1.IntroductionThe fascination with the phenomenon of natural radioactivity arose after the first radioactive elements had been discovered by Maria Skłodowska Curie,1but quickly faded away in the light of further development of nuclear techniques. Fissile products that first emerged in an uncontrolled way were tamed in nuclear reactors and quickly widely applied in techniques and medicine as so called artificial radionuclides. As an inescapable effect of their common use, some of them were released into the environment after nuclear weapon tests and either nuclear power plant breakdowns or radioactive accidents. Long-lived ones, such as isotopes of plutonium, technetium or iodine, from the perspective of a human being's life span, will remain in the environment for ever. But by far the most noticeable in the environment are medium-lived isotopes of caesium and strontium. Finally, despite the fact that artificial radioactivity in the contemporary environment has been occurring only for a few decades it is the main subject of radiation protection. Also, the overwhelming majority of present-day radioecological research is connected with the study of the redistribution in the environment and biological action of this group of radionuclides. Far less attention is paid to the radiation risk to people and especially to the environment caused by exposure to ionizing radiation originating from naturally occurring, primordial radioactive elements, such as thorium, uranium and their decay products.Actually, radiation emitted by primordial radionuclides in their natural state that has not been altered due to human activity is considered to be a source of risk neither for human beings nor the environment. There are many areas in the world having elevated content of natural radioactive elements caused either by the geological and geochemical structure of the rocks, or by the radioactive content of water flowing from underground springs.2Whether or not it can cause a negative or positive effect on human beings is a matter of opinion. But if concentrations of natural radionuclides have been changed by deliberate or accidental industrial action it is quite another matter. According to the current radiation protection principles, the related risk, excluding impact of so called natural background, must be treated as a risk caused by artificial radioactivity.The classical case where the radiation risk caused by natural radioactivity isn't negligible is uranium mining and milling. It is abundantly clear that such processes must be carried out in regions where reachable uranium ore occurs. But such activity is considered to be an immanent part of nuclear industry so that it was enclosed within the radiation protection domain at the very beginning. After the enhanced natural radioactivity had been thoroughly studied in other industries it became clear that such phenomena are very frequently present in the anthropogenic environment. Many processes beyond the nuclear industry lead to a situation when the activity concentration of naturally occurring radionuclides is enhanced. Such situations usually take place in industrial processes where a significant mass reduction of raw materials occurs. As a matter of course, these industries of concern are not aimed at the production of natural radionuclides or the deliberate use of radiation. Therefore, radioactive isotopes are usually accumulated in waste. Such alterations to the natural state can result in an increase of radiation risk to people as well as to the environment. Each particular occurrence of natural radioactivity presents a unique scenario of exposure – usually different from those caused by artificial radionuclides present in radioactive waste or spent nuclear fuel. Frequently, the amount of this type of waste can be up to hundreds of thousands of cubic metres or tonnes and they are often placed directly into the environment. In the coal mining industry, radium activity deposited in single tailing ponds may reach 300 GBq.3Probably, the biggest “producer” of waste with an enhanced concentration of natural radionuclides are phosphate processing plants where radionuclides remain associated to the phosphogypsum particles, being subsequently stored in disposal sites located in the vicinity of the factories at a rate reaching 350 MBq h−1.4As a result of the direct contact with the environment, some transformation processes, such as mobilisation of radionuclide species from solid phases or interactions of mobile and reactive radionuclide species with components in soils and sediments, may be set in motion.5Also, considerable transfer of radionuclides to biota can be observed.6,7All these result in the speciation and original distribution of radionuclides deposited in an ecosystem changing over time. Moreover, natural radionuclides are often associated with other pollutants, such as heavy metals or hydrocarbons, that can escalate the negative impact on the environment as they are dumped out of industrial plants.