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5.HUMAN EXPOSURE LEVELS

In terms of human exposure, the focus of the Expert Group on Environment report was on collective dose to persons living in contaminated areas of Belarus, the Russian Federation and Ukraine. Other reports have given attention to individual and collective doses to evacuated persons, cleanup workers and persons enrolled in epidemiological studies [2, 17].

Radiation doses were delivered to the exposed population primarily through the pathways of external dose (i.e. exposure to the radiation field created by the deposition of radionuclides on soil and other surfaces) and from the consumption of contaminated food. A feature of both pathways is that the dose received per unit time, even in the absence of countermeasures, decreases substantially with time due to natural processes. Radioactive decay is one of the important processes, but even for long lived radionuclides there is a substantial decrease with time in dose per unit time. This is explained for external dose by the weathering of radionuclides into the soil column, and the resulting shielding afforded by soil. For example, Golikov et al. [18] observed that about half of the initial external gamma exposure rate due to 137Cs decreased with an ‘ecological’ half-life of 2.4 a, and the other half is decreasing with a half-life of about 37 a (the latter value is uncertain). This relationship is shown in Fig. 10.

Similar results were observed for dose from the consumption of contaminated agricultural products. Of course, the dose from 131I decreases very rapidly due to this radionuclide’s short half-life of eight days. The dose rate from consumption of agricultural products containing 134Cs and 137Cs also declines rather rapidly due to the fixation of caesium by clay minerals in soil. Thus, soil type is an important parameter in any actual situation of interest. As a general rule, however, a very large fraction of the total expected dose from consumption of contaminated agricultural products had already been delivered within the first ten years after the accident, and only a small contribution (about 10% of the total) can be expected to be delivered in the post-2006 period [1].

Doses to humans were reduced significantly by a number of countermeasures. Official countermeasures included evacuations and relocations of persons, the blockage of contaminated food supplies, the removal of contaminated soil, the treatment of agricultural fields to reduce the uptake of radionuclides, the substitution of foods, and the prohibition of usage of ‘wild’ foods. Unofficial countermeasures included the self-initiated avoidance of foods judged to be contaminated.

Estimates of the collective, effective (not including the contribution of the dose to the thyroid) dose delivered up through 2005 are shown in Table 4 for persons living within the contaminated areas of the three more contaminated

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Time after the accident, years

FIG. 10. Reduction of the 137Cs gamma exposure rate in air due to caesium migration in undisturbed soil relative to the dose rate caused by a plane source on the air–soil interface (from Ref. [18]).

countries. An additional 3000 man Sv is estimated to be delivered to persons using the water from the Dnieper River reservoirs; such a dose would have accrued to a much larger group of persons, and its value is uncertain [19, 20]. Collective doses from the external and internal pathways are approximately equal, but there were great variations from location to location depending upon lifestyle, soil factors etc.

The thyroid dose from intake of 131I was mainly due to the consumption of fresh cows’ milk and, to a lesser extent, of green vegetables. Children, on average, received a dose that was much greater than that received by adults, because of their small thyroid mass and a consumption rate of fresh cow’s milk that was similar to that of adults.

The range in thyroid dose in different settlements and in all age–gender groups is large, between less than 0.1 Gy and more than 10 Gy. In some groups, and especially in younger children, doses were high enough to cause both short term functional thyroid changes and thyroid cancer in some individuals. The collective thyroid dose to the population of the contaminated areas of the three

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TABLE 4. ESTIMATES OF COLLECTIVE EFFECTIVE DOSE FOR RESIDENTS LIVING IN THE CONTAMINATED TERRITORIES OF THE THREE COUNTRIES OF INTEREST DURING 1986 THROUGH 2005 [1]

TABLE 5. ESTIMATES OF COLLECTIVE THYROID DOSE TO RESIDENTS LIVING IN THE CONTAMINATED AREAS OF THE THREE MORE CONTAMINATED COUNTRIES [1]

countries is estimated to be 1.6 million man Gy [1, 2]; details are shown in Table 5.

6.EFFECTS ON NON-HUMAN BIOTA

Irradiation from radionuclides released from the Chernobyl accident caused numerous acute adverse effects in the biota located in areas of highest exposure, i.e. up to a distance of a few tens of kilometres from the release point (Fig. 11). Beyond the exclusion zone, no acute radiation-induced effects on biota have been reported.

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FIG. 11. Measured exposure rates in air on 26 April 1986 in the local area of the Chernobyl reactor. Units of isolines are R/h (1 R/h is approximately 0.2 Gy/d) [2].

The environmental response to the Chernobyl accident was a complex interaction among radiation dose, dose rate and its temporal and spatial variations, as well as the radiosensitivities of the different taxons. Both individual and population effects caused by radiation-induced cell death have been observed in plants and animals as follows:

—Increased mortality of coniferous plants, soil invertebrates and mammals (rodents);

—Reproductive losses in plants and animals; and

—Chronic radiation syndrome of animals (mammals, birds etc.).

No adverse radiation-induced effects have been reported in plants and animals exposed to a cumulative dose of less than 0.3 Gy during the first month after the radionuclide fallout.

Following the natural reduction of exposure levels due to radionuclide decay and migration, populations have been recovering from acute radiation effects. By the next growing season after the accident, the population viability of plants and animals substantially recovered as a result of the combined effects of reproduction and immigration. A few years were needed for recovery from major radiation-induced adverse effects in plants and animals. One dramatic

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