###### Abstract  
In this paper, we present a useful Monte Carlo (MC)-based methodology that can be utilized to calculate the absorbed dose and the initial levels of complex DNA damage (such as double strand breaks-DSBs) in the case of an environmental ionizing radiation (IR) exposure incident (REI) i.e., a nuclear accident. Our objective is to assess the doses and complex DNA damage by isolating only one component of the total radiation released in the environment after a REI that will affect the health of the exposed individual. More specifically, the radiation emitted by radionuclide \({}^{137}\)Cs in the ground (under the individual's feet). We use a merging of the Monte Carlo N-Particle Transport code (MCNP) with the Monte Carlo Damage Simulation (MCDS) code. The DNA lesions have been estimated through simulations for different surface activities of a \({}^{137}\)Cs ground-based \(\gamma\) radiation source. The energy spectrum of the emitted secondary electrons and the absorbed dose in typical mammalian cells have been calculated using the MCNP code, and then these data are used as an input in the MCDS code for the estimation of critical DNA damage levels and types. As a realistic application, the calculated dose is also used to assess the Excess Lifetime Cancer Risk (ELCR) for eight hypothetical individuals, living in different zones around the Chernobyl Nuclear Power Plant, exposed to different time periods at the days of the accident in 1986. We conclude that any exposition of an individual in the near zone of Chernobyl increases the risk of cancer at a moderate to high grade, connected also with the induction of complex DNA damage by radiation. Generally, our methodology has proven to be useful for assessing \(\gamma\) rays-induced complex DNA damage levels of the exposed population, in the case of a REI and for better understanding the long-term health effects of exposure of the population to IR. os  
that nuclear accident found application [1] to other accidents that followed of a smaller scale, such as in Brazil (the Goiana \({}^{137}\)Cs accident in 1987) and the two accidents that took place in Tokaimura of Japan on 11 March 1997 and 30 September 1999 [2; 3], as well as of a larger scale, as with the accident that too place in Fukushima Daiichi on 11 March 2011 [4]. It is of note that environmental pollution due to man-made radionuclides began in the period 1946-1980, during the process of nuclear materials testing, when approximately 400 nuclear tests were performed in the northern hemisphere [5]. Radionuclides released in the air from a radioactive source may be in a gaseous, particulate, or multi-phase (i.e., simultaneously gaseous and particle) form [6]. These particles are generally transported through the air by adhesion to aerosols, construction particles or soil particles, as well as weather conditions such as the wind, rain and snow, which play a crucial role in transporting particles from the place of their release to the surrounding areas. Concerning the radionuclide spreading mechanism following an REI (i.e., a nuclear power plant accident, a nuclear weapon detonation, a radiological dispersal device, a transportation incident, sabotage or an improvised nuclear device), it is suggested that part of these radioparticles are deposited on the surface soil, while others are transported to the underground layers with rain, snow and watering, or they are transferred to surrounding geographic areas by re-floating [7]. The migration of radionuclides across the ground surface depends on the particle and surface characteristics. The transport of contaminants may vary depending on their physiochemical characteristics; for example, \({}^{137}\)Cs is characterized as highly mobile because of its high-water solubility [8].