Introduction

(d) Internal dose from the consumption of contaminated food and water.

Under most exposure conditions for members of the general public the two most important pathways are dose from radiation from the decay of radionuclides deposited upon the soil and other surfaces and dose from the ingestion of contami-nated food and water. If persons are evacuated quickly after passage of the initial cloud, then the most important pathways are the first two in the list, because the latter two pathways have been prevented.

5.1.3. Concepts of dose

Methods of calculating radiation dose have been refined over the years, and specific concepts have evolved [5.1, 5.6]. The fundamental measure of radiation dose to an organ or tissue is the absorbed dose, which is the amount of energy absorbed by that organ or tissue divided by its weight. The inter-national unit of absorbed dose is the gray (Gy), which is equal to one joule per kilogram. Since this is a rather large amount of dose, it is common to use units of mGy (one thousandth of a gray) or µGy (one millionth of a gray).

Since many organs and tissues were exposed as a result of the Chernobyl accident, it has been very common to use an additional concept, effective dose, which is the sum of the products of absorbed dose to each organ multiplied by a radiation weighting factor and a tissue weighting factor. The former varies by radiation type and is related to the density of ionizations created; the latter is an approximation of the relative probability that an absorbed dose to a particular organ might lead to

the production of a cancer. The sum of all tissue weighting factors is equal to 1.0.

The concepts mentioned above are applied to individuals. Where many individuals have been exposed to an event, such as happened following the Chernobyl accident, an additional concept, the collective dose, can be used. The collective dose is the sum of the doses to all individuals within a particular group, which may be the residents of a particular country or the persons involved in some type of activity, such as cleaning up the conse-quences of the accident. This concept is most often applied to effective doses, and the common unit of the collective effective dose is the man-Sv.

Finally, UNSCEAR has employed the concept of dose commitment to examine the long term consequences of a practice or accident [5.1]; for example, at the very moment that the Chernobyl accident occurred, it can be considered that a dose commitment occurred at the moment of the release of the radioactive material. This is true even though it will take many years for the doses to be received by the persons alive at that time and by persons not yet born or conceived.

5.1.4. Background radiation levels

Living organisms are continually exposed to ionizing radiation from natural sources, which include cosmic rays and terrestrial radionuclides (such as ⁴⁰K, ²³⁸U, ²³²Th and their progeny, including

222Rn (radon)). Table 5.1 shows the average annual dose and typical dose range worldwide from natural sources.

In addition to natural sources, radiation exposure occurs as a result of human activities.

Table 5.2 shows the annual individual effective

TABLE 5.1. RADIATION DOSES FROM NATURAL SOURCES [5.1]

Worldwide average annual effective dose

(mSv) Typical range (mSv)

External exposure

Cosmic rays 0.4 0.3–1.0

Terrestrial gamma rays 0.5 0.3–0.6

Internal exposure

Inhalation (mainly radon) 1.2 0.2–10

Ingestion 0.3 0.2–0.8

Total 2.4 1–10

doses in 2000 on a worldwide basis. Diagnostic medical exposure is the largest non-natural source of radiation. The residual global effects of the Chernobyl accident are now very small but, of course, are higher in European countries and especially in areas of Belarus, the Russian Federation and Ukraine.

5.1.5. Decrease of dose rate with time

To calculate the radiation dose for particular time periods, it is necessary to predict the decrease of dose rate with time. The most obvious mechanism acting to cause such a decrease is the radioactive decay of the radionuclides. Additional DRRFs are usually called ecological half-lives; for example, external gamma exposure rates decrease with time due to the weathering of long lived radio-nuclides such as ¹³⁷Cs into the soil and subsequent migration down the soil column, which results in increased absorption of the emitted radiations within the soil. Typically, a two component exponential function describes this process [5.7, 5.8].

The availability of ¹³⁷Cs for ingestion also decreases with time at a rate faster than radioactive decay. This additional long term decrease is due mainly to the adsorption of ¹³⁷Cs to soil particles from which the caesium atoms are no longer biolog-ically available. As with the external dose rate, the decrease of ¹³⁷Cs in milk or in humans living in areas contaminated by the Chernobyl accident also shows

a two component exponential decrease with time [5.9, 5.10].

5.1.6. Critical groups

In all situations that involve the exposure of large segments of the population to natural or human-made radioactive material, there is always a significant spread in the radiation dose received by various members of the population living within the same geographical area. Those individuals with the higher doses are frequently called the critical group, and these persons may have doses twice or even higher than the average dose to all members of the population considered. Usually such persons can be identified in advance, and, in some cases, special protective measures may be considered.

For external dose, members of the critical group are those who spend a considerable amount of time outdoors, either for occupational or recrea-tional reasons; also, people living and/or working in buildings with minimal shielding might be members of the critical group. For exposure to radioiodine isotopes, the critical group is often infants drinking goat’s milk. Infants have a thyroid gland weighing only two grams that concentrates roughly 30% of the radioiodine consumed; goats are more efficient than cows at secreting radioiodine into milk. For exposure to radiocaesium, critical groups have been identified as those who consume large quantities of local animal products such as milk and meat and wild products such as game meat, mushrooms, wild berries and lake fish.

TABLE 5.2. EFFECTIVE DOSES IN 2000 FROM NATURAL AND HUMAN SOURCES [5.1]

Worldwide average annual

per caput effective dose (mSv) Range or trend in exposure

Natural background 2.4 Typical range of 1–10 mSv

Diagnostic medical examinations 0.4 Ranges from 0.04 to 1 mSv at the lowest and highest levels of health care

Atmospheric nuclear testing 0.005 Has decreased from a maximum of 0.15 mSv in 1963; higher in the northern hemisphere

Chernobyl accident 0.002 Has decreased from a maximum of 0.04 mSv in

1986 (in the northern hemisphere); higher at locations nearer the accident site

Nuclear power production 0.0002 Has increased with expansion of the nuclear programme but decreased with improved practice