**1. Introduction: the essence of radionuclides—emission types and biological effects**

Radionuclides are unstable isotopes of different chemical elements. Usually, this instability is due to excess energy in the atomic nucleus, leading to the release of particles with different energies in a process called **radioactive decay**. Natural radionuclides emit three types of radiation: alpha (α), beta (β-), and gamma (γ). Of these types, α-particles have the strongest biological effects, causing 20 times more biological damage than an equivalent dose of β- or γ radiation [1, 2]. While α- and β-particles tend not to penetrate into matter, γ-radiation, especially at the higher

end of the energy spectrum, penetrates deep into living and non-living matter. This means that, when considering the biological and ecological effects of radionuclide contamination, α- and β-emitters are only relevant if incorporated into living organisms. In contrast, γ-emitters are relevant as both internal and external components of the total absorbed dose. In the context of anthropogenic contamination, it needs to be taken into account that some of the man-made radionuclides emit other types of radiation. For example, radioisotopes used in medical PET scans such as 18F, 11C, 13N, 15O are positron (β<sup>+</sup> ) emitters. Other, more exotic man-made radionuclides such as Californium-252 (252Cf) are capable of spontaneously emitting neutrons. Both positron and neutron emitters require specific equipment for handling and detection of the radiation sources [1]. Some radionuclides emit multiple types of particles. The anthropogenic radionuclide 137Cs emits β<sup>−</sup> particles at two energies: 511 and 1173 kiloelectronvolts (keV), and γ-rays at 32 and 661.6 keV [3, 4].

The biological effects of radionuclides are mainly due to the emitted ionizing radiation (IR). IR interacts with biomolecules directly by damaging them or indirectly—by producing reactive oxygen species (ROS), which in turn damages biomolecules. According to the paradigms of classical radiobiology, the principal target of IR on a cellular level is genomic DNA—it can be damaged directly or indirectly, leading to cell cycle arrest and an activation of DNA repair systems, followed by recovery, cell death, or mutagenesis [5, 6]. Sparsely ionizing radiations such as β- particles and γ-rays cause around 70% of DNA damage indirectly through ROS, while densely ionizing radiations, such as α-particles and high-energy cosmic particles, cause only about 30% of the biologically significant damage indirectly [7]. Researchers have elucidated the biological effects of high and medium doses of radiation. Nevertheless, biological effects at low doses remain insufficiently understood and a subject of much debate [1, 8]. Currently, radiation risk is extrapolated linearly to the low doses by using the **Linear Non-Threshold (LNT)** mathematical model [1, 9]. However, other hypotheses include **radiation hormesis**, which is the idea that small doses of radiation are beneficial [10], and **low-dose hypersensitivity**, which is the assumption that low doses of radiation are more mutagenic because they do not activate DNA repair systems [11]. While radiation hormesis has been well researched recently [10], it has still not been taken into account in radiation protection calculations, where every minimal dose of radiation is assumed to carry a small but non-negligible risk [12]. On the other hand, the low-dose hypersensitivity hypothesis is supported by recent studies, raising questions about the validity of current assumptions in radioprotection [13]. Living organisms tend to display different radiation sensitivity. Mammalian species are very sensitive to radiation, while insects tend to be comparatively radioresistant. The champion of radiation resistance is the bacterium *Deinococcus radiodurans*, which can withstand an acute dose of 5000 Gray with almost no loss of viability. Similarly, tardigrades can withstand 5000 Gray with 50% loss in viability (LD50 = 5000 Gy). For comparison, the LD50 for humans is around 6 Gray, for mice around 6.4 Gray, and for goats only around 2.4 Gray [14].

A significant concern in radionuclide-contaminated areas arises from the process of **bioaccumulation**. Similar to other chemical elements from their respective groups, radioisotopes are incorporated preferentially into different target organs and tissues. Thus, 137Cs, a chemical analogue of potassium, is preferentially accumulated into nerve and muscle tissue. 90Sr, an analogue of calcium, has a very strong affinity for bone and hematopoietic tissue. Some of the properties of the three most environmentally significant anthropogenic radionuclides are presented below (**Table 1**).

As evident from the table, the most significant environmental contaminants of the above are 137Cs and 90Sr due to their long half-lives and persistence in nature. 131I was

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


**Table 1.**

*The most significant anthropogenic radionuclides and their biological effects (data adapted from [3, 4]).*

only a very significant contaminant in the first year following the Chernobyl accident, causing ~4000 excess thyroid cancers in the most significantly affected populations of Russia, Belarus, and Ukraine [15].
