Currently, INERIS is involved in European and French projects regarding the CCS chain (carbon capture and storage). Nowadays, some people consider this chain as a future device to fight against the global warming due to high concentration of CO2 in the atmosphere. However, in case of massive accidental leak of CO2, this substance could be the origin of toxic effects for human. Now, carbon dioxide concentrations considered as potentially toxic get close to 10 0000 ppm. To estimate precisely the distances reached by this hazardous level of concentrations, an efficient understanding of the CO2 release phenomena, from the assessment of the mass flow rate to the atmospheric dispersion, is necessary. Whereas the carbon dioxide is often stored and handled under 2-phase or supercritical conditions associated to storage pressure, CO2 ice formation is possible in case of accidental leak in the atmosphere due to specific properties regarding its triple point. Then, this CO2 flakes creation may be followed by the liquid/solid CO2 pool formation on the ground. Due to the important pressure drop of the fluid during the leak, a significant expansion phase, a high rate of air entrainment and a huge temperature drop followed by a dense and cold cloud formation should be considered. Concerning these specific points, a few of atmospheric dispersion softwares take into account the carbon dioxide specific conditions of release. And moreover, some calculations carried out with other software seem to provide different results in terms of hazardous distances for instantaneous or continuous releases. A focus on the models used by the softwares appears interesting to let a better understanding of this difference in the risk assessment context.