**2. Radionuclide occurrence in nature: natural and anthropogenic sources**

Natural radioactivity, including external terrestrial γ radiation, internal α-, β-, and γ-radiation from naturally occurring radionuclides, cosmic radiation, and exposure to radon (222Rn) and thoron (220Rn) and their radioactive progeny molecules, accounts for ~95% of the annual radiation dose for the terrestrial biota [1]. The global annual dose for an average person is 3.6 millisieverts/year (mSv/a), of which 82% is due to natural radiation exposure, around 15% is due to medical exposure, and only about 0.8% is due to anthropogenic contamination of the environment. Natural radioactivity has been a subject of concern for decades. Globally, there are areas with increased natural radiation, often due to thorium (232Th) deposits in the form of monazite rocks. Two such areas are Guarapari, Brazil, and the state of Kerala in southern India. The area of Ramsar, Iran, has enormously increased natural radioactivity due to radioactive hot springs containing 222Rn and its progeny. Although annual doses in these areas reach an average of 35–40 mSv/a, compared to 3.6 mSv/a average in Europe and 2.5 mSv/a in Bulgaria, modern biomedical studies report no excess cancer risk, leading researchers to believe that a 10-fold increase in natural radioactivity is harmless [16].

In contrast, environmental contamination by anthropogenic radionuclides without doubt creates serious risks. The Chernobyl accident is the most prominent example of environmental damage due to technogenic sources, although it is not the only one; Chernobyl caused significant chronic morbidity and mortality in people and enormous damage to the environment and economies in Europe. This is mostly due to 131I, 137Cs, and 90Sr, and their tendencies for **bioaccumulation** and **biomagnification** in terrestrial ecosystems [17]. Although the Chernobyl accident is the bestknown example, there are many other significant contamination events in the period 1945–2011 (**Table 2**).

One aspect evident from the table is that, according to atmospheric radioactivity released, the Chernobyl accident exceeds all other INES scale 5–7 accidents combined. At the same time, during this accident, only about 30% of the core radioactivity was released, suggesting that a full-blown reactor explosion can cause even greater damage to the environment. Another noteworthy peculiarity is that most reactor accidents



*Radionuclide Contamination as a Risk Factor in Terrestrial Ecosystems: Occurrence, Biological… DOI: http://dx.doi.org/10.5772/intechopen.104468*

so far occurred either with new or experimental power plants (Chernobyl, Chalk River, Simi Valley, Beloyarsk) or military reactors (windscale). Nevertheless, the Fukushima accident in 2011 presents a new precedent—the reactors in the plant were old, nearing the end of their design life. Since this is true for many of the currently operating reactors, this presents a new, threatening perspective. Aging, crumbling nuclear infrastructure may present a new, unmitigated radiation hazard in the future.
