NRPB-W17: Non-technical summary

Non-technical summary

So that we can assess the importance of discharges of radionuclides to the environment, we need to estimate the dose that a person may receive. To do this, we need to look at all possible ways in which the radionuclides can get into the body, and in what form. If the radionuclides are deposited on the ground, some will be transferred to food plants in the same way that the plants take in other nutrients. When those plants are eaten, some of the radioactivity is transferred to the person. As well as food, people also eat soil, either unintentionally by poor hygiene or food preparation, or deliberately because of a medical condition, although this is rare. To take account of this, the calculations used to estimate doses normally allow for a certain percentage of soil to be eaten. The concentration of radionuclides in the soil is sometimes much higher than in the food. Consequently, even if only small quantities of soil are eaten, this process can account for much of the activity that is eaten.

The process of digestion breaks down the food into simpler molecules that can then pass from the gut into the bloodstream for transport around the body. However, the chemical form of any radionuclide in soil is not necessarily the same as it is in the food, and it may be less available for transfer across the gut wall and into the bloodstream. At the moment, very little information is available on this topic and generally our calculations of doses do not take any differences in availability into account.

If we knew the availability for uptake across the gut wall, we could allow for this in the calculations. Some work has been done in the past to look at the way that gut fluid from sheep and cows reacts with the radionuclides in soil, including feeding trials. However, digestion in humans is a lot different to that in cows and sheep, and so it may not be possible to apply the results of those experiments to the calculations of doses to people. This report describes the development of a simple method to simulate human digestion and the use of this method to study what happens to radionuclides in soil after they have been eaten.

In humans, food is digested in two main stages: in the stomach and in the small intestine. By using chemicals and conditions similar to those found in those parts of the body, it was possible to simulate these processes in the laboratory. The soils used were loam, peat and sand, which had been deliberately contaminated with caesium-137, americium-241, plutonium-239 and strontium-90 some years ago. These soils have very different characteristics and so were expected to provide a range of results that could then be applied more generally to most other types of soil. By measuring the activity of each radionuclide in the simulated stomach and intestinal fluids and comparing that with the total amount in the soil, it was possible to estimate the percentage that had been made available. This factor can then be incorporated into our current calculations to estimate transfer across the gut wall and into the bloodstream.

The measured availability of caesium-137, americium-241, plutonium-239 and strontium-90 associated with loam, sand and peat soils was about 3%, 3%, 10% and 50% respectively. These results show that it is important to take account of availability when assessing doses from eating soil.

The results were compared with those from two other studies that also used the same soils. The first used published information on sheep, and the aim of the comparison was to see if the data in the literature on animal studies could be useful in estimating doses to people. In the second study, we carried out a simple extraction that did not require special chemicals or conditions, the aim being to see if a much simpler process could simulate human digestion adequately. Both of these other methods gave different results, and so we concluded that the simulated digestion method would be the most appropriate for assessing doses to humans from eating soil.

Last reviewed: 1 September 2009