Purpose:The effects of ionising radiation on the human body has been discussed. The authors believe that the understanding of the radiation incidents from the perspective of its effects is crucial for better preparation, and therefore safer and more effective responses to incidents involving such threats. Introduction: The increasing use of radioactive materials and radiation producing devices in many areas of our lives carries the risk of exposure to high doses of radiation being hazardous to our health due to possible damage to radiation sources or improper handling. Exposure resulting from the intentional use of radioactive materials for criminal or terrorist purposes cannot be excluded, either. Exposure to ionising radiation may cause adverse health effects both to victims of a radiation incident and for rescuers providing emergency care. Such threats require the proper preparation of emergency medical services (EMS). Part of these preparations is to examine the specifics of radiation hazards, including radiation sources, the mechanism of injury of ionising radiation and the type of radiation damage. Methodology: The publication presents the properties of ionising, corpuscular and electromagnetic types of radiation, which are the most important from EMS's perspective. The dangers of contact with a radiation source, the problem of external and internal contamination, the estimation of the amount of absorbed radiation were discussed, and the interrelationships between them were presented. The mechanism of direct and indirect action of ionising radiation on cell structures (DNA, mRNA, cytoplasmic membranes) and intracellular enzymes was thoroughly discussed. The authors presented health consequences of radiation for the body in the form of acute (deterministic) lesions and late (stochastic) lesions. Conclusions: Particular attention was paid to acute radiation syndrome (ARS). The dependence of ARS on the amount of absorbed radiation was discussed in detail. Four stages of ARS were presented: initial, latent, manifest illness and recovery (or death) as well as the time of their onset, duration and end. The mechanism of damage to individual organs and systems was also analysed. The most common symptoms, their severity, and causes of life-threatening conditions, resulting from radiation damage in particular syndromes of ARS, were indicated. In addition to systemic effects, local changes in the form of Cutaneous Radiation Syndrome (CSR) were discussed.