Radiation exposure of the population
If the distribution of radioactive substances in the environment is given, the radiation exposure ("dose") of the population may be assessed. However, this assessment is by no means simple. Nor is there a straightforward, uniform relationship between ground contamination and radiation dose. The dose to the population includes the external gamma radiation dose as well as the internal dose received by eating food containing radioactive materials.
Different physical data are used in calculating external dose. Additional important data are biological "transfer factors" influencing the movement of elements from the ground through food into the human body. Finally, people's living conditions are important for assessing doses, such as housing, time spent outdoors, and diet. There are complex mathematical calculations which take all this into account. They always entail certain assumptions and may be designed to give either conservative or realistic results .
Certain checks can be made by direct measurements and this was done during the project. People can be equipped with dosimeter badges which, if carried at all times, record the radiation dose they receive externally. Also, by means of devices called "whole body counters", people can be monitored for the amount of caesium they have absorbed within their bodies. This provides an effective way of measuring their internal dose. Because of the nature of the work involved, such checks can be made only on representative samples of the affected population.
The project concentrated on examining data for significant radioactive elements that can affect human health such as caesium, strontium and iodine. A number of representative settlements in contaminated regions were selected for independent radiation dose assessment. Some 8000 personal dosimeters were distributed to residents of seven such settlements and were carried for two months. The dosimeters were then sent to France for evaluation.
Independent whole body counting of radiocaesium levels was done by project teams on more than 9000 people in nine settlements. Comparisons between project and USSR whole body counting facilities and data were made. Finally, independent calculations of past and future doses were made for the surveyed settlements using internationally accepted calculation methods and starting from average values for deposition of radioactivity in the soil. Since the radioactive iodine from the fallout emissions had totally decayed long before the project began, it was not possible to do independent measurements of this element for verification purposes. Because the thyroid gland concentrates iodine coming into the body, radioactive iodine can give a significant dose to that organ. Doses to the thyroid were reported on the basis of some early direct thyroid measurements as well as assumptions about iodine intake. For the seven contaminated settlements specifically studied by the project team, average radioactive iodine thyroid doses for children reported by the USSR varied widely.
- The USSR procedure for estimating radiation doses was assessed as being scientifically sound. Methodologies used in the USSR were intended to, and did, overestimate the doses actually received.
- The external dose to people from radionuclides deposited on the ground is generally the most significant contribution to total dose, especially in the areas where food restrictions are in force.
- Of the dosimeters worn for two months by inhabitants of the highly contaminated areas, only 10 percent registered exposures above the minimum detection limit of 0.2 mSv. This corresponds to 1.2 mSv per year which is a typical dose an individual would receive from the natural environment.
- The whole body counting of radiocaesium generally indicated lower contents of that element in peoples' bodies than would be calculated using theoretical models based on transfer from the environment, intakes from food, and metabolism. This is true for USSR models as well as for international ones. Comparing whole body counting with basic model calculations produced results in the USSR similar to those seen in other countries.
- Independent calculations of past and future doses, based on average deposition results for the surveyed settlements, gave doses lower by a factor of 2-3 than the USSR estimates for the same villages.
External Dose 60-130 mSv 80-160 mSv Internal Dose (caesium) 20-30 mSv 60-230 mSv Total (including strontium) 80-160 mSv 150-400 mSv