**3. Radionuclides and nature: significant risks and unknowns**

Some of the risks to ecosystems posed by radionuclide contamination are well understood. They include, at high doses >1 Gray acute dose, teratogenesis in developing embryos, stunted plant growth, and visible damage to the flora and fauna. These are **deterministic effects**, and they occur definitely after exposure to strong doses of ionizing radiation and are dose dependent (**Figure 1**).

As shown in **Figure 1**, pine trees are very radiosensitive; they can serve as a bioindicator of severe radioactive contamination at doses exceeding 3 Sv acute exposure [19]. The other depicted deterministic effect is teratogenesis in pregnant mammalian species. At doses exceeding 1 Sv acute *in utero* exposure, the number of resorbed fetuses decreases, and so does the number of offspring born with malformations [1].

Perhaps more worrying are the **stochastic effects**, which occur with a small probability even at low radiation doses. These include **radiation mutagenesis** and, as a consequence of it, **radiation carcinogenesis** [1, 12]. Based on data from experiments with specially bred laboratory mice and results from the radiobiological monitoring of humans, exposed to γ-rays and neutrons during the bombings of Hiroshima and Nagasaki, it is estimated that the doubling dose of radiation-induced mutagenesis is 1 Gy. This means that an acute exposure to 1 Gy of γ-rays doubles the spontaneously occurring rate of mutation [20, 21]. Nevertheless, this perspective is being challenged. For example, Belarussian researchers observed transmission of chromosomal damage in the progeny of wild rodents from the vicinity of Chernobyl, indicating **genomic instability** [22]. An international team observed a higher mini- and microsatellite mutation rate in the children of Chernobyl liquidators [23]. Both of these findings support the theory that even low doses of radiation can be harmful to the biota, as well as current and future generations of humans. Another, more recent venue of

#### **Figure 1.**

*Deterministic effects of ionizing radiation: Dead pine trees near Chernobyl, Ukraine in 1990 (left, taken from [19]), and experimental radiation teratogenesis in mouse embryos (right, photograph by Dr. Roberts Rugh, taken from [1]).*

research with significant progress is the radiation-induced bystander effect (RIBE) phenomenon, in which non-irradiated cells show similar cytotoxicity and genetic damage to their irradiated neighbors [24, 25]. The results from bystander effect studies generally support the theory of low-dose hypersensitivity and highlight possible molecular mechanisms for increased radiation risks in the low-dose range [24, 25]. Radiation risk is still to be taken very seriously, and every effort should be made to keep radioactive contamination of ecosystems to a minimum.
