assumed to have the same availability for uptake across the gut as those incorporated into food. Studies of the availability of soil-associated radionuclides after ingestion have been mainly conducted on ruminant animals and there are few data available for humans. As the ruminant digestive tract is totally different to that of man, the use of data from animal studies may not be appropriate in human assessments.

A simple in vitro procedure has been developed to simulate human digestion. The purpose is to evaluate the fraction of activity that is released into solution and would thus become available for uptake. The procedure was used to study the availability of caesium-137, plutonium-239, amercium-241 and strontium-90 in loam, peat and sand soils, and to derive availability factors for those radionuclides. The results showed that the availability of plutonium-239 and strontium-90 was dependent on soil type, whereas the differences for caesium-137 and amercium-241 were not so marked. For generic assessments, or where the soil type is not known, generalised values of 3%, 3%, 10% and 50% for caesium-137, amercium-241, plultonium-239 and strontium-90 respectively can be used.

Where possible, results obtained from the in vitro procedure should be used in preference to availabilities estimated either from ruminant animal studies or by simple extractions. Measurements should be carried out on freshly collected soil. The use of a simple multiplication of published dose coefficients by the availability factor should be carried out with caution; where possible dose coefficients should be recalculated.

Further research is required into the interactions between food and soil-associated radionuclides and into the association of those radionuclides with different molecular size fractions in simulated gut fluid. Other radionuclides that could be important for soil or sediment ingestion should also be studied, particularly lead-210 and polonium-210.

Last reviewed: 29 July 2009