Genetic instability leads to the accumulation of many changes necessary to transform the stable genome of a normal cell into an unstable genome. Such a transformation can be the cause of radiation mutagenesis, carcinogenesis and aging †the main long-term effects of ionizing radiation, UV light, and other harmful environmental factors. A new quantitative assessment of the genetic instability was suggested, taking into account the delay in the formation of colonies by cells surviving after exposure to harmful agents. It was demonstrated that the genetic instability of yeast cells is mainly determined by cell ploidy, but not by the sigmoid shape of the survival curve or the cell's ability of post-irradiation recovery. New experimental data confirm the participation of recovery processes in the manifestation of the relative biological effectiveness of densely ionizing radiation both for cell inactivation and for radiation-induced genetic instability. A number of universal patterns of synergic interactions are demonstrated, which did not depend on the used biological test and objects nor the interacting physical and chemical factors. Potential significance of synergy at low intensities of physical and chemical environmental factors is proved. Original data on the influence of cyclic heating and cooling on cell survival are obtained. It is concluded that osmotic homeostasis of cells, along with DNA, is an important target responsible for cell viability under the damaging action of various agents. Data on the favorable effect of the increased natural background of ionizing radiation are discussed. The manifestations of radiation hormesis were revealed for various radiation accidents, professional and medical exposure. The results obtained demonstrate that the action of high-energy ionizing radiation is accompanied by irradiation with UV light that occurs when charged particles move at a speed exceeding the speed of light in a given medium.