Before discussing the biological effects of radiation, the dose of the natural back­ground radiation is revealed. This is the radiation dose that has been present during the evolution of living organisms; they obviously somehow adapted to this radia­tion dose. Every other effect of radiation has to be compared to this effect of back­ground radiation.

Both natural and artificial radioactive isotopes were presented in Sections 13.1 and 13.2; their abundance in the different spheres of Earth was discussed in Section 13.3. These radioactive isotopes and the cosmic ray irradiate living organ­isms as external radiation sources. The cloud chamber photograph (see Section 14.5.1) of the background radiation is shown in Figure 13.5. The tracks of the alpha (thick tracks) and beta (thin tracks) are shown well. A solid-state detector picture (Section 14.5.3) of the cosmic ray exposed on the Cosmos 2044 satellite is shown in Figure 13.6.

In addition, living organisms can incorporate radioisotopes, which cause internal radiation exposure. The types and distribution of the radiation doses, as well as the mean effective doses, are quantitatively given in Table 13.5.

Figure 13.5 A cloud chamber photograph of background radiation. (Thanks to Dr. Peter Raics, Department of Experimental Physics, University of Debrecen, Hungary, for the photograph.)

Figure 13.6 A solid-state detector picture of the cosmic ray exposed on the Cosmos 2044 satellite. The thick track is probably the track of a heavy oxygen ion. (Thanks to Dr. Istvan Csige, Department of Environmental Physics, Institute of Nuclear Research, University of Debrecen, Hungary, for the photograph.)

Table 13.5 shows the part of natural background radiation that lacks any anthro­pogenic activity, which has been present during the whole history of the Earth. Since the nucleogenesis or primordial isotopes (see Section 13.1) cannot be formed under natural conditions on the Earth, their quantity continuously decreases. The decrease is obviously very slow because of the very long half-lives. Nowadays, nuclegenesis, cosmogenic isotopes, and the cosmic ray mean an effective dose rate of about 2—2.5 mSv/years, depending on the geographical location.

In addition, there is natural radiation that is present as a result of anthropogenic activity. These are also listed in Table 13.5. For example, the radioactivity of rocks in the deep layer of the Earth’s crust does not irradiate the living organisms if they are in their original place. When they are brought to the surface of the Earth (e. g., by mining of coal, phosphates, and natural gas), however, radioactive isotopes get onto the surface, increasing the natural radioactivity and the effective dose. Similarly, the 222Rn isotope is a natural radioactive isotope. It has a high atomic number, so it accumulates in closed places such as in caves. The building materials
always contain uranium and, of course, its daughter nuclides, including 222Rn. As a result, living in houses increases the effective dose rate. As seen in the decay series of U (see Figure 4.4), the daughter nuclides of Rn are solid, so they accumu­late in lungs, increasing the internal radiation dose rate. When flying by aircraft, the cosmic ray increases the external radiation dose rate. The total effective dose rate of these types of radiation (which is natural, but a consequence of anthropo­genic activity) is about 2 mSv/years.

The effective dose rate of artificial radioactivity is less than 0.05 mSv/years. This consists of the fallout of nuclear explosions and the emission of nuclear reac­tors. This accounts for about 1% of natural background radiation, and it continu­ously decreases. Medical irradiation represents about 0.5 mSv/years, but this value strongly depends on the level of medical intervention. This value can also decrease with the technical improvement of the instruments used in nuclear medicine but can increase by a larger segment of the population taking part in preventive medi­cal examinations.